Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA-9mA

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Andrew Seltzman
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Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA-9mA

Post by Andrew Seltzman »

With the new modifications to the spellman supply, I can now run at up to 9mA grid current at up to 40kV. After pumping on a turbo station for about 14 hours, the fusor pressure is down to 3e-4 torr as measured on an ion gauge. Running the ion injectors at low power without any grid bias lowers the pressure to about 2.8e-4 torr, but then it will rise back up to 3e-4 when the ion sources are turned off, that seems to be the limit of my system due to gas permeation of the viton o-rings / out-gassing of internal plastic parts ant the low conduction rate of my vacuum system. I also suspect the boron nitride grid ceramic insulators on the grid are out-gassing when bombarded with plasma. During operation, the pressure will rise towards 1mTorr, but will pump down pretty fast after grid bias is turned off.

With the anode layer ion sources off, at 3e-4 torr, there is no measurable current draw from the grid at -40kv. The injectors will run perfectly at this pressure with enough plasma current to easily hit the 9mA limit of the spellman. The fusor maintains rock solid discharge stability with these sources on all the way down to the 3e-4torr minimum achieved pressure(for reference, there is no ballast inductor or resistor in series with the grid supply), by varying the ion source bias, the current into the grid may be controlled up to the 9mA max as limited by the power supply.

4e-4torr, 1ma, 40kv, 4 injectors on
4e-4torr_1ma_4injectors_40kv.JPG
5e-4torr, 1.3ma, 40kv, 4 injectors on
5e-4torr_1.3ma_4injectors_40kv.JPG
7e-4torr, 7ma, 40kv, 4 injectors on
7e-4torr_7ma_4injectors_40kv.JPG
Vacuum during operation
plasma-pulses.jpg
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Jerry Biehler »

14 hours and you are only in the -4 range?
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Yes, I just did a pump down / valve off / pump down plot to show pressure rise, it's entirely linear after around 2mTorr. I'm almost entirely convinced it's due to gas permeation of the o-rings. All my orings are viton, I have 16x 2.75" CF, 1x 6" CF, 1x 0.75" quick connect, 1x NW40, 1x NW16 on the fusor. I helium leak checked all o-rings and welds and found no leaks (~3e-8 TL/s), but that's only inspecting where the helium is blowing over(looking for discrete leaks), I'm fairly sure it is the cumulative permeability effect of all the viton in the system.
pressure-rise5.jpg
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Jerry Biehler »

Unless you have a really tiny turbo you should be much lower than that in 14 hours even with a system that has been at air for a long time. You should be down to the -5 to -6 after that period. I bet you still might have one little leak someplace.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by John Futter »

Andrew
Itend to agree with Jerry. We have systems at work with many tens of metres of viton o-ring. All of these systems get down to the low ten to the minus eight eventually (weeks of pumping).
But we will not use any zinc plated screws, polycarbonate, as these have extremely high vapour pressures
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Dennis P Brown »

Appears your system is leaking badly (like there ever was a good leak? OK, not very good joke.) Try a can of "Dust Off" type spray to search for the leak when the ion gauge is working - you will see a jump when it hits the area of the leak(s). Nearly pure alcohol works well, too. If you have O-rings, possible you have a scratch in a metal face that cross's over to of one and is a source of the leak.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull »

Based on what I am seeing and what is reported, this system will do fusion just fine provided you just flow in D2 with the pumps running and a slight valve down on the pumps. Start with a D2 pressure of 5 microns and work up. I assume you have a good neutron detection system that can readily herald increases and decreases in fusion as the pressure and voltage are varied.

I have run a terribly leaky fusor for years with a good diff pump that just cannot ever reach 10E-5 torr. My system has produced over of a million n/sec as there are many actual witnesses on this board who can attest. No one needs to go to or below 10E-5...ever...to do good fusion. Only vacuum head purists and the anally retentive need struggle for better vacuum levels. A laudable goal to be sure, but just not needed.

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Bruce Meagher »

Beautiful pics, and a very impressive setup!

If your leak rate is truly linear then you have a real leak. You might consider running a longer test to capture data over a larger pressure range. This might help isolate the problem. As I’m sure you’re fully aware, the size of the leak is the change in pressure * volume / time. From your graph it appears to be around 9x10-4 sccm * vacuum chamber volume in liters (assuming I did the math correctly). If the pressure rise is linear then you can rule permeation, diffusion, vaporization, etc. of the o-rings. I believe these processes are all a function of pressure and therefore would not show a linear response.

How are your four ion sources plumbed to the chamber, and how is the gas connected to the ion sources? Do you have a separate larger pumping port on the ion sources for pumpdown? One area I'd think could cause some vacuum trouble would be in the gas supply system (e.g. a leaky needle valve, laser orifice not valved off, etc.).
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

If it looks like a duck and quacks like a duck...
I did give it a very through leak checking(with a He leak checker) including all welds, o-rings, and fittings, even inside the grid feed through box and inside the ion gauge. I must have missed a spot, somewhere... You would think that with such a large apparent leak it would be easy to find.

Dennis: How does the dust off work, you would think with a heavier molecule the leak rate would slow when blowing tetrafluoroethane over it.

John: Do you l-grease your o-rings?

Richard: I definitely want down to 1e-4 when operating with the ion sources to study what fraction of the ions hit the grid.

Bruce: The ion sources are mounted directly on the chamber nipples, they currently have the gas feeds capped off(VCR cap with new Cu o-ring)
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Dennis P Brown »

The "Dust Off" spray works fine for finding even very slow leaks; I have detected very tiny leaks. As for "greasing" O-rings that only works if the leak is small - ie very fine cracks or scratches. First, as I pointed out, look for a scratch or a piece of debris that is causing a leak across the O-ring. However, to "grease" the O-ring, take a very small amount of high quality vacuum grease and rub it around the O-ring. Then use a clean dust free wipe and wipe ALL the grease off. That will leave more than enough to seal tiny cracks on the O-ring or micro-scratches on the metal surface. Remember that for a while, the grease will "out-gas" a good bit so give this method some time before thinking the system is still leaking. Also, use as little as possible - large amounts do more harm than good since a large leak stopped by grease will most likely start very soon and then a large amount of "grease" vapor will deposit all over your formerly clean vacuum system ... . Only do "grease" if all other methods fail - checking for scratches or debris and re-mounting all O-rings/re-tightening bolts to make sure they they are uniform and not over tightened.

Still, if you HE-leak tested I would think you didn't miss a large leak. Still, it happens so use either a "Dust Off" can or alcohol (70% or higher.) when the ion gauge is running. The gauge will "jump" when the area leaking is subjected to the test. While 10-4 torr is good for fusion, a large leak can introduce water vapor and as I understand, this can causes issues in achieving fusion. Whether that really is an issue or not, still, best to get rid of any major leaks.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Leak raised the fusor pressure to about 500mTorr after about 12 hours, pumped down to 3.1e-4 torr in 3 hours.

Tried the dust off spray at 3.1e-4 torr(I gave everything a solid spraying,), still haven't found the leak.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Jerry Biehler »

Very strange. Sure the turbo is running at full speed? On my sem which has a similar volume and turbo size, I can be down to the -5 in minutes, even after the system has been up to air for a while. And that is all o-rings, not a metal seal on it.

Maybe you have some internal component that is porous and is holding a whole bunch of air or water vapor. Any blind holes inside the system with non-vented screws?
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Dennis P Brown »

Wait, 12 hour and the system rose up to just half a torr? That isn't a very large leak if it is a leak ... if the system had been exposed to air, that could be normal out gassing. Maybe its the turbo (not up to speed as mentioned) but are you certain the ion gauge is correct? My system too, gets to mid 10^-5 torr in minutes and I have a small turbo and lots of O-rings. To be on the safe side, I have two independent ion gauges for pressure check so maybe (?) the gauge is lying? My system was bottoming out in the very low 10^-5 torr range and the "Dust Off" spray detected a leak that keep me from reaching mid 10^-6 torr so the stuff works - if you are reading no leaks maybe you have no leaks! Check the accuracy of the ion gauge.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Jim Kovalchick »

I agree with Dennis. Pressure rise to only a half torr in 12 hours is not bad at all. If possible, you should check your turbo and instrumentation by dead heading your pumps onto just the instrumentation without all the chamber attached.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

I brought the chamber up to air and swapped ion gauge systems.
Deadheading the gauge against just the turbo brought it down to the low E-5 range pretty rapidly(minutes).

Leaving the fusor pumping down over night resulted is a vacuum of 2.1e-4 after the gauge tube swap.

The turbo pump is a pfeiffer TPU 055, pumps about 30L/s in the 1e-4 range.

Leak rate by pressure rise method is a pretty constant 1E-5 TL/s
Leak rate by He leak checker was about 3E-8 TL/s

Things that I suspect for the gas source(other then potential leaks):
--------------------------------------------------------------------------------
The grid system:
It has some trapped volume(but not tightly trapped) There is a 0.75" OD, 0.5"ID, 6" long quartz tube with a BN insert sitting on the top
The grid insulator ceramics: there are 95% Boron Nitride (BN), 5% Boric Oxide
If memory serves correct, the "leak" was there without the grid was mounted

The ion sources:
They have NdFeB magnets
The casings are 410 stainless(sulfur free though)

Plastic insulator:
There is an acetal copolymer standoff(2" length, 1.5" ID)
It was baked in a vacuum oven for 1 week in the 10s of mTorr to outgas any condensable volatiles


pumpdown-gauge2.jpg
pumpdown-gauge2-pressure-rise.jpg
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Jerry Biehler »

Plastics are generally a bad idea inside the chamber. Some are usable like teflon (internally only, it is somewhat permeable by air). It is best to stick to ceramics.

Boron Nitride is just fine.

Magnets and the stainless should be fine.

A not terrible list of vacuum compatible materials is here: https://en.wikipedia.org/wiki/Materials ... s_to_avoid
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Latest pictures:
There was an ion beam hitting Pyrex view port and causing it to fluoresce, magnets were added to sweep away the beam resulting in a much clearer picture.
Still no progress with the leak/outgassing Contemplating replacing the acetal insulator with a PEEK/stainless insulator and possibly re-cleaning the grid ceramics and ion injector parts.
SAM_4400.JPG
SAM_4402.JPG
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Dennis P Brown »

OK - still don't think it is the chamber; now the turbo works (but as you know, speed is the issue and ultimate chamber pressure) but what about the fore pump and/or lines? A leak back against the turbo will surely look like a chamber leak and/or does the fore pump work well? Have you tested just the fore pump on the chamber? If its performance is very poor (can't pump fast, or oil out gassing or a leak in the fore line system) the turbo will not pump your system down against a high fore pressure. Not necessarily the problem but very well worth looking into.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

I think it may be outgassing.

The turbo has a valve on the inlet, and the chamber also has a valve on the inlet (both nw16 valves), they are connected by a 2ft nw16 stainless flex hose. Deadheaded against the ion gauge with no chamber, the turbo will pull down into the low 10^-5 torr range very fast so I don't think it's the turbo or backing pump. Closing the valve on the chamber or the turbo will cause the pressure to rise at about the same rate, so I don't think it an issue with the lines connecting the turbo station to the fusor.

Valves are pictued here:
http://www.rtftechnologies.org/physics/ ... hub-v2.htm

I'm sure I take a rather substantial hit in conductance from the valves, but it does seem that there is a gas source inside the chamber.

2 other things that i thought of:

I have a pair of ZnSe viewports, made out of AR coated lenses for high power CO2 lasers, though ZnSe in it self shouldn't out gas.
http://www.rtftechnologies.org/physics/ ... ometer.htm

There is a woven alumina fabric shield for the chamber o-ring to prevent plasma bombardment, though it was baked out at very high temperature(red hot)
bottom of:
http://www.rtftechnologies.org/physics/ ... sphere.htm
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Dennis P Brown »

From the picture your turbo is attempting to pump your chamber through a very long, very small diameter (16 mm) line. While for a well sealed (your's appears to be), clean system that will work it is really creating unnecessary issues. Why not connect the turbo directly to the chamber with as large coupling as possible? Adding a 16 mm valve just makes it worse. While not necessarily the cause, that just makes any minor leaks/out-gassing very slow to pump. My turbo has a 16 mm diameter opening for the fore line port but I use an adapter and connect my fore pump to the turbo using a 40 mm diameter SS tube. As for my fusor, I use a water cooled DF pump and the DF connects directly to my fusor chamber via a gate valve that is the same diameter as the DF pump mouth. My chamber is, however, very much bigger; despite this, it pumps down in under five minutes to mid 10^-5 torr once my DF pump is at temp.
You have checked the fore line pump and it is getting under 10 microns?
As for your insulators you show in a pic - exactly what do they accomplish? That is, once you strike a plasma in your chamber, the insulator section is worthless since all metal parts connected to the insulator are now connected directly to the plasma and will share in the voltage.
By the way, you have one extremely professional looking system! Your machine shop work is absolutely fantastic and the results really show it.
Also, unless your metal flanges have a scratch, I agree it is highly unlikely they are leaking. Out gassing in a long, small diameter system with a large number of components can really take time but if it has been under vacuum for a number of days, then really, a clean system will not out gas significantly from then on even if exposed for short periods to air - do consider an air dryer for admitted air. I have one and it really makes a difference. My accelerator when back filled for an hour or so with dry air then pumps down to mid 10^-5 torr in under 5 minutes.
Can't really tell but do you do have a oil vapor anti-back streaming component for your fore line pump?
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

The hose used while pumping the fusor is larger then the one shown in that picture, I use a 2', 0.75" ID stainless hose to connect the turbo station to the fusor.
SAM_3401.JPG
The reason for the dual valves is to allow the turbo station to be disconnected from the fusor without bringing the fusor up to air so the station can be connected to other hardware. The turbo side valve allows the turbo to be isolated from whatever it is connected and the roughing pump so the vacuum system can be brought up to air/roughed out without stopping the turbo. Hopefully someday I'll have gate valves instead of the 16mm solenoid valves.

As far as an air drier, I'm thinking about buying one of these:
https://secure.drierite.com/catalog3/page5.cfm
How important is it to add a fine filter on the drier outlet(eg a 40um) to prevent dririte particles from entering the system?

As far as outgassing there seems to be a thermal component associated with the heating of the grid boron nitride insulator ceramics. As they get hot, the fusor pressure rises.

The plastic insulator electrically separates the CF cube from the fusor shell (at very low pressures where there are predominantly ion beams rather then a diffuse plasma filling the fusor), both hemispheres are electrically insulated from each other as well to allow a future upgrade that can monitor collected shell current in each hemisphere independently.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Dennis P Brown »

I just use aquarium grade filter floss to filter my drying agent; as for the insulator - a fusor operating at 5 microns is in a plasma state, not isolated ions and all parts will then conduct electric power from the plasma very well; yes, significantly below 10^-3 torr it starts to become very non-conductive ions but I didn't know that was were you were operating (exactly how can it operate as a fusor, then?) Good idea to show the current picture when asking questions on problems. I find it difficult to believe your system leaks that badly - then the only possible reason is the turbo or fore pump. As for the fore pump, if the turbo fore line pressure is below 100 microns than all that remains is the turbo. That said, a bad turbo with "zero" load can make 10^-6 torr but fail with a small out gassing. I suggest you check the fore line pressure (we had a SS tube like yours have a pin hole leak!) and then, the turbo on a smaller system that has more limited out gassing. If the turbo is somewhat better but still in the 10^-4 range than it is the turbo ... . Maybe your system has a tiny bit of liquid water in which case out gassing could continue for days ... just guessing here.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Here is the roughing line pump down with the turbo on and the fusor open. Backing line at about 15mTorr
roughing-line-turbo-on-chamber-open.jpg
Turbo pump down with turbo pumping on gauge only (pumps down to about 8e-5)
turbo-pumpdown1.jpg
Turbo pump down with turbo pumping on gauge only (pumps down to about 8e-5)
turbo-pumpdown2.jpg
Gauge valved off
gauge-valved-off.jpg
This leads me to believe the gas load is somehow coming from the inside of the gauge. I have suspected this for some time but have no way of proving it. Two different gauges give similar results. Is it possible the gauges are contaminated?

The turbo spins up fine to full speed. Two different turbo pumps have produced similar results on this system. Is it possible the turbo is contaminated?
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull »

One reason I like the diff pump is that water in the system can and does wind up in the diff pump and its oil and is pumped out of it as the boiler reaches its operating range between 140-180 degees C. Once a first pumping and long run occurs, the water is pretty much out of my system unless I leave it un-attended for a month or more. (common after HEAS)

The common water signature and assurance that it is gone is as folllows:

Turn on the forepump and open the basllast valve fully and evacuate the fore line to about 15 microns. Open the valve to the diff pump and pump it down to a foreline indication of about 20 microns. Open the fusor valve and this raises the foreline pressure to almost a full torr, but instantly plunges back to about 25 microns and stabilizes there. Now turn on the diff pump boiler. As the boiler temp reaches about 80 deg C. the foreline pressure rises, (yes, I have a PID controller on the boiler to read the temp and control the fan)

This pressure continues to rise to about 40-50 microns as the boiler temp goes above 120 deg C. (volitiles and water being removed from the diff pump and its oil) The pressure in the fusor doesn't rise more than about 2 microns during all this as the jets in diff pump go active and the mechanical pump ditches the water. Quickly, within seconds the fusor chamber pressure plunges to 0.5 microns or (5X10e-4 torr). At this point I let the boiler continue to heat to about 160 deg C and by this time the foreline is back to about 20 microns and I shut the ballast off.
As the ballast is closed I'll usually see about another 3 or 4 micron drop in the foreline to 15 microns or so. Ultimately, the diff pump drags the system down to the 7X10e-5 range and fusor operation with flowing D2 commences.

Fusion will heat the fusor to about 100 deg C and water molecules are continuously boiled out of the fusor walls. This is, inturn, pumped out of the system. Thus a three day group of separate runs will have the bulk of the water out of the system.

Note I never use S.I. pressure units and also refuse to use torr in my discussion unless well below 1 micron. The fusor in operation and in most pump down scenarios is always in the "micron range" It is a micron device. I religiously go to scientific notation torr for diff pump bottoming figures. However the fusor system never, ever dwells there long as fusor operation shoots back to the multi-micron level of real fusion operations.

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Nick Peskosky »

Andrew,

I think I might have an answer to your virtual leak problem for you from experience. I just looked at the way you have the MKS gauge head connected to your vacuum head CF-cube. It looks like you're using a Conflat->VCR->KF->KF flange sequence to connect the gauge head to the chamber itself. You might be facing a conductance issue with reducing the throat of the head from KF-to-VCR in the molecular regime, in effect differentially pumping the sensor. I did something similar on my chamber when I had my combination Pirani/Ion gauge hooked to the small male 1/4" VCR via a ~5" length of 1/4" ID SS tubing->KF->KF. I could never get the chamber pressure (as measure during this setup) to dip below the mid E-4 Torr range but when I dead-headed the gauge my turbo would quickly drop the pressure below E-6 Torr. When I re-plumbed the gauge to a KF->CF nipple on my 2.75" tee the measured pressures dropped well into the lower E-5 Torr range after minimal pumping. The leak probably appears to come from the gauge because it isn't bottoming out at the chamber's ultimate pressure (you wouldn't see this when you dead head the gauge because in-effect it is your total 'chamber' volume). Hope this helps!
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

The picture with the VCR adapter is pretty old, currently it is a 2.75" CF to NW40 adapter on the cube(the gauge has a NW40 flange).
When deadheading the gauge it is an NW40-NW16 adapter.

I'm going to install some bake out heater tape on the fusor shell to assist with pumpdown.
Would it work to install a small(~2l/s) ion pump on the CF cube on the fusor for maintaining low pressure on a valved off system when not running(so the backing/turbo pump don't have to be continuously run)?
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Ion beam deflecting magnets have been added to all 4 view ports on the fusor. In addition to preventing burn in on the view ports, it also eliminates view port fluorescence under ion beam bombardment leading to considerably clearer pictures.

the magnets used are:
https://www.kjmagnetics.com/proddetail. ... CS&cat=173

The aluminum clamps are modified from:
http://www.ebay.com/itm/CBM-10923-Bille ... 70&vxp=mtr

Modified and unmodified clamps
SAM_4408a.jpg
Clamps and magnets
SAM_4411a.jpg
Mounted clamps
SAM_4413a.jpg
View port with magnets
SAM_4414a.jpg
Viewport without magnets
SAM_4417a.jpg
Any guesses to the dendritic pattern around the edge of the view port under ion bombardment?
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Jerry Biehler »

That delrin has got to go. I thought you were just using it internally, not between atmosphere and delrin. I talked to a prof friend of mine and plastic just has no place in a vacuum system.

Also you need at least a KF40 line going from the turbo to the chamber and the line really should be smooth bore. Your conductance like you have it set up is just awful.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Jerry Biehler »

Andrew Seltzman wrote: I'm going to install some bake out heater tape on the fusor shell to assist with pumpdown.
Would it work to install a small(~2l/s) ion pump on the CF cube on the fusor for maintaining low pressure on a valved off system when not running(so the backing/turbo pump don't have to be continuously run)?
2 l/s is probably a bit small. They are good for keeping really small spaces down. Look at the size of ion pumps they put on field emission SEMs, that is closer to what you want. They just keep the pressure down in the column, it needs to be in the -9 to get good life out of the tip. And ion pumps can be very finicky, sometimes they won't start till down to the -6, -7 range.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

We use delrin / acetal insulation brakes fairly extensively on Madison symetric torus, all the vacuum engineers agree it is fine as long as there is no direct plasma bombardment, and it is not operated at very high temperature. They say it has similar outgassing properties as viton.

It is however critical to pick the correct type of acetal casting/extrusion to eliminate porosity leaks: an acetal round will have axial porosity down the center(useful as the type of insulating break I have, since the center is bored out, useless as a blank-off), while an acetal plate will have porosity along the center of plane(useful as a blankoff, useless as round insulating break since it would have radial porosity)

Outgassing mass loss percentages are listed here, with the plastic Semitron® ESd 225 noted as filled acetal (1.00% TML, 0.05% CVCM and 0.60 % WVR), it as not that much more then PEEK ( 0.31% TML, 0.00% CVCM, 0.06 % WVR)
http://www.boedeker.com/outgas.htm

To quote from a previous thread where I discussed the Acetal insulator
To further eliminate problems, the acetal copolymer insulator was baked out at 110C for 24H under 30" of vacuum in a vacuum oven. The mass of the insulator before and after baking was measured and the total mass lost was determined to be 0.4%. This includes condemnable volatiles (out gassing mass) and absorbed water. Mass changed from 135.8g to 135.5g. Additionally a stainless plasma limiter was added to the insulator to prevent plasma bombardment of the material.

Previous discussion threads on vacuum use of plastics are here:
viewtopic.php?f=6&t=2968&hilit=acetal
viewtopic.php?f=6&t=2943&p=18165&hilit=acetal#p18165

Hopefully I'll swap out the acetal with a short stainless spool with a PEEK insulator to make sure, and it will also increase conductivity by allowing the removal of the baffle that shields the acetal from plasma bombardment.

----------------------------------------
Current times / pressures are:
3min from 160mtorr to 2.8mtorr
20min from 2.8mtorr to 1.2mtorr
30min from 2.8mtorr to 3e-4torr
3hours from 3e-4torr to 1.8e-4torr
Best vacuum 1.2e-4torr

Once valved off, pressure rise from 1.2e-4torr to 9e-3torr in 20 min

Stable plasma discharge in the 3e-4torr range with slow rise as components heat up.
To be clear, I have not done a high temperature(of the shell) run of the fusor, so there still might be significant trapped water on the walls.

------------------

I've noticed a steady pressure rise during operation when starting at 3e-4torr. I think the very tip of the cone may get extremely hot due to plasma bombardment, and it may be vaporizing the boric oxide binder which is then decomposing in the plasma and contributing to the pressure rise.
pump5.jpg
I've tracked down the vapor pressures of the boron nitride and boric oxide binder at various temperatures (I'm using HBN grade boron nitride for the insulator)
https://www.momentive.com/WorkArea/Down ... x?id=27480

Significant heating of the top of the insulator confirmed by thermal imaging, but I lack the resolution to differentiate the temperature at the very tip.
Note: the temperature readout is incorrect due the the attenuation of the ZnSe viewport; the insulator is actually much hotter.
https://www.youtube.com/watch?v=8DfPozHL37A
SAM_0350a.jpg
SAM_0314a.jpg
Last edited by Andrew Seltzman on Sat Sep 19, 2015 9:11 pm, edited 1 time in total.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Bern's logon is not currently working, but he sent me some useful suggestions:
----------------------------------------------------------------------------------------------

Hi Andrew,

I read your recent fusor.net post. My login is not working there tonight, but I thought I would provide some food for thought for you to check out this weekend regarding your vacuum issues. I believe you are chasing not one, but several issues and the combined effects are what you are seeing.

1) It does look like you have a oil contamination problem from your Pfeiffer Duo 1.5A backing pump that has affected your MKS Quattro 999 gage. This appears to be the most likely significant problem. The reasons that backing pump oil contamination are suspected are: a) you do not have a metal sieve trap between your backing pump and your TPH 055 turbo pump and b) the reading you are getting on your MKS Quattro 999 gage when connected directly to the TPH 055 turbo pump (with a specified ultimate vacuum pressure of 1E-6 mbar or 7.5E-7 Torr) is only 8E-5 Torr after ten minutes of operation. Also, in addition to reviewing your current setup, from reviewing the older images of your early ion injectors, it looks like you may have had oil contamination issues plaguing you for a while. The brown residue on the old ion injectors and in the chamber sure looks like deposited oil film.


You're probably thinking that the backing pump oil is rated at a vapor pressure of 1E-6 Torr, so that probably isn't an issue. Think again. That rating is at room temperature and the backing pump gets hot when operating for a while. If you take the hose connecting your backing pump to your turbo pump off, I am sure that you will find it has a bit of an oily smell on the inside and this is a not good thing. When you bought the preconfigured system, it likely didn't have a metal sieve trap, so you are probably thinking it shouldn't need one. It never ceases to amaze me where some manufacturers cut corners to keep pricing down on an initial system. Anyway, you will save yourself a lot of grief and continual cleaning of oil film out of your system by putting a metal sieve trap with a stainless steel gauze filter (for filtering hydrocarbons) between your backing pump and your turbo pump. Either that or buy and install a very expensive dry pump capable of operating at vacuum pressures required by your turbo pump. The metal sieve trap is a much less expensive solution. You should also install a anti-suck-back valve between the metal sieve trap and the turbo pump. Even if the backing pump has a built in valve or holds vacuum pressure well enough after being turned off, the trapped oil in the metal sieve trap filter may eventually escape in small amounts. What works well is to use a anti-suck-back valve between the metal sieve filter and the tubing to the turbo pump. You will save yourself a lot of headaches in hunting for problems and cleaning oil out of the rest of the system by installing these items. As examples, check out the MKS Vacuum Sentry and Norcal metal sieve traps.


Once you have a compatible metal sieve trap with stainless steel gauze in place and anti-suck-back valve, then put a known good MKS Quattro 999 (or similar) gage on the end of the turbo pump and test the vacuum pressure. Your last reading was 8E-5 Torr after 10 minutes. You should see something closer to 7E-6 Torr (or a bit lower) and you should see a continued drop to 7.5E-7 Torr. Oil contamination affects both ion gages and capacitance-based gages, usually causing them to read a bit over an order of magnitude higher pressure than they should be. Don't go through the trouble of trying to clean the turbo pump, except as an absolute last resort. See what you can improve in the system by cleaning everything else first - you probably won't need to touch the turbo pump. You should be able to find instructions on cleaning the MKS Quattro 999. You will find it far easier to take a part and work on than a turbo pump. Also, assuming the vacuum gage is now registering where it should be, consider cleaning the inside of the vacuum chamber to remove any oil residue prior to testing again with the vacuum chamber.


2) Non-optimized turbo pump and backing pump combination - it also amazes me how many manufacturers put out turbo pumping stations with less than adequate backing pumps. Oh sure, they work well enough, but the manufacture is not always concentrating on performance, but in some cases looks more at the bottom line on cost (usually due to competition). So take a look at the manual for your TPH 055. It is a TPH 055, because it has a NW63 port and not a conflat flange (also, the images of the placard on the unit on your web site show it to be a TPH 055). You can find the manual here http://www.idealvac.com/files/ManualsII ... Manual.pdf . So take a look at the graph in figure 2.2 Volume Flow Rate on p. 9. Note that the ultimate pressure rating on your Pfeiffer Duo 1.5A is 6E-3 mbar and that means that the maximum volume flow rate you can get out of the TPH 055 connected to it is right at about 30l/s. Hmmmmm ... that sure sounds like exactly what you measured. Now you know why and that the volume flow rate from the turbo pump won't get any better, unless you change the backing pump. If you still have that Leybold D2.5 sitting around, it has an ultimate pressure of about 2E-3 mbar and has approximately the same pumping speed as the Pfeiffer Duo 1.5A. Because it has a lower ultimate pressure and assuming it will get close to that, you should be able to get a volume flow rate out of the TPH 055 of around 45 l/s when using it (that is 15 l/s more!). You will still need the metal sieve trap in place to keep oil vapor from the backing pump from contaminating the rest of the system. One thing to note on backing pumps. Do check the vacuum pressure going to the turbo pump and know the rating of the oil that the bearings on your turbo pump use. I don't think it will be an issue for your TPH 055 turbo pump given the ratings indicated in the graph of figure 2.2, but some turbo pumps have bearing oil with a vapor pressure of around 5E-3 Torr, so pulling the exhaust port vacuum pressure lower on some turbo pumps can reduce the amount of oil on the bearings over time and freeze up the bearings. Given that the manufacturer is indicating that their TPH 055 turbo pump needs at least 1E-3 mbar (7.5E-4 Torr) backing vacuum pressure to get to its rated volume flow rate of 55l/s, then you would expect the bearings to be in good shape with a backing pump vacuum pressure down to that; however, you should always check with the manufacturer to be sure. Anyway, it looks like you can improve the volume flow rate on the TPH 055 from 30 l/s to 55 l/s by changing the backing pump to one better matched for the TPH 055 operation.


3) Losses through the tubing. It is best to keep all vacuum tubing as short as possible. As far as the diameter used, this will depend on cost, mechanical fit in your system, and the amount of time you want to spend waiting for the system to pump down. Given your background I suspect you are already familiar with the wealth of information on this subject on the Internet and in text books, so I don't plan to cover it here. However, do note that some improvements can be had at reasonable cost. For example, you have a 2ft tube connecting the turbo pump to the vacuum chamber. You could change this to a 19 inch tube (standard size or possibly shorter) and gain some benefit. An option that many find reasonably economical and sufficient is to use CF2.75 connections with a relatively short piece of 1.5" dia. tubing. To keep vibration down to the turbo pump, it is best to use bellows tubing as a connection directly from the turbo pump exhaust port towards the backing pump.


4) That big o-ring connecting each half of your chamber. Note that the reason the TPH 055 has an ultimate pressure rating of only1E-6 mbar instead of a far lower ultimate pressure (orders of magnitude lower) is that it uses a viton o-ring as a seal on its NW 63 port instead of a metal-to-metal conflat flange seal. So consider that your even larger viton o-ring sealing the two halves of your vacuum chamber is causing more of an issue. I know that for some reason you were not measuring an appreciable leak near the big o-ring you are using, but if you do some research on atmospheric gas permeation through viton o-rings, you will wonder why you didn't. If reaching a higher vacuum is more important than measuring current separately on each half of your vacuum chamber, then you may want to consider putting a metal seal between your vacuum chamber halves instead of a viton o-ring. Also, if you have not already done so, research the amount of outgassing you can get from new viton o-rings.


5) Ionizer impact on the cold cathode ionization gage. You did some nice work on developing your ionizers. However, consider that these can affect your vacuum gages, particularly, if they are ionization gages (such as cold cathode gages). Your MKS Quattro 999 appears to have a KF25 connection. You can acquire KF25 o-rings with a stainless steel screen and use these to connect your vacuum pressure gauge to the KF25 port on your chamber. While this may not totally eliminate errors in vacuum pressure readings caused by your ionizers, it will likely reduce the issue quite a bit.


6) Impact of ionized gas and high voltage electrostatic forces on polymers, epoxies, and sealants in the vacuum chamber. Even if you have plastics, polymers, and sealants rated not to outgas down to very low vacuum pressures, these ratings do not apply when you subject these substances to high electrostatic charges. For example, if you apply some epoxies designed for use in sealing vacuum chambers in an application to hold your cathode of your fusor in place (attached to the cathode itself), they will outgas like crazy when you turn up the voltage to 20kV or so, even though they don't outgas at all otherwise. So it would be reasonable to expect that plastics, polymers, and sealants could do the same when exposed to the beams from your ionizers (probably what you are seeing when you have the ion beam pointed at your viewport is outgassing from viewport sealant). You also noted that increased temperatures tend to cause the plastics to outgas. This sounds plausible as well. So in your selection of plastics, polymers, and sealants you not only should consider their operation under a vacuum, but also their operation under a vacuum combined with high temperature and high electrostatic voltages. So that you are not chasing outgassing issues as much when you are working to achieve a lower overall vacuum pressure, leave the ionizers and the cathode grid voltage off, until you get the vacuum level to a suitably low pressure. At least that way you can tell where the leaks are and then once the leaks are resolved, you will know that what is left are outgassing issues to track down (that will likely be more prevalent when you turn on the ionizers and high voltage to the cathode).


Now for some comparisons. It is usually helpful to have something to compare to in order to get some idea what to reasonably expect.

Take a look at the chamber configuration at: viewtopic.php?f=18&t=10032

The backing pump has since been changed to a Welch 8912 (3.8 CFM/Ultimate Pressure 1E-4 Torr). As you can see in the picture, the chamber has 5 total ports (1 KF25 and 4 CF2.75). The more ports, the more chances for leakage, so the number of ports was kept to a minimum. All seals are metal except the KF25 on the MKS421 (viton o-ring with SS metal screen) and the NW63 (viton o-ring with SS metal screen) on the MDP-5010 (7.5l/s and ultimate vacuum pressure of 1E-6 mbar or 7.5E-7 Torr).

In that configuration:

From atmospheric pressure (at 643 AMSL) to 5E-3 Torr using only the backing pump takes about 17 minutes.

The MDP-5010 is then turned on and in 10 more minutes, the vacuum pressure in the chamber goes from 5E-3 Torr to 5.9E-5 Torr. In 50 more minutes, the 6 inch diameter chamber is at 1E-6 Torr.

Just my two cents.

Kind Regards,

Bern
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

TLDR: Conductance of pumping system is terrible(~0.35L/s), gas load in fusor predominantly(about a factor of 10 over the o-rings) due to delrin surface outgassing, insulator will need to be replaced(cheaper then re-working the turbo station valves).

To address Bern's suggestions <thanks Bern, very well thought out suggestions :) >

1) oil contamination
--------------------------------------------------------------
I bought the gauges(3 of them) used on e-bay, used, unknown history. I got them for $50 ea, but they each cost $1120 new
http://www.capovani.com/iinfo.cfm?itemno=172700
http://www.mksinst.com/product/Product. ... ductID=448
so replacement is not really a practical option. 2 were working, 1 had a burnt out ion gauge, the 2 that are working have similar performance.

I do not necessarily suspect oil contamination in the fusor since I turn on the roughing and turbo pumps on at the same time(roughing from the 1 torr range or thereabouts) and rough through the turbo, that way the turbo will have spun up by the time the roughing line is at a pressure where oil back streaming would occur and back streaming through a running turbo is unlikely.

To prevent oil back streaming when the pump is shut down, the pump has in integral anti-suck back/vent system(identical in function to the MKS Vacuum Sentry) upon pump shutdown, the pump is vented to atmospheric pressure, and the roughing line is isolated (pictures here http://www.rtftechnologies.org/general/ ... uo15a.html). As you noted, this may still leak and oil and a sieve trap would still backstream

To prevent this I have an additional high vacuum valve to isolate the turbo connected directly to the backing flange on the turbo, upon shutdown the roughing line is also vented to atmosphere, leaving the turbo isolated. The high vacuum valve on the turbo inlet and the valve on the fusor is closed as well. Eg, for low pressures(under ~100mTorr), the fusor never sees anything but a running turbo, or closed high vacuum valve, it is never left with a direct line to the roughing pump.

In as far as the backing line sieves go, this seems like a worthwhile addition, I definitely like the nor-cal right angle versions:
https://www.n-c.com/ECommerceSite/media ... eTraps.pdf
these are on ebay, what do you think:
http://www.ebay.com/itm/331264350423?_t ... EBIDX%3AIT
the other option is a metal sieve filled roughing line(which I already have)
SAM_4432.JPG

2) Non-optimized turbo pump and backing pump combination
---------------------------------------------------
I no longer have the Leybold D2.5, nor would I go back to it, I like the pfeiffer better as the oil lasts longer. I also rebuilt it after buying it so it's in very good condition. The TPU 055 manual specifies a 1.5m^3/h (the rate of the Pfeiffer Duo 1.5A) pumping rate as sufficient.
I think you are misreading figure 2.2 Volume Flow Rate on p. 9 (http://www.idealvac.com/files/ManualsII ... Manual.pdf), that specifies pumping speed vs turbo intake pressure, not outlet pressure.


3) Losses through the tubing
---------------------------------------------------
Right on the nose there, conductance is absolutely dip, see calculations below. I might go to a nw25 flex hose and valves, hopefully gate valves if I can find them.


4) That big o-ring connecting each half of your chamber
---------------------------------------------------
The big o-ring stays, I'd love if they made non-conductive copper o-rings, but they don't. Also not nearly the predominant gas load in the chamber(the 16x 2.75CF orings are greater length), and the outgassing of the delrin is a factor of 10 greater then the total oring permeation rate. see calculations below


5) Ionizer impact on the cold cathode ionization gage.
---------------------------------------------------
The MKS Quattro 999 is actually a hot cathode bayard-alpert type. It also has a built in fine mesh stainless screen on the flange(an nw40)


6) Impact of ionized gas and high voltage electrostatic forces on polymers
---------------------------------------------------
The main gas load in the chamber though is due to the delrin surface outgassing combined with the very low pumping rate, see calculations below.

Only polymers in the vacuum chamber are the viton o-rings(hemisphere midplane shielded with a ceramic limiter, all others well out of the line of sight of the plasma) and the delrin insulator(also shielded from direct bombardment). The viewports use no sealants(epoxy, torrseal, etc) other then direct contact with the 2.75 CF viton o-rings. The fusor runs are done at very low pressure(and consequently low power) and the chamber does not heat up much, only slightly warm to the touch. I do believe the pressure increace during operation is a temperature driven outgassing, but of the Boric Oxide binder in the HBN grade boron nitride grid insulator.

Comparisons
---------------------------------------------------
Very nice chamber and system Bern, thanks for the well though through input

=============================================================================================================
OK, and now for some math

Problem: sub-optimal final pressure, and poor pumping rate, chamber pressure rise when valved off.

Fusor volume:
--------------------------------------------
Main 6" diameter sphere: 1.85 L
13x 1.5" diameter, 2.5" length cylindrical segments(10x half nipple, insulator and cube): 0.9411L
Total volume: 2.79L

Gas source rate
----------------------------------------
Pressure rise when valved off: 2E-3 Torr in 250 sec
Gas source rate into fusor volume: 2.24e-05 TL/s

Viton o-ring atmospheric permeation
-------------------------------------------
Oring quantity: 1x 6.5", 16x 1.875", 1x kf16, 1x kf40
total_oring_length: 122.5"
permiation rate for viton=2.5E-8 TL/s/(linear inch) (http://www.vacuumlab.com/Articles/Gas%2 ... 0Seals.pdf)
o-ring permeation rate: 3.06e-06 TL/s

Delrin insulator outgassing
-------------------------------------------
Insulator internal dimentions: 1.5" dia, 1.5" length
delrin_area: 45.6 cm^2
Surface outgassing rate: 8e-7 TL/s/(cm^2) (from acetal: http://vacuumcursus.nl/casussen/Vacuum_Systems_UHV.pdf)
Gas load rate: 3.65e-05 TL/s

gas load
-------------------------------------------
assuming dominated by permeation of orings and delrin outgassing
total_gas_load=delrin_outgassing_rate+oring_leak_rate
Total gas load: 3.95e-05 TL/s

Pumping system conductance
-------------------------------------------------
Edwards PV16 valve conductance: 2 L/s (https://shop.edwardsvacuum.com/Viewers/ ... &lcid=2057)
Valve quantity in series: 2
Conductance for a 20" long 2cm ID straight smooth tube: 1.57 L/s (http://www.physics.ucdavis.edu/~kliu/Phy250/Vacuum.pdf)
Conductance of 20" long, 2cm ID bellows: 0.52 (using a fudge factor of 1/3 the conductance of the straight tube)
conductance=1/(N_valve/C_valve+1/C_bellows)
Total system conductance from turbo to fusor: 0.34 L/s

Ultimate pressure (calculated)
-------------------------------------------------
ultimate_pressure=(total_gas_load)/conductance
calculated final pressure: 1.15e-04 torr
Actual measured pressure after about 10 hours of pumping: 1.2e-4 torr

Math checks out, it's the delrin+low conductance
disclaimer: fudge factor assuming conductance of bent bellows is 1/3 that of straight smooth tube of same dimensions

=============================================================================================
Solution: the delrin has to go, the insulation break must be maintained, it will have to be replaced with PEEK+ stainless or ceramic

A new ceramic break can be bought for about $200
http://www.lesker.com/newweb/feedthroug ... fm?pgid=cf
but it's a bit long to fit my system at 2.65" length, I'll replace the 1.5" long delrin break with a 1.5" stainless short nipple+ a 0.25" PEEK insulator which should fit fine, though it will add 1 2.75" viton o-ring to the system.

Gas source from delrin is 10x that of viton o-ring permeation, the o-rings can stay. I'd be happy to pump down to a final pressure of 1e-5 torr(calculated 8.9e-6 torr with the delrin removed and no changes to the conductance) and operate the fusor at a pressure of 5e-4 torr

Matlab calculations:
===================================================

%gas permiation rate

format COMPACT
clear all
clc

vol_sphere=4/3*pi*(3*0.0254).^3*1e3 %vol of sphere in L
vol_ports=pi*(1.5/2*0.0254).^2*(2.5*0.0254)*13*1e3 %vol of 10 ports+cube in L
volume_fusor=vol_sphere+vol_ports %vol of fusor in L
measured_leak_rate=2E-3*volume_fusor/250

%-----------------------------------
rate_viton=2.5E-8; %TL/s/(linear inch)

N_275=16;
N_600=1;
N_kf16=1;
N_kf40=1;

D_275=1.875;
D_600=6.5;
D_kf16=1;
D_kf40=1.5;

total_oring_length=pi*(N_275*D_275 + N_600*D_600 + N_kf16*D_kf16 + N_kf40*D_kf40)

oring_leak_rate=total_oring_length*rate_viton

%-----------------------------------
delrin_outgassing=8e-7; %TL/s/(cm^2)
delrin_area=pi*(1.5*2.54)*(1.5*2.54) %(cm^2)
delrin_outgassing_rate=delrin_outgassing*delrin_area

%-----------------------------------
total_gas_load=delrin_outgassing_rate+oring_leak_rate

%-----------------------------------
C_valve=2; %valve conductance l/s
N_valve=2; %valve number
C_tube=10*(2^3/(20*2.54)) %straight smooth tube conductance l/s
C_bellows=C_tube/3

conductance=1/(N_valve/C_valve+1/C_bellows)


%-----------------------------------

ultimate_pressure=(total_gas_load)/conductance


format LOOSE
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Dennis P Brown »

Really great posts and as I warned earlier - a oil vapor anti-back flow device is critical on any fore pump. All I will add is that you keep saying the plastic components are fine since you did "X" but your system isn't getting to any were acceptable vacuum - so, when in doubt, remove all suspect components and test your system without them. If the problem is solved, slowly, carefully, reinstall removed components one at a time, testing each time. This will remove that unknown. As pointed out, a fusor with its intense plasma is no place for plastics and even teflon should be considered questionable.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Bern Bareis »

Andrew,

1) If you are not obtaining measurements reasonably close to the ultimate vacuum pressure of the TPH 055 when you have the MKS Quattro 999 attached at the input (i.e. only elements in the system being the backing pump and possibly a metal sieve trap, the TPH 055, the MKS Quattro 99, and reasonably high conductance tubing/couplings in between), then there is something that is not quite right in that system. If you are indicating that you are getting 8E-5 Torr in that configuration, you should be obtaining a reading that is quite a bit lower, given that the ultimate vacuum pressure for the TPH 055 is 7.5E-7 Torr. My thought is that the most likely cause would be some sort of contamination in the MKS gauge.

Regarding the metal sieve trap, if you can find a larger diameter and can still fit that on your system, it would not need to be cleaned as often, if you will be using your system a lot. That being said, the 2 inch diameter metal sieve trap with stainless steel gauze/mesh works well on the system that I provided the link for earlier. The 2 inch diameter metal sieve trap appears to have considerably more filtering than the metal sieve filled roughing line that you provided a picture of. By putting in a metal sieve trap and a valve between the metal sieve trap and the exhaust port of the turbomolecular pump on your system you should be able to pretty much eliminate the potential for oil vapor from the backing pump contaminating the rest of the system. The reason that a Vacuum Sentry was applied on the system from the link I provided earlier is that in the event of a power failure or simply shutting down the backing pump, the line between the metal sieve trap and the MDP-5010 is closed off within 30 milliseconds and the side leading to the backing pump is vented to atmosphere from the Vacuum Sentry towards the backing pump. Consider that if you are using a manual valve and the vacuum is not properly vented between the manual valve and the valve in the backing pump when the pump is turned off, then you will maintain a vacuum across the metal sieve trap and if it is still quite hot from running the backing pump for a long time you may have some amount of oil vapor awaiting entry into your system the next time you open up the manual valve. The Vacuum Sentry eliminates this potential since it both closes off the path to the chamber and vents the path towards the backing pump.

I am making the assumption that when you bring your vacuum chamber up to atmospheric pressure and are venting it you are doing this from the vacuum chamber end of your system and not the other way around. Otherwise, you could end up sucking oil vapor from tubing leading to the backing pump, if present, into the chamber and gauges.

2) I agree. Though if you can find a Inlet Pressure versus Fore Pressure graph for the TPH 055, this may provide some additional useful information. After reviewing a few of these graphs for other turbomolecular pumps I am familiar with, I am convinced that you would have to be pretty far off on the Fore Pressure requirement not to get close to the ultimate vacuum pressure, so this is probably not an issue.

3) Looks like you have a good handle on the conductance issues.

4) Depends on how low you want to take the vacuum pressure in the chamber as to whether or not at some point you will decide to remove the o-ring. At your target vacuum pressure, it does not look like an issue. Given the length of time you have been operating your vacuum chamber and producing results with that o-ring in the design, I suspect you have a good idea where its shortcomings are. A check of the permeability of the o-ring was useful.

5) Since the MKS Quattro 999 has a built in screen to help isolate any impact the voltage and current transferred through the plasma may have, then I agree that the difference in the measurement between the ionizers on versus off must be predominantly from something else.

6) You are exploring new territory. Keep at it.

Kind Regards,

Bern
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Dennis P Brown »

Hold on a minute. I am very confused by this post:
The big o-ring stays, I'd love if they made non-conductive copper o-rings, but they don't.
If this is a fusor there is absolutely no way that the two halves will not be in full electrical contact in a plasma in the micron range. If your goal is to build an ultra high vac system and not do fusion then ok. If you are doing fusor fusion, then an insulating vacuum seal between these components is useless.
Its possible that your gauges are out-of-calibration and you are getting to the low 10^-6 torr? I really have doubts about the gauges because you posted:
Turbo pump down with turbo pumping on gauge only (pumps down to about 8e-5)
Your turbo just on the gauge should easily bottom out the 10^-6 torr range in a few minutes - my entire system (a small chamber and "leaky accelerator tube and 1 meter of teflon tubing) pump down to 5*10^-5 torr in under five minutes and I use a near identical turbo! Your turbo is neither not working properly or you have very bad contamination in the turbo/chamber (which I doubt) or the gauges/unit is not reading correctly.
Your pressure rise is sufficiently slow - this tends to tell me that your system isn't really contaminated (2 microns in 30 seconds isn't bad at all and your top pressure after a 12 hours was a very acceptable half torr.) If that is true then I suspect that your turbo is at fault or the gauge system - and frankly, high end of 10^-5 torr with no load is very poor performance for a turbo. There is really no way you will determine your problem until you calibrate your ion gauge somehow - sorry but until you know for certain the gauges (with control box) is working, you will not solve this problem - it is the turbo or gauge system is my guess.

If a university physics department is near by, call them and talk with one of their physicist - I am sure they will be able to let you test your complete high vac gauge system on one of their high vac systems.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

I have a planed upgrade to measure the current collected into each hemisphere. Both hemispheres will be grounded, but the collected current will be measured separately. I'll also be operating in the sub-micron range.

I'll look into the gauge and turbo
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

A trap has been added to the turbo station.
https://www.youtube.com/watch?v=4F4qzPbcFiA

I'll need a few more adapters to plumb it to the turbo pump, but the station should be up and running in a week or so.
Backing pump with vent valve removed
SAM_4436.JPG
Stainless seive trap
SAM_4437.JPG
Trap mounted on pump
SAM_4439.JPG
Bakeout heater for the turbo pump
SAM_4440.JPG
I also have bought a tank of deuterium for use some time in the future
SAM_4438.JPG
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Gas drier purchased(for dry air venting). This is the one with the built in 40um outlet filter
SAM_4441a.jpg
Gate valve purchased(for higher conductance)
SAM_4442a.jpg
In other news, the turbo and gauge have both been confirmed good, the problem is purely based on conductance and the viton gas permeability. Today I was installing a replacement vacuum gauge fresh from the manufacturer on one of our turbo stations for my PhD work and used the opportunity to test the hot cathode ion gauge as well. The gauge was compared to a known good/calibrated vacuum gauge on a similar known good turbo station with a low conductance valve and the readings matched perfectly. The pump down times were also equivalent, reaching 7.5e-5 torr in several hours
c.jpg
The gauge was then attached to a separate, known good turbo station, with higher conductance hardware and a second known good gauge.

The gauge reached about 2e-5 torr in 1.5 hours, with the gauge nearer to the turbo inlet was considerably lower in pressure
b.jpg
pumpdown4.jpg
The gauge was then attached directly to the turbo inlet, resulting in a pump down to 2e-6torr in a few minutes
a.jpg
It's shocking how much pressure difference a single very short NW16 adapter can make, compared to an NW40 directly coupled to the turbo inlet. The difference in pump down rate and final pressure is enormous. At UW we use these turbo stations to pump out diagnostics and probes before opening the high vacuum valves to madison symmetric torus, but we do the pump downs through 4 feet of 0.25" ID plastic hose. We might as well not even have a turbo on the pumpdown station at that point. We let them pump down over night, but all of our de-mountable seals are viton as well, we probably are not even getting much past the mtorr range at that point. Our continued use of the stations in this manner has apparently given me a skewed sense of system performance expectations when using low conductance tubing with viton o-rings.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Jerry Biehler »

I did the calculations when I was connecting my big turbo with an ISO-250 (10") fitting to my chamber with a 6" ASA and it was amazing how much conductance you loose. If I recall I lost over 500l/s and that was with about 6" length of 6" passthrough. Still it did not turn out to be bad. I can get down the low -7, high -8 without too much trouble. I can get down to -5 within just a couple minutes of firing up the turbo, it still pulls down even when it is not at full speed. If I remember it takes about 5-6 minutes to get up to speed (27000 rpm).

Moral of the story, mount your high vacuum pump as close to the main chamber as you can. Thats why you see semiconductor tools with the turbo mounted right to the chamber. The portable pump units are nice but you really cant constrict the line down any if you want to see good vacuum at the other end.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull »

We've been yappin' about conductance for years in this forum. The rule is use the largest diameter plumbing up to the diameter of the smallest throat and jam alll vacuum components together as tightly packed as you can within the limits of easy operation and disassembly.

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

The fusor has been upgraded to help combat the outgassing/permeation problem by replacing the delrin insulator with a PEEK/stainless insulator and using copper o-rings. Fastener hardware was chosen to be silver plated 12pt bolts additionally coated with moly based anti-seize compound.

Currently the modification changes the system from 16x to 11x 2.75" CF viton o-rings, 1x to 0x 8" CF viton o-rings, 45.6 cm^2 delrin surface area(8e-7 TL/s/(cm^2)) to 7.6 cm^2 PEEK surface area(2.9e-7 TL/s/(cm^2))

Parts for upgrade
SAM_4479a.jpg
Conflat cube
SAM_4480a.jpg
Replacing viton with copper o-rings
SAM_4481a.jpg
0.25" high, 1.5" ID PEEK insulator and 1.5" high stainless coupler to replace 1.5" high, 1.5" ID Delrin insulator, reduces outgassing rate from 3.6e-05 TL/s to 2.2e-06 TL/s
SAM_4485a.jpg
Anti-seize coated stainless studs, used with silver plated nuts
SAM_4486a.jpg
Re-fitted cube
SAM_4488a.jpg
Core hemisphere
SAM_4489a.jpg
Deposition on ceramic fabric o-ring shield
SAM_4491a.jpg
Condition of grid insulator
SAM_4495a.jpg
Pre-annealed(for easier tightening) copper o-ring
SAM_4497a.jpg
Port alignment jig, to ensure alignment of opposing conflat ports(optical and ion source)
SAM_4498a.jpg
New stainless/PEEK insulating break
SAM_4499a.jpg
Core bolted together
SAM_4500a.jpg
Additionally a lead glass disk has been added to the pyrex viewport for x-ray shielding, previously there was a lead glass rectangle that had some standoff distance reducing the field of view for taking pictures, the new disk fits into the conflat ring giving the disk more protection and making it easier to take pictures.
SAM_4466a.jpg
SAM_4467a.jpg
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull »

Again, fabulous, plus ultra rig. It is going to work brilliantly.

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

With the new modifications the fusor achieves a base pressure of 8.9e-5 torr after 10h of pumping (compared to 1.9e-4 before), so the gas source was not the delrin, though the system feals like it runs a lot cleaner. It will pump down from 70mTorr to the low E-4 torr range in about 10min as compared to 1-2 hours before, and will sustain operation in the low E-4 range without the pressure climbing up to the mTorr range.

Vacuum pumpdown with new system
pumpdown1-10-18-2015-copper-orings-peek-insulator.jpg
pumpdown2-run-10-18-2015.jpg
Vacuum rise after valve-off with new system
10h-ultimate-vacuum-rise.jpg
10h-ultimate-vacuum-rise2.jpg
Star mode operation at 1E-4 Torr
10h-ultimate-vacuum-star-mode.jpg
SAM_4505aa.jpg
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Jim Kovalchick »

Andrew,
I know its too late now, but did you try conditioning your delrin prior to use?

A couple years ago my son did some work that required him to use silicon rubber inside a vacuum chamber. Predictably, the stuff was a massive outgas source. He ended up sonicating the rubber in pure isopropanol for hours followed by a slow bake out in the kitchen oven. The results were amazing and outgassing was significantly reduced. I have no idea if that would work for delrin or any other outgas sponge material, but it's a technique to keep in the toolbox if you ever "have" to use a material that is normally a problem.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Yes I did, the delrin was baked in a vacuum oven at over 100C for 24h.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Bern Bareis »

Andrew,

If the images above are representative of your system in the configuration that you were testing, you may want to consider putting bolts in all 2-3/4" CF mounting holes and tightening to the proper torque. It has been my experience that not doing so results in leaks. For the 2-3/4" CF bolts, the industry applied torque is typically 12 ft-lbs and for the 8" CF bolts, the industry applied torque is typically 15 ft-lbs. The tightening method applied is generally a crisscross pattern to finger tight or snug with all bolts in place and then a round-robin pattern going around at least 3 times tightening more on each successive time around, until the proper torque is reached.

Nice work on your upgrades.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Bern,

All 2.75" cf flanges with copper o-rings have all bolts used. there are some 2.75 CF ports that use viton o-rings that don't use all the bolts(like the ion injectors and viewports). The 8" CF flange uses a pre-annealed copper o-ring(for easier tightening) and all but 4 bolts. Those bolt holes were needed to be free for the insulated mounting bolts to hold the core to the frame (still using only one ground return path through the grounding wire to avoid ground loops).
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Bern Bareis »

Hi Andrew,

Noting from the images above it appears that the ion injectors are attached with metal bolts (eliminating electrical isolation), is there a compelling reason not to use copper o-rings on the 2-3/4" CFs for the ion injectors?

If you don't require a lower vacuum pressure, then perhaps it is not an issue at this point. However, with copper o-rings for almost all connections I would expect that you would be able to get close to an order of magnitude lower vacuum pressure after 10 minutes of turbomolecular pump operation (i.e. E-5 Torr range based on your turbomolecular pump specifications and vacuum chamber size, assuming no significant leaks).
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Bern,

Unfortunately all copper o-rings are not an option at this time. There are 5 viewports, 3 pyrex, 2 ZnSe that require viton o-rings, as well as 2 o-rings for the insulating spacer. There is also an NW40 for the vacuum gauge, and an NW16 for the valve. There are some places that sell aluminum NW o-rings, how well do these work?

I do plan to replace the ion injector viton o-rings with copper o-rings, but have not done to it yet, there are few things I want to do first.

There is a trapped volume problem with the mounting standoff holding the injector to the CF flange. The standoff and screw are both vented, but when the standoff is screwed into the flange, the thread bottoms out, potentially sealing off the vent, I have built some spacer washers(also vented) to fix this problem.
SAM_4472a.jpg
SAM_4471a.jpg
SAM_4475a.jpg
Secondly, the injectors have a movable central cathode pole to adjust beam focus, currently they are set for a cylindrical beam, but I want to change it to focus at about 3" from the injector so they are focused at the center of the grid. I have a fifth identical injector on a test bed that I can try this out with.

Thirdly, I would like to have some custom alumina insulator standoffs machined for the anode ring support, currently they have steel clips holding the alumina rods in place, but they can slide, so I'd like to have one with a step machined.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Bern Bareis »

Hi Andrew,

According to the specifications for the aluminum metal seals "aluminum o-rings" for replacement of Viton o-rings, the leak rate is expected to be less than 1E-11 standard cc/sec. That leak rate is also likely measured using the chain clamps that go with them. Obviously, there are tradeoffs in what you want to accomplish for your experimental setup versus the type of gasket seal or o-ring that you can apply. If lower vacuum pressure is of interest to you, then replacing more Viton o-rings with metal gasket seals will help. You don't necessarily need to replace all of the Viton o-rings, but if you can get the number down to 1 or 2 from in your vacuum chamber through to the turbomolecular pump, then I would think you would get an order of magnitude or more reduction in the vacuum pressure in the first ten minutes of operating the turbomolecular pump, assuming that there is not any excessive constriction in the tubing connecting your turbomolecular pump to the vacuum chamber.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Some new upgrades:

Deuterium supply and control:
Got a better quality lecture bottle regulator on ebay
SAM_4520a.jpg
Testing regulator with argon
SAM_4523a.jpg
A piezoelectric valve is being outfitted to control pressure in the fusor.
SAM_4431a.jpg
Converting the valve from swagelok tube to VCR inlet and outlet ports
SAM_4430a.jpg
Deuterium/dry air admit(for venting the core to atmosphere) manifold to attach to the back of the conflat cube, used swagelok SS-4H bellows sealed valves w/ 1E-9 TL/s leak rate
SAM_4513a.jpg
Bakeout system, to drive water/contaminants off the chamber walls without running plasma(reduces ware on grid / metal deposition from using ion bombardment to heat the chamber walls)

20W stick on heater
SAM_4519a.jpg
8x heaters(160w total) attached to the core
SAM_4518a.jpg
Heaters connected to fusor 24v power supply
SAM_4516a.jpg
Bake out of core after up to dry air to install deuterium manifold, bakeout had to be stopped to prevent damage to the magnets shielding the viewports from ion beam damage(80c max), core reached 90-100C, though cooler out on the nipples
bakeout-130c-annotated.jpg
bakeout-130c-annotated.jpg
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Latest test runs:

Pump down time after the bakeout has decreased to 10min to get from 500mTorr to 3e-4 torr

A second lead glass shield on the topside viewport has been added allowing pictures of the upper segment of the grid

Operation at 40kV, 2.2e-4 Torr
SAM_4528.JPG
Interestingly enough it seems like only the bottom viewport fluoresces under ion bombardment(residual ones not deflected by the shield magnet), not the top one. Also there is in interesting fluorescing pattern on the top of the boron nitride insulator
SAM_4528b.JPG
It looks like secondary electrons from the grid are hitting the top of the insulator causing the fluorescence, however there is a dark area between the grid rings, apparently where there are no electrons hitting. The incoming ions hitting the outer surface of the grid are generating secondary electrons streaming off the grid, however the electric field directs them radially outward causing the dark area between rings.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Operation in the 9e-5 to 1e-4 torr vacuum range
SAM_4534c.jpg
Video of the plasma during voltage ramp up
https://www.youtube.com/watch?v=4pOdWpW ... e=youtu.be

You can see the structure of the ion beam coming from the opposing sources, compared to a picture of the ion beam from the injector in a test bed with no grid potential, particularly on the lower left beam
SAM_3244a.jpg
The cylindrical beam is being focused by the grid potential into a conical beam. I think it might be useful to put some focusing directly on the injector to get a tighter beam at the focal point.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

First fusion test runs with the new system

System setup with piezo control valve and deuterium tank(an older one with about 90psi left in it) and regulator)
SAM_4536.JPG
Deuterium tank and regulator
SAM_4538.JPG
Piezo control valve, takes a 0-100V signal, accurately admits gas to the fusor allowing operation in the 3e-4 to >10mTorr range
SAM_4539.JPG
Valve control circuit, 90v boost converter and potentiometer to control valve voltage
SAM_4537.JPG
Gas inlet manifold on back of cube
SAM_4553.JPG
Pressure during fusion test run
fusion-run.jpg
Star view, bottom, ~6mTorr
SAM_4542.JPG
Star view top, ~6mTorr
SAM_4546.JPG
Videos of test run
https://youtu.be/sZnh9uPCKvU
https://youtu.be/3hJ6Pn14SL4
https://youtu.be/dqJbiwHoiNk


Some counts detected on a BF3 type neutron detector, no exact calculation of neutron flux yet
Operation in the 3e-4 torr to 8mTorr range
40kV on the grid, ~5-8mA current
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Steven Sesselmann »

Andrew,

Beautiful engineering as always on your projects.

Did you have the ion sources running during this run?

I am really interested in your ion sources, is there a post describing these in more detail?

Steven
http://www.gammaspectacular.com - Gamma Spectrometry Systems
https://www.researchgate.net/profile/Steven_Sesselmann - Various papers and patents on RG
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Yes, all 4 ion sources were on. you can see the distinct difference in a gunned fusor, in that the star mode only has 4 rays(for an 3 ring grid with 8 openings), with no visible rays coming out of the segments of the grid where there are no ion sources feeding in ions.

The ion source designs are
viewtopic.php?f=12&t=5011
and
http://www.rtftechnologies.org/physics/ ... source.htm
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Steven Sesselmann »

Andrew,

Thanks for the links, it seems my memory is not so good, I was active in your ion source thread back in 2009, phew time flies when one is doing fusion...

If your ion source can operate with negative bias, I think it would be a worth while experiment to do a fusion run with a) Positive bias and b) Negative bias, if I am not mistaken you should see more fusion with a negatively biased ion source. As you are only running a few kV the central grid will still be able to extract the ions.

Read my recent post to understand my reasoning (viewtopic.php?f=50&t=10443)

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Neutron output is estimated to be in the 25k-30k per sec range based on an ~500sec run at about 8mTorr, 40kv, 8ma, 8.9cm radius from BTI to center, both a BTI bubble dosimeter(pretty old, won't recompress, 8 bubbles present before run started, ~29 after run). Flux estimate from BTI matches estimate from BF3 neutron detector
SAM_4559.JPG
the neutron rate drops off pretty rapidly as things heat up and start to outgas, requiring reduction in D2 flow and thus reducing plasma purity. I might need to redesign the grid to remove the BN ceramic which I suspect may be the main source of temperature driving outgassing.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

I've been getting my bubble dosimeter to re-compress by inserting a pencil eraser in between the screw on tap and the rubber membrane(still looking o buy a new one). This one was purchased in 2009 and is still working with some effort to get it to re-compress. The only thing that i notice is the bubbles are smaller then when it was new.


Best neutron numbers to date 2.1E5 n/s at 8mTorr, 40kV, 8.9ma, 4 ion injectors at 700v anode voltage
24 bubbles over a 100sec run
SAM_4568a.jpg
Calculations
http://www.gammaspectacular.com/fusion_calculator.html
fusion11-21-2015.jpg
Something is still heading up and outgassing which requires the reduction of D2 flow to maintain pressure and kills the neutron numbers after about 100sec.
Best vacuum to data 5e-5 torr, 96mTorr rise after 5 days, the bake out heaters really seem to help keep the system clean

I also swapped the grid cooling pump with one with lower displacement, allowing the pump motor to provide more flow without hitting the torque(current) limit on the pump. Flow through the grid is now at most 200ml/min

On the to do list:
Replace NdFeB magnets in the ion sources with SmCo for higher temperature operation and comparability with deuterium without disintegrating like NdFeB magnets eventually do.
Replace hhe viewport on the vacuum hub with one that uses a cu o-ring
Re-design grid insulator to vent virtual leak from body and use a non-outgassing material(0 porosity ceramic)
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull »

Fabulous image and fabulous results. It just gets better and better as operaational experience grows. It is always true and the key to real fusor operation. For the simple amateur fusor there is more that science and engineering, there is operational artifice required due solely to the simplicity of the device and the fact that we are working on a "razors edge" located at a fine point in the gas/electric, ionization routine, at rather high pressures. It all comes together solely due to operational control excellence found through experience.

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Daniel Firth »

I don't have much to add... just wanted to say it's been interesting watching your progress in this thread.

I think this got buried in a thread about my construction, but I based my grid off of yours. It's not water cooled, but I used your bending method with the slotted pipe.

Also, my BTI bubble detector was done in about a year. I got it in 2012, and now it has many gigantic bubbles. Do you keep yours stored compressed, or uncompressed?
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Bob Reite »

If you got a full year out of your bubble detector, you did well. Mine was gone in 9 months. They are supposed to be stored compressed in the factory supplied tube between 15-20 degrees C. I was shocked when I took mine out of storage and found large bubbles in it, even though it was stored compressed. That seems to be the failure mode. The bubbles from the last measurement "grow" even while under compression.
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

I think there might be a lot of variation based on how well the dosimeter seals when assembled.

They used to manufacture the dosimeters with an anodized aluminum cap and compression system, but then switched out to a plastic one which seams to seal a lot better, probably since the epoxy that they fill the threads with bonds well to the plastic. The first one that I had was made with the aluminum handle and always had a "fishy" smell inside the tube. It lasted about 1year range before it was leaking and would not recompressible. By 2 years it had huge bubbles as well
viewtopic.php?f=13&t=6121&p=34565#p34565

The second one that I bought had the plastic handle and it looked very well epoxied in place. It never had any hint of a "fishy" smell in the storage tube. I got it in 2009 and it would recompress until 2012, with the built in compression chamber, and after that it would still recompress by placing a mechanical pencil eraser in the compression chamber before tightening it down to increase compression.

I always stored both in the tube, compressed. It's possible there is a lot of variation in life due to problems with the sealing system.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Here is an excellent publication on leak rates for viton/kalrez/teflon o-rings for UHV systems(atom optics traps), the vacuum tests start on the 3rd page of the publication
viewport-oring-leak-rate.pdf
(440.42 KiB) Downloaded 498 times
And a reference of various vacuum compatible ceramics, I'm thinking of making part of the grid insulator out of macor, or LAVA, does anyone have any experience using these in a fusor under ion/electron bombardment?
http://www.technicalproductsinc.com/ceramics.html
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

All 4 ion sources have been cleaned and rebuilt and baked out at 200C before instalation. They now use the angled anode rings and SmCo magnets instead of NdFeB magnets for compatibility with Deuterium with out disintegrating, and to allow high temperature operation(up to 300C) without loss of strength. I decided to go with the original style pole piece for now since it provided a near parallel beam with the angled pole piece but may switch over to the other style later. The SmCo magnets have lower field strength though and the ion sources require higher voltage to operate and produce a less intense beam which is fine since they are still significantly over powered for the fusor.

Ion sources before cleaning
SAM_4686a.jpg
In addition, one of the glass disk viewports was replaced with a conflat viewport eliminating one more viton o-ring (5 removed in this modification)
SAM_4691a.jpg
They now use copper o-rings instead of viton and have pump out ducts to avoid virtual leaks.
SAM_4692a.jpg
SAM_4689a.jpg
One of the swagelok valves was outgassing something when it was getting hot and was also replaced, and a 0.5um filter was added to the dry air admit line for venting the fusor to atmosphere for servicing
SAM_4694a.jpg
Some burning on the inside coating of the ZnSe viewports was observed when running with deuterium(doesn't happen with air). There seems to be some charged particle coming from the grid that is not readily deflected by the magnetic beam deflectors. I'm assuming it's a negative deuterium ion
SAM_4695a.jpg
I've doubled the field strength on the beam deflectors by adding a second magnet but it doesn't help much. I may have to remove the ZnSe ports for deuterium runs or re-design them or the grid to deflect the ion beam away from that area.
SAM_4697a.jpg
A feedback control circuit has been built to regulate fusor pressure by feeding back from the vacuum gauge analog output, It will stabiily control pressure from the 1e-4torr range to the 10s of mTorr. This considerably reduces workload during fusion runs and increases neutron output by tracking the optimal pressure. It's a very simple circuit.

Pressure controller circuit
SAM_4698a.jpg
An NP10 boost converter similar to this, or other boost converters used to drive nixie tubes:
http://www.ebay.com/itm/SHV5-5Vin-400Vo ... SwwE5WVDQB
provides 87V from the 24V supply, then an LR12 adjustable regulator steps the voltage down to 70V to feed the positive side of an OPA445 high voltage op amp which is configured as a gain 21 non-inverting amplifier. A recom RS3-1212D DC-DC converter provides -12v to the OPA445 and an AD826 that is used as an input differential amplifier. The vacuum gauge output(quattro 999, 0-10v, logarithmic to pressure) is connected to the negative side, while a potentiometer supplying 0-12v is connected to the positive side. The valve will now accurately control fusor pressure. There is still some drift and overshoot since it's only a proportional feedback controller, but the final version will be a full PID with valve dither ontop of the analog signal to improve performance.
12-5-2015-pressure-control-close.jpg
The fusor will now pump down to the low E-4 range in about 5-10 minutes and max out at about 5e-5 torr after several hours.
12-3-2015-pumpdown2.jpg
The cleaning of the ion injectors and removal of 5 viton o-rings seems to have really helped, though the pressure still climbs when running as the grid insulator heats up. It's made of boron nitride with a boric oxide binder which is fairly hydroscopic and will soak up water every time it sees atmospheric air. It will have to be replaced with a combination of macor/alumina/fused quartz.

Fusion rate is now at 3e5 n/s
12-5-2015-calculations.jpg
Star mode at ~3mTorr during fusion fun
SAM_4704a.jpg
Last edited by Andrew Seltzman on Sun Dec 06, 2015 5:19 pm, edited 1 time in total.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Steven Sesselmann »

More nice work Andrew, efficiency is improving you are now in the E-9 range and it should get better with use. If you can get unto the -65kV range I thing the numbers will really shoot up.

If I understood correctly you are using feedback to control a vacuum valve, another possibility is set the vacuum valve and use a combined pressure and mass flow controller, to regulate the pressure with the gas inflow. Something like an MKS 649 or similar.

I look forward to see the more run data.

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Actually it controls the inlet gas flow by controlling the voltage on the piezo valve.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull »

Getting "betterer and betterer"!
Good work

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

The gas control system has meed modified to include a full PID feedback system and in instrumentation amplifier to buffer the input.
SAM_4780a.jpg
This results in better long term pressure stability and lower overshoot
12-18-2015-PI-feedback.jpg
step-responce-log.jpg
A dual layer circuit board has been designed in expressPCB for the final configuration of the control system
SAM_4790a.jpg
gas-pid.jpg
The circuit board holds 2 independent PID control circuits, for the gas valve control only one will be used, the version with both in use will be used to control the HV power supply and in injector current. The one connected to the power supply will compare the grid current to a setpoint and adjust the ion sources accordingly to allow operation at any pressure/voltage/current point with all 3 adjustable independently.

The ion source control will use this circuit to control the input voltage to a set of emco F40 proportional HV supplies driving the ion sources:
http://electronicdesign.com/power/simpl ... ers-output

And assembly is in progress
SAM_4781a.jpg
SAM_4782a.jpg
The boards will be housed in Hammond 1590BB boxes, one for the power supply/ion source control, and the other for pressure control
SAM_4783a.jpg
SAM_4789a.jpg
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Roberto Ferrari »

Hi Andrew

Your comment about the SeZn screen and its damage when running on D2 is consistent with the reductive power of hydrogen.
Please comment on any chemical alterations.
Congratulations for your excellent work!
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by DanielSandner »

Hello,
there is something very effective against long pumping times with a Turbo.
A Turbo can pump heavy molecules like hydrocarbon very fast in contrast to light molecules like D or He.
So if you can i would try to fill the chamber with an heavy gas and 1% D. The Turbo will remove the heavy molecules much faster and after 1-2 hours pumping you should have only Deuterium at a very low pressure.
kind regards, Daniel
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by John Futter »

Daniel
your post seems to be arm chair posteuring.
most pumping time on vacuum chambers is getting rid of gas and water molecules that have stuck to internal surfaces
with water vapour being one of the hardest to remove as the molecule is polar and sticks to the chamber walls with great tenacity.
Just backfilling the chamber with a dry inert gas ie argon will save hours of pumping when restarting a vacuum system
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Roberto,

I'm not sure about any chemical changes to the lenses, It looks like beam burn in there the D- beams hit the window surface. There is some evidence of the same thing happening to the pyrex glass viewports as well, though to a lesser extent. I plan to remove the ZnSe ports and may replace them with germanium viewports at a later time to see if that makes a difference.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Roberto Ferrari »

Thanks Andrew, we wait for future reports.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Final construction and tuning of the PID gas control system is complete

The control system is now in a complete box,
The controls in the front are(from left to right), power LED and switch, valve purge button and internal/external pressure setpoint reference, Direct output voltage control pot and direct/feedback control switch, internal pressure setpoint pot, process variable and output voltage displays
SAM_4793a.jpg
On the back panel, valve output connection, power input, error voltage monitor, external pressure setpoint, pressure input from gauge
SAM_4794a.jpg
Final circuit values
SAM_4795a.jpg
The final circuit is a full PID controller with output dither, there is a 6v, 90Hz triangle wave superimposed on the output to improve accuracy
valve-dither.jpg
Tuning between variable pressures
PID-final-tune.jpg
Holding constant pressure
PID-holding-presure.jpg
step response, achieves final value in about 8 seconds
PID-step-resp.jpg
The pressure control system holds pressure very stability, now onto the automatic control system for the power supply and ion injectors.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Steven Sesselmann »

Nice work, so you basically made your own pressure controller from a vacuum gauge and a piezo pressure valve. I guess this would be not so different from using a ready made MKS pressure controller with a built in baratron gauge. My only concern about using a pressure controller is accidental loss of gas if for some reason a gate valve is opened and the pressure drops rapidly, causing the gas to vent too fast.

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

By limiting the maximum valve voltage, the maximum flow rate can be limited, preventing a runaway in case of control system failure.

The maximum conductance of my vacuum system is 0.3L/s, with only an open and closed valve
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull »

Fabulous coup on your pressure controller, Andrew. A real win on that one.

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

The ludlum model 12 counter has been modified to provide a 0-10v output based on meter movement to allow the power supply control system to feed back on the neutron signal and provide a constant neutron flux

A boost converter provides +12.5v to a rail-rail opamp
SAM_4802a.jpg
Amplifier board converts 0-1.25v signal to drive meter to 0-10v signal on output
SAM_4803a.jpg
Boost converter and amplifier installed
SAM_4801a.jpg
Voltage and current control system construction in progress
SAM_4808a.jpg
Right now it can display power supply voltage and current and control power supply voltage. The final version will have a 2 channel PID controller for current and voltage control. The system will either directly set output voltage, or control voltage using the neutron flux for feedback. For current control, it will look at the power supply current monitor output and adjust ion source current to track a specific grid current. This will allow the system to accurately track a given voltage/current/pressure operating point with minimal operator tuning.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull »

Wow! Shooting for a constant neutron source in a fusor. Lots of feedback loops here. I hope they don't go "slinky" on you. Fine tuning such systems can range from moderately simple to near impossible. Damping system wide swings can be the real win on such efforts. Good luck.

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Steven Sesselmann »

Andrew,

Nice work and AFAIK a first here on fuser.net, I echo Richards concerns, you have quite a few loops happening there, so I guess you need some kind of hysteresis to compensate for delays. If anyone can do it, I am sure you are the one.

Looking forward to your next run..

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

I have the control system successfully holding a constant neutron flux based on feedback from the neutron detector. At the lower rates that the fusor is producing the integrator time constant has to be fairly long to prevent the system from oscillating. For these tests the ion injectors are just holding at constant emission without feedback control from current since that part has not been built yet.

Video of operation
https://youtu.be/BiEOAbmksow

Circuit schematic
SAM_4810a.jpg
Control system close up
SAM_4815a.jpg
Control system and neutron detector
SAM_4813a.jpg
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull »

This appears to be a first in electronic fusor control that appears to be rather successful. Do you have a current fusion level at which the system seems stable over about 20 minutes or longer? What is your longest stable, controlled run? Just curious.

Really a super effort.

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Based on comparison between the old BTI(it finally did fail) I had and the BF3 detector, I would estimate neutron rate in the 2e5 n/s range. More fine control will be possible with higher neutron rates as the effect of a single count becomes less. As of now, the control system is running with mostly integral feedback with a very long time constant to prevent oscillations. If the neutron rate was higher, the time constant could be decreased.

At high proportional gain or lower integral time constant the system does start to "slinky" rather noticeably, but with low Kp and long Ki, it runs stably.

The longest run would be in the 2-3min range. I'm still having problems with out gassing of the boron nitride grid insulator and possibly the ZnSe viewports as things heat up which limits run time as the pressure controller reduces gas flow to maintain constant pressure until no deuterium is being admitted to the chamber. Removing the ZnSe ports and upgrading the grid insulator design is on the to-do list.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Dennis P Brown »

I am a bit concerned by the ZnSe window - powerful electron bombardment could release Zn into your system and cause long term contamination - Zn, if it does deposit will re-vaporize again and again - not good for electronic sensors, I'd think. I assume you need that ZnSe window as an IR clear window but if you aren't using it at the moment for that purpose I'd suggest replacing it with a glass window for now. Why keep an unknown that might be causing issues until you have the system fully operating in the manner you want and then installing the ZnSe window? Just a thought.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

That is one of the things I worried about with the ZnSe viewports. There are defector magnets to prevent the electron beam from hitting the surface, but it doesn't really deflect the D- ion jets.

I have removed the ZnSe viewports and still have the problem. It's most likely the BN on the grid insulator, the binder has a tendency to soak up some moisture.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

The ZnSe windows have been removed and the damage analyzed. Both top and bottom windows showed deposition from sputtering considerably more visible then that on the glass viewports.
SAM_4818a.jpg
SAM_4817a.jpg
SAM_4863a.jpg
This is likely due to the deposited material interfering with the BBAR(broad band antireflective) dielectric coating on the window causing considerably more change in transmitted light. Examination of the window surface under a microscope showed pitting of the BBAR coating where the D- beam was hitting the window.
top-lens-d-ng-beam-centera.jpg
Subsequent cleaning of the window was able to remove most of the BAR coating(and the deposition with it). Post cleaning the window is transparent again.
top-lens-d-ng-beam-center-cleana.jpg
None of the pitting or damage extends into the ZnSe material, it appears only the BBAR coating is strongly affected by D- bombardment, however to err on the side of safety, the ZnSe windows were not re-installed to prevent any potential sputtering of zinc from the now unprotected window surface into the vacuum system.

The viton o-ring sealed glass disk viewports were replaced with AR coated vacuum rated viewports.
SAM_4859a.jpg
Due to the previously observed interaction of the sputtered material and D- beam with AR dielectric coatings an internal shield system was designed to prevent exposure of the viewport internal surface with plasma or sputtered material.
SAM_4852a.jpg
A stainless adapter holds a 1.25" diameter 1/8" thick glass disk inside the vacuum system in front of the the viewport. The glass disk is held in place with a internal retaining ring. No groove in machined in the adapter, the friction force of the ring is sufficient to securely hold the disk in place.
SAM_4857a.jpg
SAM_4854a.jpg
The adapter allows the glass disk to hang off of the free space inside the copper o-ring between the two conflat flanges.
SAM_4858a.jpg
A pump out duct in the side of the adapter vents the volume between the shield and the viewport.
SAM_4853a.jpg
The small distance between the shield and viewport, and large aperture of the shield prevents any restriction in field of view.
SAM_4883a.jpg
The lead glass x-ray shield is held in place with an o-ring but a plastic holder will be machined to securely hold it in pace.
SAM_4884a.jpg
Dimensions of the shield holder are as follows
Glass disk is McMaster 8477K28
Retaining ring is McMaster 92479A790
SAM_4864a.jpg
Additional upgrades included the design and installation of a faraday cup to measure D- beam current reaching the end of the CF nipples on the vacuum chamber
SAM_4843a.jpg
SAM_4848a.jpg
SAM_4845a.jpg
SAM_4846a.jpg
The collector plate is recessed inside a secondary electron suppressor shell. The shell is also isolated from chamber ground so that both the collector and shell can be biased for ion energy analysis.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Robinson »

Outstanding work as usual Andrew!
I can wire anything directly into anything! I'm the professor!
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Robinson »

Ha... Sounds weird saying that. Feels like I'm patting myself on the back lol.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull »

This is the kind of advanced work that we would hope to see more of here. Again, thanks for the continued sharing.

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Some power supply upgrades are coming to the fusor allowing increased voltage operation. Earlier this year I got a second spellman PTV power supply on ebay for $200 with the intent of paralleling the outputs for increased current
http://www.ebay.com/itm/Spellman-Lunar- ... 7675.l2557
Unfortunately the seller had one positive and one negative polarity and sent me the positive polarity one, which had to be returned. Kuba got the other one that had the negative output.

Fortunately Bern spotted a spellman x-ray supply on e-bay which I managed to win. It's a -70kV, 8.56mA spellman power supply(DXM70N600X3547)
http://www.ebay.com/itm/Spellman-Power- ... 7675.l2557
The manuals are listed here
http://www.spellmanhv.com/DXM
http://www.spellmanhv.com/~/media/Files ... XMMAN.ashx

It has digital control of the output(serial, usb, and ethernet)
s-l1600.jpg
though it does use an unusual connector, a Claymount Mini Federal Standard X-ray connector, identified as a model CA11
http://www.claymount.com/en/products/hi ... /connector

I managed to find some short cutoff cables for testing of the supply
http://www.ebay.com/itm/Claymount-Xray- ... xyMZ5RnAlY
But I am looking for the screw ring for the connectors and longer cables that I can adapt to my system if anyone knows where to get them.

My grid system is in the process of being upgraded to prevent arcing from the tip of the ceramic insulator and prevent outgassing by replacing the boron nitride with macor and alumina. With the grid removed the fusor will easily pump down into the 1.5E-5torr rang and into the high E-6 range with help from the bakeout heaters and ion sources to desorb water from the walls, so the grid BN ceramic was definitely causing the vacuum problem. With the bakeout heaters on and the fusor shell in the 80-90C range, the vacuum will hold in the mid E-5torr range, so it should still have 99% deuterium purity when operating in the mid E-3torr range. Leakage through the few remaining viton o-rings still lets in some water, but the vacuum is getting progressively better.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

The old grid was modified to remove all boron nitride components that were causing vacuum problems due to their porosity and water retention.
Deposition on old boron nitride grid insulator
SAM_4889a.jpg
Arcing damage to top of old grid insulator cracked the quartz tube
Still0003a.jpg
Still0008a.jpg
Still0012a.jpg
New grid system with macor deposition shield, alumina insulator stalk, and molybdenum field control ball
SAM_4900a.jpg
molybdenum field control ball fits over the tip of the alumina stalk preventing any sharp edge from being exposed to the plasma like on the last grid design. Sputtered metal deposition will eventually make the lip of the insulator conductive leading to arcing. The molybdenum ball fits like a cap over the tip of the ceramic rod presenting only smooth surfaces to the plasma.
SAM_4899a.jpg
The new macor bushing adapts the quartz vacuum feed through to the alumina rod and includes a deposition shield, the bushing has a machined in pump out duct to vent the inside of the grid assembly to prevent a trapped volume
SAM_4898a.jpg
Design of bushing and molybdenum cap
SAM_4908a.jpg
SAM_4909a.jpg
Operation of new insulator
SAM_4907a.jpg
Unfortunately the new design arcs at 25-30kV at 10mTorr at the base where the alumina rod meets the macor bushing. This only occurs when exposed to plasma, in a neutral gas at 10mTorr with the ion sources off to prevent any plasma breakdown the insulator will hold off 40kV. This design needs to have additional bushings outside the alumina stalk like on the boron nitride grid. These will be machined out of macor and added to the system.
grid-rev4-down2a.jpg
The vacuum performance of this grid is perfect with no outgassing or water retention. The system will easily hit the mid 5E-5 torr with the grid installed, macor is definitely the way to go.

Preliminary tests are being conducted on the new Spellman DXM supply. It turns out the supply is brand new, with no logged hours on the HV on hours counter. The supply can be controlled and monitored over an ethernet connection, or with analog inputs.
SAM_4911a.jpg
Matlab program generating a 5kV ramp waveform of the voltage output
spellman_kV_rampa.jpg
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by John Futter »

Andrew
I have had various results with the ethernet control program as supplied by Spellman with our 6kW 140kV unit
it disappeared from our intranet when we needed to shut it down urgently due to excessive x-ray production in our experiment ie counters went over 300mSv.
So we now have a kill switch wired through the interlock connector.
Not sure why you are having so much trouble with the BN insulator.
As for o-rings some of our equip has nearly a 100 meters of viton o-ring and it sits at around 2x 10^-8 millibar base pressure with a single 300L/S turbo, the RGA does not show appreciable water signal we see a weak air signal N2 O2 H2 Co2 and heavier hydrocarbons probably fingerprints and grease ,backing pump oil etc.
Pyrex makes very good shields as it is slightly conductive and defines the "E" field with a nice gradient.
I do not like MACOR for this as it is too insulative and the field can concentrate in the wrong places IMHO
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

John,

I definitely plan to have a hardware interlock/kill switch on the final setup.
The problems with the BN insulator were mainly due to it having 7% porosity and a boric oxide binder, which is hygroscopic. It soaks up water vapor and then released it when it starts to get warm.

I'm trying to avoid any non-quartz glasses like pyres since they benin to get conductive when heated by direct ion bombardment. This is especially a problem at the lip next to the grid. The quarts has much less problems and can tolerate much higher temperatures, but still needs to be shielded from direct bombardment at the top lip. It's also kind of brittle and can fracture during an arc. Alumina will not fracture like quartz, but has much lower dielectric breakdown strength.

The macor is something new I'm trying due to it's machanability, I hope to make something similar to the BN insulator but with macor shields, an alumina core, and a molybdenum field control ball at the top to prevent any ion bombardment if the insulator lip. I really think the re-entrant molybdenum ball will greatly increase grid insulator life by preventing conductive deposition on the insulator lip by the grid from initiating field emission discharges.

Do you have any experience using macor under intense ion bombardment, like near the focal point of a fusor? Does it begin to melt/degrade/outgass etc...?
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Robinson »

Great to see you're still making progress on this design. We're running into some of our own operational challenges with our liquid feed through. Always great to see the challenges you are running into. It has helped us immensely in the past to avoid some costly mistakes. Thanks Andrew!
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Latest upgrades to the mark 3:

A complete rebuild of the grid cooling system has been completed.
The old system was removed
SAM_4966a.jpg
and replaced with an upgraded version consisting of a compact thermometric heat exchanger designed around an all in one waterblock pump combo (swiftech apogeedrive 2) http://www.swiftech.com/ApogeeDrive2.aspx
SAM_4976a.jpg
as well as a new radiator and low noise fan
SAM_4975a.jpg
An acetal block is machined to hold a second copper waterblock that fluorinert flows through
SAM_4989a.jpg
SAM_4980a.jpg
A thermoelectric cooler transfers heat from the fluorinert to the water cooling loop
SAM_4999a.jpg
A CPU retention plate is outfitted with extension standoffs
SAM_4988a.jpg
Allowing the apogeedrive cooler to clamp down to the thermoelectric cooler and fluorinert waterblock
SAM_5002a.jpg
Dimensions of the acetal block
SAM_4994a.jpg
SAM_4995a.jpg
The newer system is considerably more compact
SAM_5003a.jpg
A stainless reservoir stores fluorinert for the cooling loop
SAM_5039a.jpg
SAM_5040a.jpg
Complete system mounted in reactor frame
SAM_5044a.jpg
SAM_5046a.jpg
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Upgrades to the grid system have also been completed

The arcing problem on old grid design was traced back to the area between the alumina insulator and the macor bushing
SAM_5024a.jpg
SAM_5025a.jpg
This was only a problem when exposed to plasma bombardment. In a vacuum it would hold off 40kV, but arc at 25kV under plasma bombardment

A new design with a macor insulating cap and new bushing with a overlapping interface that prevents any scattered plasma from forming a conductive path
SAM_5011a.jpg
SAM_5019a.jpg
SAM_5021a.jpg
The new version will operate without arcing at 40kV in a plasma and will not produce any outgassing(unlike the boron nitride version) allowing continuous operation of the fusor
SAM_5054a.jpg
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Robinson »

As always, fantastic work Andrew! Both myself and my team always look forward to updates on your grid design and have been following them with great interest.

You have caught my particular interest with Macor. Will need to read up on this ceramic. Thanks!
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Latest operation data from the new grid and cooling system:

The new macor insulated grid allows significantly better vacuum performance due to it's zero porosity and lack of binders. There is no discernible outgassing as the ceramics heat up, allowing continuous operation.

Latest operation at 7.2mTorr, 40kV, and 8.8mA
SAM_5051a.jpg
Generated 70 bubbles (30bub/mrem) in 60 sec at 3.5"(8.89cm) from the center
SAM_5068_counta.jpg
SAM_5067_counta.jpg
Current best neutron output is 1.08E6n/s
fusion-6-26-2016-7,2mtorra.jpg
Next on the to do list is connect the second PTV supply in parallel to run at 40kV, 17mA, and hook up the new 70kV, 8.5mA spellman supply for higher output.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull »

Fabulous report and great work across the board. Few folks who have done fusion here have hit the mega mark. Probably as few as 10%. You kinda' have to stick with it once you have done fusion to get numbers up. You system is clean and very well executed. You can now easily do activation on a significant scale, far from any need for satisitcal back up.

You might try to calibrate a fixed silver activation set up against your BTI bubble detector, if that is your only neutron indicator. When the bub dosimeter craps out, you can rely on the silver activation as a crude but calibrated indication of future performance. (assuming you don't want to blow the coin needed to replace the BTI when it goes south.)

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Steven Sesselmann »

Andrew,

Great engineering and incremental improvements and it's obviously working. Your fusion rate and quotient is improving with every iteration, you are now sitting somewhere between Richard Hull and Jon Rosenstiel on the list, despite running at lower voltages then Jon. I shall hold off putting name on the list until you report back again after a 70 kV run.

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Best run to date is 1.4e6 n/s, at a slightly increased pressure of 7.8mtorr, 40kV and 8.8mA. 1e6n/s is now easily achievable with 1.4e6 n/s possible with some tweaking. Best to add this Q value to the list for now, it might be a while before the system is capable of 70kV operation.

The molybdenum cap under the grid is also heating up during the runs, getting into the dull red range. I might switch to a stainless cap and braze it onto the grid tubes for better heat conduction to the coolant.
SAM_5073_crop.jpg
Has anyone noticed how the bubbles seem to be clustered towards the center of the dosimeter? Is anyone else seeing this of their BTI?
SAM_5091_count.jpg
rate.jpg
Here is a list of Q values for university fusors, mine is currently at 4e6 n/s/kw
UW-Q.jpg
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull »

Keep pushing that pressure!! Old hands who have been here for years, and still working with their fusors, will tell you.......Once you have a fusor working over 35+kv, GO FOR PRESSURE!! My very best runs are always over 10 microns and my best run, ever, of 1.58 e6 neutrons was at 42 kv applied and 15 microns! Right after doing fusion, the secret is to run and run and run and learn with each run. Learn operational technique!

We are working on a ragged edge, bull-heading fusion in a very narrow zone not found or to be had casually for the finest results. Even the finest constructed systems from the best materials have to be considered crude based on our modus operandi in doing fusion. In the end, the science allows it to happen, but operational artifice makes it happen. Improved artifice makes it happen better.

Newbs who come, see, marginally do and then leave, never develop the artifice for any number of reasons. Yes, the fusor, as we typically configure it, can only go so far, but pushing it becomes an art and craft.

I have trumpeted this for years here in numerous postings. It is good to see someone pushing their individual envelope in continued work with a good fusor system.

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Steven Sesselmann »

Interesting, I just looked up some information on the SIGFE reactor and I never realised how close the construction is to my current FICS design, as in FICS the cathode is semi enclosed with ions entering through holes, forming almost a hollow cathode, it also has electron suppression. SIGFE and Hirsch design clearly stand above the rest in terms of efficiency.

The only critical mistake they made IMHO is to accelerate the ions into the cathode, obviously they believed ions need speed to overcome a coulomb force and fuse, quite to the contrary I think the ions need to stand still before they can fuse, but time will show.

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Latest run updates:
2.4E6 n/s at 39.8kV, 17.68mA, 9.24mTorr
SAM_5106.JPG
SAM_5109_count.jpg
SAM_5110_count.jpg
neutron-flux.jpg
This higher current run was made possible by paralleling 2 spellman PTV power supplies to double the output current
SAM_5114.JPG
A new plastic insulator was machined that allows 2 HV cables to be plugged into the grid via a splitter adapter. This insulator will also allow use of the 70kV supply that has a larger cable.
SAM_5101.JPG
SAM_5095.JPG
SAM_5111.JPG
A control connector is then made to allow the master supply's current monitor line to drive the slave supply's current program line
SAM_5113.JPG
Both supply's are then run in constant current mode at slightly below the voltage setpoint

There are definitely heating issues on the moly cap, with it now becoming bright red. The macor is still holding up very nicely with no discernible outgassing, even when in contact with the red hot moly cap. I'll probably have to switch to a stainless cap brazed to the grid structure if I want to continue at these current levels.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Steven Sesselmann »

Andrew,

You are really cooking now, heading towards the 5 meganeutron mark, I trust you are keeping a safe distance.

Even at this emission rate the efficiency has remainded roughly the same.

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Runs are kept short for safety(and thermal issues)
x-ray rate at the viewport surface is 10-15mR/h through the lead glass shield
neutron rate at the surface of the vacuum vessel is 140mR/h at 2.4E6n/s
Runs are 30s with the two parallel supplies, about 60s with the single supply, and I only run long enough to get good counting statistics from the BTI dosimeter, so about 2-3mR of neutrons at the surface of the fusor(~3.5" from focal point) per run. I'm usually about 1-2ft from it so it drops off a lot due to inverse square law.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Steven Sesselmann »

Andrew,

I know only too well how fast the minutes pass when one is fiddling with the knobs on a fusor, big difference between 30 seconds and 5 minutes. At those emission rates I would want to be further away than 2 feet.

My new fusor may not achieve 5x10^6 neutrons like yours, in any case I have built it for remote operation with a 10 metre fibre optic control cable extending to my laundry behind a 2 foot sand stone wall.

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Agreed, adding more shielding is next on the to do list. In this case there isn't really any way to push the runs any longer, the difference between 30s and 40s of operation is the moly cap not glowing and glowing bright red, it's a pretty reliable timer.

More automation is also in progress to allow remote operation, I bought a labjack T7
https://labjack.com/products/t7
to monitor and control the fusor. It has an ethernet interface and numerous digital and analog outputs for control.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull »

Great Work! I guess the best of all current operating fusors!!! Very, very clean system and star. Those ion sources are making a real impact. Plus ultra all the way.

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

I've tracked down the source of x-rays coming from the fusor. I had initially thought the 15mR/h was some of the higher energy x-rays coming through the lead glass, or stainless shell, however it turned out to be coming from the viewport near the vacuum gauge. Some of the secondary electrons from the grid were getting up into the conflat cube and generating x-rays. A lead foil cap over that viewport has now eliminated any measurable(on a ludlum 17 ion chamber, minimum resolvable rate ~0.5mrem/h) x-ray flux from the fusor. I'll still need to add lead shielding around the vacuum chamber when I start running at 70kV, but for now the x-ray issue is solved.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull »

The shell starts to go transparent on the standard .060 inch thick SS shell at about 35kv applied. At the shell, working at 40kv, using a victoreen ion chamber, I get about 10mr/hr. It will only go up from there. Any window or view port will be much higher at lower voltages as the lower energy x-rays produced will pass through the glass as electrons strike the shell on the opposite side. This is much the same as old x-ray tubes.

The lead cap trick over the view port was used by Joe Zambelli in his 2001 fusor. I avoid it by pointing my view port straight down into the concrete floor through the video camera. View ports are the most dangerous emitters of radiation in the amateur fusor and all that danger is x-radiation. Neutrons from fusion are of no real consideration for the average successful amateur fusioneer as few will use more than 40kv applied.

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

I recently scored an URSA multichannel analyzer on ebay and am hooking it up to do neutron activation analysis
SAM_5159.JPG
The system was calibrated with a Cs137 check source
SAM_5161.JPG
Cs137.jpg
And an Am241 source(smoke detector)
Am241.jpg
All of which showed distinct peaks.

Spectra was also taken of the x-ray emission of the fusor
At 25kv (with the lead shielded cap off)
fusor-25kv.jpg
At 30kV (with the lead shielded cap on)
fusor-30kv.jpg
At 40kV (with the lead shielded cap on)
fusor-40kv.jpg
It's interesting to see how the 30-40keV x-rays are just starting to get through the 1/8" stainless shell, while the 25keV ones are blocked entirely unless the lead cap is removed.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull »

Great work Andrew! I have noted this above and for some years. Your work puts a real fine point on this. Thanks for the excellent report.

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Additional progress is being made on the liquid cooled grid to address the heating of the field control cap that covers the lip of the ceramic by brazing it to the cooling lines on the grid. Cusil brazing alloy (72% silver and 28% copper eutectic) was used in the brazing with a boron modified flux(Superior No. 601B/3411)
SAM_5190.JPG
The allow has reasonable wetting properties on stainless and only contains vacuum compatible materials(no zinc or cadmium)

A test braze was completed on an older grid, shown after sandblasting and some cleaning
SAM_5191.JPG
SAM_5192.JPG
SAM_5194.JPG
The braze seems structurally and thermally sound and generally wetted ok on the stainless, though the gap may need some tuning to get good flow down into the area between the cooling lines and the cap.

Additionally, the neutron detection system is being upgraded to use He3 detectors instead of the older BF3 detectors that were previously in use. The new detector is about 7x more sensitive then the BF3 tube on the system. This should allow the time constant on the constant neutron flux control system to be reduced for tighter regulation while still remaining stable.
SAM_5195.JPG
SAM_5196.JPG
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Steven Sesselmann »

Andrew,

It's really good to see you make these small incremental improvements it means the technology is being refined and becoming more mature.

By the way I have one of those LND He3 tubes, and I'm interested in knowing where you got the connector. Does it come like that with the threads or is it something you have custom made?

Steven
http://www.gammaspectacular.com - Gamma Spectrometry Systems
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

The connector is a TNC bulkhead mount adapter to clamp on coax, the clamp on section has the same 7/16-28 as the TNC and the LND tube. the other fitting is an Amphenol 79675 Adapter TNC to BNC adapter, a small spring connects the pin on the end of the He3 tube to the center pin on the other adapter
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Pictures of the molybdenum cap glowing under ion bombardment after about 40sec of operation at 17mA
SAM_5200.JPG
Pictures of the molybdenum cap glowing under ion bombardment after about 40sec of operation at 17mA
SAM_5201.JPG
The old grid was removed and inspected prior to the upgrade to the version with the brazed cap. The macor ceramic has held up extremely well with no signs of thermal or arcing damage.
SAM_5210.JPG
SAM_5211.JPG
A cooled grid with a brazed field control cap has been installed and is currently being tested
SAM_5217.JPG
SAM_5218.JPG
The new grid was tested for several minutes at ~2.5e6n/s with no discernible heating of the field control cap
SAM_5220.JPG
The current limit for operation time is now the heating of the vacuum vessel which will require some additional fans to keep cool .
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull »

I have never really worried about the shell heating which tends to reach about 200 deg F after 1.5 hours of running. (electron bombardment and neutral crash area).

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Mine seems to have localized spot heating near the ion sources. I think the fraction of the deuterium beams that is neutralized or charge exchanged at the focal point is being accelerated outward and striking the shell near the injector ports on the opposite side.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Steven Sesselmann »

Andrew,

Did I understand this correctly, the cap was initially not brazed to the cooled grid and so became heated from the ion bombardment, so you have now brazed it to the grid so it now stays cool?

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

That's correct. The previous version had a molybdenum cap for shielding the lip of the ceramic. The cooling lines just stuck through, making electrical contact, but without making good thermal contact. The new version has a brazed stainless cap.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull »

Thanks for the info. My shell has no ion sources pointing at one another across the sphere. Those beams on delecate parts in the chamber would be a bad thing.
So much for taking the time to critically align stuff during assembly.

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

The beams seem very well aligned, the fact that they are hitting near the ion injector ports is due to the electrostatic lensing effect from the grid. The grid consists of 3 rings, and while it resembles a spherical source at a distance, nearer to the grid the discrete effects of the grid rings are noticeable. An ion beam fired directly radially inward at +-45 degrees to the midplane of the vacuum vessel will pass through the grid and bend towards the midplane. When exiting the grid it will be at an angle nearer to horizontal and will strike the shell about halfway between the midplane and the opposing injector port.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

I've been working on converting my older CAD files on the fusor into autodesk inventor, here's what I have so far
core1high.jpg
Cross sectional view
core3high.jpg
Grid Detail
grid.jpg
Grid insulator box detail showing cooling lines
grid_box.jpg
Detail of machined macor grid insulator
grid_tip.jpg
grid_tip2.jpg
grid2.jpg
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull »

Nice CAD work. Very professional. Fabulous images of what the finished product will resemble. Boy, times, they do change.

I took two classes in engineering drafting while in college in the mid-sixties. We joked about the drafting requirements, saying that should we fail as electronics engineers, we could always find work with International Havester designing self-propelled manure spreaders.

I bought the drafting table and a super nice set of drafting instruments and used them into the early 70's for my own work and pleasure. I still pull them out on occasion, but tend to use simple Computer drafting programs for mechanical stuff and schematic programs for electronic circuit printouts. I have never considered CAD, as I can doodle out a 3-D mechanical impression and see issues develop on that front. Without free access to CAM, I always felt CAD, by itself, a bit over blown. CAD-CAM need to be fully wed and implemented together. My thoughts.

As the brain ossifies, I currently only attempt to learn that which is supremely valuable immediately which is free of any cost. As I am retired, the only new stuff I need to know is that which presents itself in the press of advancing my own personal efforts. I don't have to impress any one person or employer any more. I have a lifetime accumulation of hard won knowledge, about 60% of which is useful now only for campfire stories about how it was in the "old days". This sort of specific, focused knowledge and its application, which the young folks here strive for now, will be part of their campfire story collection in later life, as well. There will always and forever be "this new thing" leaving the bulk of your past efforts and hard won experience, a relic on the trash pile of what was once the cat's pajamas.

All that past effort in life, as you advanced, served to put bread on the table, pay for the car, etc. However, just as the bread ultimately ended up in the sewer system as consumed waste product and the car, in the junk yard, having ferried the family around so valiently, so hot, current, specific skill sets in focused endevors will also fade from being of any real value in future.

This is why I can see the sage, yet sad, Yogi Berra quote in my closing salutation as truly humorous on many levels.

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Robinson »

Glad to see you pushing into some newer CAD software Andrew. Nice attention to detail. I keep trying to reinforce the usefulness and application of CAD here. With your level of past attention to detail, I know I don't need to try to convince you. I know it can be a daunting task to model everything but very rewarding in the end. We're now pushing into the 1000's of hours on our current model. I'd be happy to talk CAD with you. I'm sure we both have some tips and tricks we can share. Keep it up and as always, I continue to look forward to your progress.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Robinson »

Andrew,

You may be particularly interested in the thermal and flow characteristics we're modeling currently off your original design. Note the continuous feature via 3D sketches to enable the flow and thermal simulations of the grid. This was an early (and incomplete) 3D test and unfortunately I'm not at the office currently so I don't have our updated models or data to share at the moment, but like I said, send me a message sometime. We're investigating heavily into this area and building on your previous success. I would love to share data between teams.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Robinson »

Hey Andrew,

I fired off a few emails to you, but I'm not sure if I have the correct email address. I wont post it here in plain text due to crawlers but are you still at the university/using that email address? Looking forward to hearing back from you.

Cheers,
Andrew
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

I'm here(same address), I sent you a response via email.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

The fusor has been tested with the new molecular drag pump and dry diaphragm pump. The MDP was mounted to the high vacuum solenoid valve on the fusor and was running off of it's built in AC power supply, while the diaphragm pump was running off of the fusor's 24Vdc supply. Vacuum performance was generally good. The solenoid valve between the fusor and MDP was opened at ~100s while the fusor was at ~100mTorr, pump down into the high 1e-4 range occurred with generally good performance. Deuterium pressure was held at ~8mtorr and the fusor generated ~2.3E6n/s at 40kV and 17.6mA, also generally about the same as running with the turbo station.
fusor-mdp.jpg
MDP mounted on fusor
SAM_5865a.jpg
Plasma
SAM_5866a.jpg
My last deuterium tank had run out of pressure, having only ~90psi when I got it, it lasted about a year of intermittent use. New deuterium tank has ~1750psi in it and should last a significant time. The tank is now mounted inside the fusor frame(more compact and better protected) and all VCR fittings have been upgraded to copper o-rings rather then the nylon washers I was using for testing.
SAM_5870a.jpg
The fusor's going to be hooked back up to the old turbo station until I can get the new one with the MDP built though.
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Re: Mark3 operation and upgrades

Post by Andrew Seltzman »

A recent upgrade to the mark 3 has replaced the 6 way conflat cube with an 8 way kimball physics vacuum hexagon. The hexagon is considerably smaller and more compact, and has 2 more ports then the cube. The hexagon is connected to the core with a brazed ceramic isolator with copper conflat o-rings, which replaced the PEEK plastic isolator with viton o-ring. In addition the hexagon mounts an MKS 901P with integrated display, x-ray shielded 1.33CF viewport, deuterium inlet valve, Pfeiffer EVI 005 vacuum valve, an electron collector probe on a BNC feedthrough, and a germanium viewport for IR measurements. This upgrade replaced 1 plastic insulator, 2x 2.75CF viton o-rings, 1x KF40 Viton o-ring, and 1x KF16 o-ring with all metal parts, is about 4kg lighter then the previous unit, and significantly more compact, which will allow a diaphragm pump / turbo pump system to be integrated into the frame. The MKS 901P is the RS485 version which will allow several vacuum gauges and the turbopump to talk on the same bus.

The Pfeiffer EVI 005 valve was the one tested in this post
viewtopic.php?f=10&t=12155

The deuterium inlet valve was welded with this procedure:
viewtopic.php?f=6&t=12284

CAD designs on the hexagon are attached below
vac-hex.jpg
vac-hex-half.jpg
6way-frame.jpg
Pictures of the completed hexagon
SAM_6445.JPG
SAM_6448.JPG
Comparison to the old vacuum cube:
SAM_6452.JPG
Vacuum tests of the new system are very encouraging, the lower conductance of the smaller valve is offset by the removal of the viton and plastic; the system now archives considerably better vacuum performance, and will max out the gauge at below 1e-5 torr.
First pump down with bakeout starting at ~700s and pressure rise with valves closed.
6way-vac.jpg
6way-rise.jpg
In addition a new digital control system is in the process of being designed, which will replace the older analog feedback controllers with an arduino controller and provide a touchscreen display for monitoring and control
SAM_6456.JPG
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA-9mA

Post by Richard Hull »

Fabulous upgrade Andrew!! One of the better systems out there and a fusioneer that has been around a while. Thanks for the upgrade. I am envious of the valved off 1000+ second hold to about 50 microns. I am at 100 microns after about 360 seconds, but I have operated like this for about 9 years now.

Again, nice stuff! ( as an old friend used to comment to me).

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA-9mA

Post by Andrew Seltzman »

Latest upgrade progress:

Diaphragm pump, rough vacuum gauge, and turbo pump to be installed on fusor frame.
SAM_6520.JPG
CAD design of mounting position:
turbo-mount.png
Vacuum gauges and turbo pump will share an rs485 bus for communication. The USB to RS485 dongle used for turbo pump testing is the following:
https://www.amazon.com/gp/product/B010K ... UTF8&psc=1

Pump actual speed, set speed, and controller current, voltage, and power.
turbo-test.jpg
Matlab control program for pfeiffer turbo pump:
Pfeiffer_TC100_rs485_plot_data.zip
(2.27 KiB) Downloaded 321 times
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA-9mA

Post by Andrew Seltzman »

More upgrades for the fusor:

Upgrades for mounting internal vacuum pumps, custom backing and mounting bracket for the TPD011 turbo pump
SAM_6850.JPG
Front and back of bracket, replaces the NW16 backing connector and provides 4x 10-32 tapped threads on a 1.75"x97mm pattern
SAM_6849.JPG
Turbo pump and diaphragm backing pump
SAM_6852.JPG
Mounting plate to hold vacuum pumps
vacuum-system.png
Locations of vacuum pump within fusor frame
vacuum-system-location.png
Thermal camera mounting system on vacuum hexagon to observe grid temperature
vacuum-hex-optics.png
Right angle bending mirror
SAM_6853.JPG
Optics path
optics-path.png
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA-9mA

Post by Andrew Seltzman »

The new CNC'd mounting plate for the internal vacuum pumps have arrived and the turbo and diaphragm pump are installed within the fusor frame.

Diaphragm pump, turbo pump and MKS-901P gauge for roughing line pressure measurement are mounted on the aluminum plate.
SAM_6860.JPG
Plate mounts on top of fusor
SAM_6862.JPG
Vacuum line connects turbo pump to foreline solenoid valve
SAM_6863.JPG
Pumps powered up and backing vacuum gauge added to the RS485 bus
SAM_6866.JPG
Since both gauges are now on the RS485 bus, backing vacuum and high vacuum can be measured simultaneously, pump down was measured with the turbo at standby speed, at full speed the pumping rate increases. On standby, ultimate vacuum is below 1e-5 torr.
dual-pumpdown.jpg
Testing of the thermal camera was also successful.
O202057287.jpg
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA-9mA

Post by Richard Hull »

Wow! You are already in the show-piece class and I sense a move into museum grade exposition. This is how it is to be done if one is "long-term serious" about making a world class fusor system and sticking with it. Great work Andrew! Turbo standby mode is more than adequate to do fusion........

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA-9mA

Post by Dennis P Brown »

Very impressive system. When money is no object, one can create a very professional system!

Your thermal camera might need optical filtering to reduce the intensity since that filament will be in the thousand centigrade range. So you should keep that in mind to provide room for any such pre-lens filtering. Also, noise might be an issue from the EMF.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA-9mA

Post by Andrew Seltzman »

The thermal camera is viewing the grid during operation, my grid is liquid cooled so it will remain at fairly low temperatures as long as there is coolant flow. There is a little EMI noise on the video, though I have not yet cleaned up the wiring and the video is being sent over twisted pair wire.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA-9mA

Post by Andrew Seltzman »

I have recently added an x-ray camera to one of the fusor's viewports to image the locations electrons are striking the shell through bremsstrahlung intensity distributions.

The x-ray sensor is a Hamamatsu S10814 CsI crystal needle array coated on top of a fiber optic plate collimator/shield coupled to a 1700x1200 CCD detector, controlled by a C9266 USB readout box. This is the industrial equivalent of the modern digital dental x-ray sensors. The sensor has 20x20um pixels, 42x30mm size, 20Lp/mm resolution, and probably on the order of 60 LSB/mR sensitivity.
http://dtsheet.com/doc/569698/hamamatsu-c9266-04
https://www.hamamatsu.com/resources/pdf ... d1124e.pdf
IMG_20190201_221822889a.jpg
The sensor gets mounted on a thorlabs optical assembly holding a 1.5mm molybdenum pinhole looking through a Pyrex glass viewport which has negligible attenuation for the more energetic x-rays.
IMG_20190201_195306450a.jpg
IMG_20190201_194536571a.jpg
IMG_20190201_194546131a.jpg
The first x-ray images from the camera are shown below
xray_img.png
Remember that my fusor has magnetic deflector yokes on 4 of the nipples, shown circled in red.
SAM_4425a.jpg
http://www.rtftechnologies.org/physics/ ... shield.htm

These magnetic deflectors were installed to prevent the electron beam emitted through the grid openings from striking and damaging the optical viewport. Instead of striking the viewport, the beam is deflected into the wall of the nipple. In this case, x-rays from electrons hitting both the near and far nipple walls are visible, along with the x-ray attenuation shadow due to the grid insulator and molybdenum field control cap. The far electron beam is striking the top of the nipple.
x-ray-fig.png
Rotating the far deflector field 90 degrees moves the electron beam strike point from the top to the side of the nipple
xray_img4.png
Rotating the far deflector field 180 degrees moves the electron beam strike point from the top to the bottom of the nipple
xray_img3.png
Removing the far deflector from the nipple causes the electron beam to illuminate an area of the far nipple behind the grid
xray_img5.png
In this case the grid wires and insulator are more clearly visible
x-ray-fig2.png
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA-9mA

Post by Dennis P Brown »

Nice apparatus! A side possibility; you can get single crystal silicon wafers super cheap (even select from the company what plane it was cut - 001, 011 etc.). Set it at the correct angle relative to your camera (maybe a micrometer rotation mount) and use it to get specific energies from the electron x-rays. The wafer need not be in the vacuum just line of sight to the x-ray source. Closer, of course, the better.

A side thought: one could "focus' the x-rays by placing the wafer on a circular vacuum ring and apply a moderate vacuum to deform the wafer into a spherical surface. This will weakly focus grazing x-rays off its front surface to "image" the source and provide an energy spectrum.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA-9mA

Post by Andrew Seltzman »

I'm planning to do something similar for energy. I have some CZT detectors which I'll mount on an x-y drive to position over the part of the image that I want to probe the energy at.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA-9mA

Post by Andrew Seltzman »

Some upgrades to the x-ray camera now include:
Diffrent molybdenum pinhole sizes 1.5mm, 1.0mm, 0.5mm to sharpen the image
IMG_20190202_184858028a.jpg
A 3D printed sensor holder to allow zooming in by varying the distance from the sensor to the pinhole
IMG_20190202_185341488a.jpg
IMG_20190202_185019702a.jpg
Picture taken with a 1.5mm pinhole for 30s:
1.5mm 30s.png
Picture taken with a 1.0mm pinhole for 30s:
1.0mm 30s.png
Picture taken with a 0.5mm pinhole for 60s:
0.5mm 60s.png
Picture taken with a 0.5mm pinhole for 60s (brightened):
0.5mm 60s bright.png
Reducing the pinhole size noticeably sharpens the image at the expense of intensity.
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