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

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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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