Finn Hammer, fusor update.

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Richard Hull
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Re: Finn Hammer, fusor update.

Post by Richard Hull »

Great system Finn and a fine sequence of repetitive pump downs, well graphed for all of see. Seems like the system is well sealed and relatively dry for a first pump down....(thanks to the oven work). In use with a bit of ion bombardment with air it should dip a bit lower.

Sorry to hear about the snafu with customs on your baratron. Hopefully, this too shall pass.

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Re: Finn Hammer, fusor update.

Post by Finn Hammer »

I have been reading the FaQ over at Leskers:
https://www.lesker.com/newweb/faqs/question.cfm?id=491
relating to the leaks and adsorbtions that can spoil the function of a vacuum system.
Since I have no prior experience with a vacuum system, but am trying to do things the right way, I decided to replicate the Rate of Rise test which is described on the page I linked to.
In short, it gives the student an ability to gain insight in the health of the vacuum environment, particularly related to leaks and contaminations, by looking at the shape of the leak up curve, and by comparing the Rate Of Rise number obtained by solving this equation:
Using the formula:

Q = ((P2 – P1)*V)/T where the result is in Torr.Liters/Second Where:

Q = Throughput, or Gas Load, or Rate of Rise
P1 = Pressure at the beginning of the test
P2 = Pressure at the end of the test
V = The volume of the chamber, in Liters
T = The time between readings of P1 and P2 , in Seconds.

The numbers I have as a basis are in this time/pressure sequence which I recorded today, from a low 3.6E-7 torr to, 100sec. later, 3.7E-5 torr
The volume of the chamber is 0.24 liters consisting of 480mm of 25mmØ pipe and 100mm of 16mmØ pipe.

Here is the time/pressure sequence:

00:02 3.60E-07
00:04 3.80E-07
00:06 1.20E-06
00:08 2.30E-06
00:10 3.40E-06
00:12 4.30E-06
00:14 5.10E-06
00:16 5.90E-06
00:18 6.70E-06
00:20 7.50E-06
00:22 8.20E-06
00:24 8.90E-06
00:26 9.70E-06
00:28 1.00E-05
00:30 1.10E-05
00:32 1.20E-05
00:34 1.30E-05
00:36 1.30E-05
00:38 1.40E-05
00:40 1.50E-05
00:42 1.50E-05
00:44 1.60E-05
00:46 1.70E-05
00:48 1.70E-05
00:50 1.80E-05
00:52 1.90E-05
00:54 1.90E-05
00:56 2.00E-05
00:58 2.10E-05
01:00 2.10E-05
01:02 2.20E-05
01:04 2.30E-05
01:06 2.40E-05
01:08 2.40E-05
01:10 2.50E-05
01:12 2.60E-05
01:14 2.60E-05
01:16 2.70E-05
01:18 2.80E-05
01:20 2.90E-05
01:22 2.90E-05
01:24 3.00E-05
01:26 3.10E-05
01:28 3.20E-05
01:30 3.30E-05
01:32 3.40E-05
01:34 3.40E-05
01:36 3.50E-05
01:38 3.60E-05
01:40 3.70E-05
01:42 3.80E-05


Here is the leak up curve:
ROR 22-04-2021.JPG
This curve looks a lot like a pure gas desorption curve, at least this is what I would like to think.

When I plug my numbers into the equation:

((3.80E-05 Torr - 3.60E-07 Torr)*0.24 Liters)/100 Seconds = 9.07E-08 Torr.Liters/Second

Now, I am not complaining, 9.07E-08 is a fantastic ROR number, but on the other hand, I find it improbable that I should have a system which is several orders of magnitude better than the base numbers quoted on Leskers site, where they talk about 1E-5 as the turning point for a good system.

I assume that going through these calculations is something a 1st year student does once or twice, and then forgets about it and relies on them as proof that he is doing things right.

I would like to arrive at that same conclusion particularly if some kind soul would give my calculation a sanity check.

Cheers, Finn Hammer
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Re: Finn Hammer, fusor update.

Post by Joe Gayo »

Is your system baked? (Extensive time at 100degC+)
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Re: Finn Hammer, fusor update.

Post by John Futter »

Finn
I see you are using a bellows type connection from your turbo pump to your chamber

Lose it!!!!! and use smooth bore pipe fittings your conductance will be awful due to the surface roughness of the bellows. Your pump speed is only probably a few liters per sec because of this

the bellows pipe is ok for backing /roughing connections but at molecular flow they are a hindrance
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Re: Finn Hammer, fusor update.

Post by Mark Rowley »

John, is that a condition specific to TM pumps? I have a bellows from my diff to the fusor and has no issues dropping the chamber to the sub micron zone. In fact, the diff is throttled back almost 90% because it’s too thirsty.

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Re: Finn Hammer, fusor update.

Post by Finn Hammer »

Joe,

Not baked while assembled. Before assembly I washed components in dishwash, then rinsed in water, skipped the alchohol rinse, baked in oven @150degC. And assembled, using gloves. I did my best to avoid fingerprints. Used a very thin smear of Apiezon M on a couple of the viton rings.

John,
My intension with the bellows was to avoid applying force to the pump, but I will look into getting a smooth bore elbow.
Pumpdown does get awfully slow in the low end, but before I shut it down last night (cannot run overnight due to whine from turbo) it reached 2.3E-7, after an 8 hour pump.
I don't have dry nitrogen at my disposal, for purging, but have tried Argon from the tig welder. This opens the question, which atmosphere am I measuring with the hot cathode gauge.... Can of worms in a way.

Cheers, Finn Hammer
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Re: Finn Hammer, fusor update.

Post by Joe Gayo »

Bellows have lower conductance regardless of the pump, especially in the molecular flow region. How much lower depends on several factors (length, minor diameter, etc)

Mark, Your system may be in the transitional flow region and the impact may not be severe.
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Re: Finn Hammer, fusor update.

Post by Joe Gayo »

@Finn

I find your numbers surprising. I would be sceptical of the high vacuum gauge accuracy.
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Re: Finn Hammer, fusor update.

Post by Finn Hammer »

Joe,
I am sceptic to the core, but also reluctant to shell out the big $$$ for a brand new transducer.
However, there is a 100 millitorr calibrated Baratron on the way from overseas. Hopefully it passes under the radar this time.
Not that it measures way low, but at least down to 5.0E-5 that should give at least a pointer in the right direction.

@ bellows and molecular flow: I see the point, a particle heading towards the pump has every opportunity to hit a wall at an angle where it bounces back instead, or is scattered in all directions except towards the pump.
I have ordered proper smooth elbows for that important transition.

Cheers, Finn Hammer
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Re: Finn Hammer, fusor update.

Post by Maciek Szymanski »

Finn!

I've made some calculations, and your numbers are very good, but not unrealistic for such a small outgassing area.

Assuming wall outgas rate Or = 1e-8 mbar*l/(cm2*s) (according to Leybold's "Fundamentals of Vacuum" in the midle of the rate for metals)

The wall area of S = 52 cm2 (KF25 10x10 cm cross and 10 cm tube plus 10% margin)

The PV virtual leak rate rate will be Qo = 5.2e-7 mbar*l/s

The pressure rise can be derived from:

p2*V = p1*V + Qo*t

as:

p2 = (p1*V + Qo*t)/V

Using p1 = 3,6e-7 Tr, t = 100 s and V = 0,24 l

The final pressure is p2 = 2,17e-4 mbar = 1,67e-4 Tr. Your numbers are better, but by less than an order of magnitude, not a huge difference in the high vacuum world.
“Begin at the beginning," the King said, very gravely, "and go on till you come to the end: then stop.” ― Lewis Carroll, Alice in Wonderland
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Re: Finn Hammer, fusor update.

Post by Maciek Szymanski »

And one note regarding the corrugated tube. Generally the conductance between the high vacuum pump and the chamber is the key factor for the pumping speed. In fact, if the conductance is smaller than the pump throughput the pimping speed will be limited to the conductance value independent of the pump size (a "hole to ideal vacuum" case). The pump throat diameters are usually matched to the pump throughput. For other hand the fusor chamber pumped by passage matched to the pump throat will be evacuated very efficiently but the deuterium will be consumed and pumped out with the same efficiency...
“Begin at the beginning," the King said, very gravely, "and go on till you come to the end: then stop.” ― Lewis Carroll, Alice in Wonderland
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Re: Finn Hammer, fusor update.

Post by Joe Gayo »

For turbos, there can be a large difference in pumping speed between heavy (N2, O2, CO2, etc) and light gases (He, H). This can be used to great advantage to conserve D2 but reduce background pressure.
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Re: Finn Hammer, fusor update.

Post by Richard Hull »

For the most part, ideal conductance on smallish chambers can rip a chamber down to the basement real fast, in a well sealed system. Likewise, as noted, you then wind up strangling off the wonderful vacuum conductance so often expensive to achieve once D2 is introduced to literally destroy the beautiful vacuum you have created with ideal matched and maximized conductance. The result, in the end, is at least a fabulously pure vacuum into which D2 is introduced. The silly result is that you must ruin the chamber vacuum by filling the reactor with flowing D2 at a pressure equal to what might be expected of a well used mechanical pump!

I have always been amused by this sad fact in fusor operation. The concept of a sealed vessel fusor, properly pressurized, will work for but a moment in time. I learned this in 1999. Such a sealed device will fuse well, but will bury the D2 in the walls in a few minutes decreasing pressure steadily in a well sealed spherical device. This phenomenon is at the very heart of the early X-ray tubes with no filament, operating at high pressures. Ultimately, these early tubes went "hard" and the power supply could not produce the amount of x-rays needed. (Current through the tube at any voltage determines the x-ray flux) Technicians were kept busy in big hospitals delicately regassing those early tubes with side arms meant to accomplish this task. The modern high vacuum "Coolidge tube" put them all out of work, for the most part.

If one is not hung up on the best conductance in molecular flow regime and has the time to pump to an acceptable level using a less than ideal conductance, the purity of D2 during operation will be acceptable and pinching off the D2 flow will have a much broader adjustment range. Also, the D2 purity will increase steadily as you load the walls to boost fusion.

This is not a suggestion for deliberately limiting conductance, but more of a reasoned methodology specific to what we know as "the fusor operational experience". In the end, do as you wish as it all works in the hands of any experienced operator of their specific fusor. Learn as you gain experience in operation.

The Farnsworth system of the mid-sixties in their best mark II prime with hot filament pigatron ion guns often had reports of 1-2 microns of D2 with 140kv applied at a few ma. This produced great numbers mostly due to the focused ion guns. All of the original team members that I interviewed noted that it took a couple of hours fiddling to balance and set up the "guns" to achieve those results. In some cases, during these tedious adjustment sessions, as the voltage was raised, burn-throughs, melted screens, arc-overs, and other issues ended the session that never came to be as shut down, disassembly, and repairs were made. (1-2 day downtimes)

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: Finn Hammer, fusor update.

Post by Finn Hammer »

Maciek, Joe, Richard,

Thank you for taking the time to do calculation for me, and writing it up in a way that I can understand.

With regard to the corrugated tube, it probably does not matter a whole lot in fusor operation, although it is always nice to hit low pressures quick. And the purity of H2 is determined by the low pressure to start with.
The Turbo can run in standby mode, around 60% or 54KRpm, but even this is probably too fast for comfort, I wish I had a controller with full speed controll, but that will be for another time. I hope to get away with an almost fully closed gate valve, turbo on standby (because without the dual vane pump, I cannot work against the 2 torr of foreline pressure) feed 1ccm of H2 and see.

But frankly, from now I had better stop fantasising, and wait untill I have got hands on experience. You guys all know howto, I have yet to try it.

I have had a good day in the workshop, where great sacrifices were made to the god of swarf:

IMG_20210424_192441.jpg

I finally got the bulkhead clamp for the viewport manufactured, in copper and phenolic, to stay in line with the steampunk theme. The copper ring holds the 6mm lead glass which I schrounged from a Lesker wiewport. The paper phenolic clamps a KFviewport glass against the o-ring.

IMG_20210424_210215.jpg


I know that during the first learning experience, I will be looking at the plasma directly, but by restricting myself to the 30kV supply, and by having lead glass between plasma and myself, I can be safe.


Cheers, Finn Hammer
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Re: Finn Hammer, fusor update.

Post by Finn Hammer »

In the spirit of documenting the build as it progresses stepwise, here are a few additions to the build:

I got the grid manufactured, 1.6mm tig rod and copper.

IMG_20210506_163250.jpg

Nested nicely inside the chamber,

IMG_20210506_164936.jpg

Another thing that was mentioned was to get rid of the ugly corrugated tube from turbo to main control valve: It did help on the pump down time.
.

IMG_20210521_084406.jpg

Then I got the flow controller mounted, and the piping routed:


IMG_20210522_111710.jpg
This was interesting, because I could now start to get an impression of the range of pressure control I will have, and it fits the bill nicely.
with the turbo on full speed and the main controll valve fully open i get from 3 - 15 micron with 1ccm to 5ccm
with the control valve almost shut, I get from 11 to 42 microns with the same flow figures.
I am feeling a lot more confident about the ability to controll pressure with this addition under the belt.


This is the rig at present, notice that the electricals are temporary, shielded controll cable is waiting to be fitted.

IMG_20210522_111800.jpg

The trained eye will notice that I finally received the calibrated Baratron, and it is a marvel to behold, even though it has the penaltry of a 4 hour warmup time.

I think that I have talked about field controll before, this is something I feel strongly about, since I have decided to run this system dry, without any oil to insulate, or cool for that matter.

The feedthrough looks like this:

IMG_20210523_124718.jpg

Look at all the ugly protrusions just begging to present themselves to the first rogue partial discharge, and create a grounded arc. Uff, ugly!

IMG_20210523_124814.jpg

That was more like it, but only half of the story, 1/3, really :-)
field.jpg

Here is the finished article. Perhaps overkill, I don't know, because the feedthrough may well puncture through the porcelain (I think it is allumina, really) but it is a cool addition and if all fails, I can still use it in another episode.

That's all for now, it is the power supply that is being put into a nice box, that awaits attention now, and a couple of Ludlums in the mail too.

Cheers, Finn Hammer
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Re: Finn Hammer, fusor update.

Post by Mark Rowley »

It’s a work of art Finn!
Did you custom spin the lower flange toroid?

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Re: Finn Hammer, fusor update.

Post by Finn Hammer »

Mark,

Thanks for your kind words.

The 3 toroids are turned out from solid bar, spinning is something I am looking into, but for this one-off it was simpler to just turn them out of bar stock.

Cheers, Finn Hammer
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Re: Finn Hammer, fusor update.

Post by Richard Hull »

Wow! You have really documented far better than any others have do on their first pass. The system is plus ultra looking and you have exchanged a lot of money for "the right stuff". I like the field control, but Bar stock!!! You are a better man than I Gunga Din.

Fortunately, we have one of the original Tesla coilers from the first years of the 1989 Teslathon attending regularly to date. He perfected the art of spinning and always shows up at every Teslathon and HEAS October event with toroids for sale at the flea market. Last Year, due to covid, he did not attend, thus, breaking his perfect run over the 32 years of attendance. All of my lab's toroids were spun by John Freau. I think the largest he ever spun was a 10" and the smaller ones like 6 to 4 inch are the ones he normally brings now. He did note that he isn't doing it so much now and only spins a couple for sale at the HEAS flea market. He once sold them by the dozens. We are all getting old now and the tedium of such work for limited rewards is weighed much more seriously as we all have a limited time remaining.

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Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: Finn Hammer, fusor update.

Post by Finn Hammer »

Sorry, double post, see below
Last edited by Finn Hammer on Mon May 24, 2021 3:38 am, edited 1 time in total.
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Re: Finn Hammer, fusor update.

Post by Finn Hammer »

Richard,
I have 2 of John Freau's 6 x 1 1/2 inch toroids, and a single 13 inch. Great stuff. John was a regular and very valued contributor to the Pupman list.
I also have tooling for 600mm x 170mm toroids and a good relationship with the spinners shop, so can deliver....
I even had tooling made for reentrant Van de Graaff 400mm top electrodes made. Still some in stock of those too, but the tooling got lost, unfortunately.

It takes 2 pieces of tooling to spin a toroid shell, one for the concave portion, and one for the rest, so if I was going to spin these toroids, I would be looking into even more turning from bar. The tooling can be made from tufnol, or other phenolic types of thermosetting material, but the stench from machining these materials keeps me from doing that, for the most part.
I had planned to make the grid from EDM graphite, but one hole drilled, and a single pass on the bar, on the lathe, convinced me that turning graphite is for another day, when I get a proper point dust extractor setup. Just don't want to foul up the shop with carbon dust.

Cheers, Finn Hammer
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Re: Finn Hammer, fusor update.

Post by Richard Hull »

Great work Finn. I am glad you already have some of John's fine work. I wish you could meet him in person. He is a calm, very retiring and reticent person who is incredibly easy to like. Perhaps if you ever make what few HEAS October events are left in me, you can meet John.

Your effort is one that mimes my own in its temporal journey, which I find the best way to get into fusion. (Slow and easy - Read, do, absorb, correct the do, read some more, do some more, repeat) Of course, you have the best of us to soak up a lot of hard won knowledge. This can shorten the journey, but still, you move at the right pace, taking advantage every wrong thing made right over the years here. It is the right way to do it.

I wish others would enter this way.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: Finn Hammer, fusor update.

Post by Finn Hammer »

I had first light the other day.

At this point, I knew that the solutions I would come up with would probably have to be altered along the way, possibly in short order, so I allowed myself to delve into the fine art of jerry rigging, the ballast resistor being the first item to receive my left hand attention:

first ballast.jpg

You may be able to see the 3x16 matrix of VR68 resistors, they did not last long......


Next attempt was a 5X10 matrix, this time potted with heat conducting potting in a FR6 tube, it lasted for one hour of testing.
The long thin tube is the voltage divider 1:1000, it seems to be a good one.

ballast vd.jpg

One success to report is the voltage control on the feedthrough, it is good.

convenient attach.jpg

I had made a squirrel cage grid with 9 pins in it, and this was for the sole reason that I wanted a spanking cool 9star picture as my future avatar, well that turned out to be a bad choise, at least since the axis pointed right at the viewport. I had some bad expectations beforehand, and for that reason I had put sacrifical glass in front of the viewport, this should prove beneficial.

9star.JPG

I got my nine star, but in the wrong place, as a blue reflection on the viewport, but what was worse, the front hole in the grid shou out a death ray directly on the glass, which got damaged.

cracked.JPG

You can see how the glass (quartz) got chips blown out of the surface where the death ray hit.


Ok, back to the lathe and make a new grid:


DSC_4802.JPG

This one showed out to perform better:


cool grid.jpg


and soon It got all red hot.

hot grid.JPG

If I could only figure out how others manage to catch theit fusor internals without all the haze, perhaps it is a photoshop thing

Cheers, Finn hammer
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Re: Finn Hammer, fusor update.

Post by Jim Kovalchick »

Finn,
Great looking plasma pics!

Regarding haze, fusors tend to look hazy until the chamber conditions through heating. I'm betting yours will look clearer soon.

Jim K
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Re: Finn Hammer, fusor update.

Post by Mark Rowley »

Finn, Ive always had similar problems taking distance shots of the fusor and BoT. Seems that the autofocus latches onto the exterior hardware rather than the slightly more distant plasma within the chamber. My remedy was to place the camera right up against the viewport (using a makeshift insulator) allowing for focus on the grid or BoT target. Sometimes taking a video from the same distance will work better. Then it’s just a matter of taking a screenshot from the video.

Regarding the squirrel cage (SQ) I always wondered how folks kept the axis beam from blasting a divot out of the viewport. The latest videos of Dougs fusor display some great imagery of the SQ in action but there’s no evidence of any beam hitting the viewport. Quite possible there’s a sacrificial lens there but knowing the high power levels he ran I can’t imagine it lasting long. I also can’t imagine him cracking open his chamber all the time to replace it either. It’ll be interesting to find out exactly what he did.

The diameter of your new copper electrode appears to take up a bit of real estate within the chamber. I’m curious if you’ve noticed any problematic interaction with the chamber wall. One of the BoT targets I used which incorporated fixed magnet electron suppression was of similar size. Just big enough to cause a Paschens Law issue with the nearby chamber wall. I know BoT dynamics differ greatly from the fusor but I’ve had similar issues a couple years ago when experimenting with larger fusor grids.

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Re: Finn Hammer, fusor update.

Post by Finn Hammer »

Mark, Jim

Regarding the photos, I am stunned. It is not an autofocus thing, I set the focus manually, by shining a flashlight on the grid, so I can see the focus well, camera on a tripod, lens is the famous 200mm Nikon Micro, sharpest lens available. But still there is that blue ionizing thing. I saw it in the big triodes on my single ended tube amplifier too, but not to this extent. I am considering to afd a grounded mesh in front of the sacrifical glass in the viewport.
Regarding the grid diameter, no problems as I see it, no flashovers to ground or anything like that. The first couple of minutes, there was a sort of sprinkling on the surface, as if small specks of dust provoked breakout, but were extinguished in the proces, that is all.
But mind you, I have no idea whatsoever, which diameter is the right one, or more suitable, I use the rule of TLAR ( That Looks About Right) and with my limited experience, that does not say much.
But I did go to the junkyard to get short pieces of stainless tubing in 15 - 22 mm range, and I am quick on the lathe, so many grids will be tested, as soon as I get a suitable ballast resistor. Those power mox resistors look tempting, to end all experimenting on that part.

Cheers, Finn Hammer
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