Plasma!

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Anze A Ursic
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Plasma!

Post by Anze A Ursic »

I think this was around 4.75kV, the old HV feedthrough was failing around there and we kept blowing up the power supply. This was last year, around October. It's a cylindrical fusor and I hope it'll work good for the fusion run we're planning soon!
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Rich Feldman
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Re: Plasma!

Post by Rich Feldman »

Nice light. Congratulations.

If the glow covers the surface of cathode wires, as in a Nixie tube,
then the pressure is too high for fusion by orders of magnitude.
I'd guess the voltage is much lower than 4.75 kV, unless it's measured on the supply side of a ballast resistor.
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Dennis P Brown
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Re: Plasma!

Post by Dennis P Brown »

As Rich has pointed out, you are in the torr range and can't do fusion at all - you'll need to reach high vacuum levels to begin before adding deuterium. Do you have a method to measure vacuum?

And why can't your pump get lower then this vacuum level you are showing? Your fore pump should be able to get below 20-30 microns at least for most decent two stage oil based pumps. Is it a bad pump or do you have massive leaks? These are issues you really have to address before you can even think about trying to do fusion. Maybe show a picture of your entire system so we can get an idea on why your vacuum is so poor - have you measured the vacuum at the pump head (no load)?
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Anze A Ursic
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Re: Plasma!

Post by Anze A Ursic »

The second picture was in the 15-20mTorr range if I'm correct. We have a Balzers transcuder and gauge that we use. Although now I'm curious how accurate it is. Although we probably have something better I could check out. I should say, the chamber is quite large...

I know it's too high for fusion. But with the turbo, I can go down to 0.1-0.5mTorr range which is hopefully a low enough starting point for fusion.
chamber.jpg
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Anze A Ursic
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Re: Plasma!

Post by Anze A Ursic »

Also, please don't mind the mess in that picture haha! I've been tweaking this system for the past week, once we do any testing it's very tidy and clean.
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Dennis P Brown
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Re: Plasma!

Post by Dennis P Brown »

Nice set up and certainly my system gets messy all the time. You have some big ports on that chamber.

As for reaching 0.1 - 0.5 micron being enough for a fusor, well yes and maybe - that is, if leaking is occurring that is preventing the system from doing better then that with the turbo running - i.e. outside air is being drawn into the fusor - then the air will dilute the deuterium gas and reduce your ability to do fusion. I'd think your turbo would do far better - like 10^-5 torr (or 0.01/0.05 or so mtorr.) Otherwise, either heavy out gassing, or a possible leak. But accurate vacuum measurements are essential to better clarify the issue.

A well sealed chamber (those are a lot of bolts to tighten properly!) like that and if fairly clean, your fore pump should get it well below 20 microns. That doesn't appear to be the case.

As Rich pointed out, the first picture looks more like the torr range rather than milli-torr* (microns.) The second still does not look to me to be in the 20 micron range. Just saying.

We tend to use microns there - mtorr is a micron but it is just common usage.
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Anze A Ursic
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Re: Plasma!

Post by Anze A Ursic »

The lowest pressure I've ever gotten with this thing is 19 micron. The greatest outgassing factor here is the top flange. It is a non-standard size and so we cannot find the proper copper ring and it is also the part of the chamber that is always being opened / closed when my fusor is placed in there so we just use that home made rubber seal as you can see in the picture.

With that thing we've never gotten past 19-20 micron. But it is what it is, with the turbo, we can go way lower.
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Dennis P Brown
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Re: Plasma!

Post by Dennis P Brown »

Have you tried leak detection? Just getting to 19 microns may or may not be out gassing (through, that should change with a few over nights/days sealed but pumped. How long does it hold vacuum - like in an hour does it go from 20 microns to 100 microns (arb. values here.) You should tell us the rise rate/time after you close off the fore pump and what the vacuum gauge reads as time passes. The best way to determine a leak is to do a simple test. While pumping, use a spray bottle of near pure alcohol and spray each seal area when at your lowest vacuum, one at a time. When you do this, if the vacuum gauge rises at all, then you have a bad seal.
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Anze A Ursic
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Re: Plasma!

Post by Anze A Ursic »

I'm back in lab right now. Yesterday I ran just the mechanical pump for roughly 2 hours and I got to 78 microns (it was fluctuating a lot, it would probably go lower had I left it for a longer period of time). I just came back and after almost exactly 14 hours the pressure in the chamber was 1.1 Torr. It's not fantastic but also not too bad I don't think. Again, that top flange is probably the biggest issue.

I've never heard about the spraying alcohol test, I will have to try it, thanks!
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Dennis P Brown
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Re: Plasma!

Post by Dennis P Brown »

Only 71 microns in two hours is not good with a good two stage vacuum pump (is that pump ok? Maybe test that at some point, too.); even out gassing shouldn't cause that slow a pump down. Climbing to a torr over night isn't good but certainly isn't terrible. Definitely, test the main seal and then move to other seals. Alcohol works very well at these higher pressures.
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Anze A Ursic
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Re: Plasma!

Post by Anze A Ursic »

Well hahah that's the other issue... the pump sounds like a dying pig, it is definitely on its last legs. This project isn't sponsored or anything so we're making due with what we have. We may actually have another 220V pump that would be way better but we'd have to test it. But I was never worried about the mechanical pump since I just end up using it as a backing pump for the turbo. I figured as long as the turbo is good, it doesn't matter that the mechanical pump is kind of shotty. I am probably wrong though.
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Dennis P Brown
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Re: Plasma!

Post by Dennis P Brown »

Well, likely that turbo is ok with that back pressure; however, you barely get into 10^-4 torr (0.1 - 0.5 microns) and that is a concern. Do test for leaks but maybe your turbo is having trouble with that back pressure. This is where leak testing is useful.
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Anze A Ursic
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Re: Plasma!

Post by Anze A Ursic »

Well actually the 0.1-0.5 micron range was when I had a massive leak (too many extensions / adapters from chamber to mechanical pump and from turbo to mechanical). At that point I was able to get only 110 micron with the mechanical. Then I turned on the turbo and went to that 0.1-0.5 micron range. I have since removed all those adapters and just used a single flexible tube from the chamber to the mechanical pump and now is when I get 70 microns. I assume now that I'm getting lower pressures with just the mechanical pump, the turbo will take me quite a bit lower too.

However, I found some old data from July 2021 where I took down this chamber to 29 micron with just the mechanical pump, closed off the chamber and waited 30 minutes. It was exactly at 58 microns after 30 minutes, so a change of 29 microns in 30 minutes.

Today I ran down to 77 microns and capped the chamber, waited 30 minutes and got to ~108 microns, so the same exact pressure change over the same time!

This data was useful because July 2021 was before I had ever taken any flange or other thing off the system. Since then I've installed a gas feedthrough, replaced the old 5kV feedthrough with a blank, installed a massive 4 input 50kV feedthrough etc... Over all of these "newer" changes, I also did the alcohol pressure change test you recommended and the pressure did not change when it was sprayed.

I would also like to add that the system, when I open up the valve to the mechanical pump, goes down to 45 microns for a few minutes and then starts slowly rising. It goes to 110 microns and then started slowly decreasing in pressure. I fear the chamber is dirty and it outgassing all the junk inside because when I came back after 5 hours today it was jumping between that 60 and 70 micron range quite rapidly. I suppose had I left it on overnight I could have gone down to about 40 microns but I don't like leaving that pump along for such extended periods of time.

Anze
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Dennis P Brown
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Re: Plasma!

Post by Dennis P Brown »

Looks like your system has no significant leaks - out gassing is easy to deal with. One just gets the system to where one wants it (configuration, wise), seals it up, leaves it under vacuum (repump it down every day) and it cleans up nice in no time. Not an issue if you open rarely, keep the open times short, and don't open on really humid (rainy) days. Sounds like your good to go.
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Anze A Ursic
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Re: Plasma!

Post by Anze A Ursic »

Great!

I've also tested the gas inlet system today - no leaks! Bubble neutron detectors are also ordered on expedited shipping... Next two weeks should interesting! Thanks for the help, Dennis and sorry for straying away from the "Images du Jour" topic but this was all excellent info!

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Rich Feldman
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Re: Plasma!

Post by Rich Feldman »

Are you paying attention to the ballast valve on the rotary vacuum pump?
https://www.vacuumscienceworld.com/blog ... as-ballast
https://www.vovacengineers.com/uncatego ... uum-pumps/

You could also try changing the pump oil and see what happens.
When pump starts cold, it takes a while for oil to get up to its steady state temperature.
All models are wrong; some models are useful. -- George Box
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Anze A Ursic
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Re: Plasma!

Post by Anze A Ursic »

Thanks for the heads up, but I just double checked and we have no gas ballast on that pump. We have these on some of the other pumps but not this one. This is just an old Edwards mechanical pump.
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Re: Plasma!

Post by Dennis P Brown »

No problem - but do be safe - don't know the voltage you intend to use but getting above 35 kV, and that chamber you have might start to get transparent to x-rays. Do keep an eye on that danger (depending on your voltage.)
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Anze A Ursic
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Re: Plasma!

Post by Anze A Ursic »

It's all remote controlled from another room, so we are certainly doing our due diligence! We also just got 280 kilograms of lead shielding delivered from the safety folks, so we will be separated by around 5m of air, 1/4" of steel from the chamber, several inches of lead and 6" of reinforced concrete!
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Dennis P Brown
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Re: Plasma!

Post by Dennis P Brown »

Keep forgetting the overkill you have to deal with - just the distance you have is enough even if you were using 70 kV. So, good luck on the testing.
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Anze A Ursic
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Re: Plasma!

Post by Anze A Ursic »

Haha, yup! But, I don't mind the overkill as long as they just allow us to do it really. We're scheduled for next week on friday so I really hope it all works then... I wish I could test everything before hand but they won't allow me to go past 10kV without radiation officers present lol.
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Re: Plasma!

Post by Anze A Ursic »

Very quick update, after running my turbo pump today for roughly 2 hours, I achieved a pressure of 0.17 micron (1.7E-4). Still not ideal, but better than before. I will try to get to the E-5 range, but that top rubber gasket must be placed with extreme precision and tightened in a precise manner too otherwise the pressure is too high. Would a base pressure of 5-9 E-5 Torr be good for a semi-decent fusor run? I know people say to aim for 1E-5 or lower, but again, I doubt I'll come close to that with a home-made rubber gasket.

Anze
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Richard Hull
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Re: Plasma!

Post by Richard Hull »

1.7e10-4 torr is just fine to start to admit deuterium and do fusion I did this at this base pressure for 16 years with fusor IV. I never hit 10e-5 torr as a base pressure. I can do it now with my Turbo Which I have only had 4 years now. The old leybold diff pump did just fine for those 16 years with a rather leaky fusor and still dragged the fusor down into the 10e-4 torr range.

You are good to go, I would think.

This assumes you develop a good operational technique which should come quickly and naturally once you do detectable fusion.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.
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Anze A Ursic
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Re: Plasma!

Post by Anze A Ursic »

Geez, talk about ougassing... I reached 1.7E-4 Torr the other day after 2hrs and 10 minutes, and today I got to the same pressure in just over a third of the time (46min). i plotted my measurements in an Excel sheet (see below). My fusor isn't in the chamber yet, we're still doing some pressure tests, but I'll make sure to clean the hell out of the chamber once I re-open it. Currently running the test but based on my calculations, I should reach below 0.1 micron in roughly 1.5 hours, so hopefully with more outgassing procedures like this we'll be in the proper range for fusion!


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Liam David
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Re: Plasma!

Post by Liam David »

What type of rubber is your large seal? I would highly suggest you use viton, or at least buna-n. Silicone and especially natural rubbers can outgas terribly.
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Anze A Ursic
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Re: Plasma!

Post by Anze A Ursic »

It is Viton. I just hit 7.9E-5 Torr, so 0.079 microns. I think we're good for fusion, I'll be installing my device in probably tonight and then re-do this outgassing procedure. I'm certain with this we can get betwen 0.05-0.07 microns on day of fusion. Exciting stuff!
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Dennis P Brown
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Re: Plasma!

Post by Dennis P Brown »

When my fusor hits 10^-5 torr, not only is it clean (and all major out gassing is finished) but that is when I back fill with deuterium for a run. You certainly are there. The only addition or caveat is the time it takes - generally, my system hits 10^-5 torr in under 10 minutes. A bit longer if opened to air.
Last edited by Dennis P Brown on Thu May 05, 2022 6:51 pm, edited 1 time in total.
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Anze A Ursic
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Re: Plasma!

Post by Anze A Ursic »

Do you think there is any point to keep pumping down for another week to minimize outgassing and get the chamber as clean as possible? Someone in my lab mentioned that I should do that but I'm currently at 6.9E-5 Torr and it's stable (slowly falling), but there's not quick jumps in pressure that would indicated disastrous contamination or something.
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Dennis P Brown
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Re: Plasma!

Post by Dennis P Brown »

If the system is getting to that pressure in under fifteen or twenty minutes, it is fairly clean and likely fine for a fusion run. I'll add, my system has a rise of less than a micron a minute. Again, certainly out gassing but does not affect my system's ability to do acceptable fusion.
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Anze A Ursic
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Re: Plasma!

Post by Anze A Ursic »

Not quite, I get to about 0.2 micron (2.0E-4) in that time. Will continue to pump down then.

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Re: Plasma!

Post by Dennis P Brown »

Sorry, been busy so not keeping up of late (had been on travel some time and just now catching up on all the things that fell by the way side - lol.) Your results are fairly good and that isn't too bad a vacuum that has been open to the air. I saw that your system is large so out gassing and getting it very clean takes time - frankly, that will always be an issue for your system that you will need to live with. The key is not opening it to air for long periods of time. However, burning a deuterium plasma in it for a short time at a significant fraction of a torr would likely do wonders after opening and might be worth trying if your deuterium supply isn't too limited - this is ideally done below 15 kV so x-ray isn't an issue.
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Anze A Ursic
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Re: Plasma!

Post by Anze A Ursic »

Dennis, thanks for the idea, however, because of the nature of our DF3 supply, nothing below 15-16kV is possible. the power supply just shuts off lol.

But, we're doing a preliminary test on wednesday and burning some deuterium at 16kV more so to check everything including the HV setup, the ballast, the pressure control, my LabView remote control gui, etc,...

I did hit 3.3E-5 Torr yesterday and the vacuum expert was right, the chamber and specifically the turbo were dirty and we had a lot of outgassing. I plotted the pressure vs time again and you can see drastic improvements day by day, to a point where we're getting below E-4 torr in fractions of the time. Our fusion run date is scheduled for 5/16/2022 so if we continue pumping and purging the system, I wouldn't be surprised if we hit E-6 Torr.

Anze
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Anze A Ursic
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Re: Plasma!

Post by Anze A Ursic »

Since the viton gasket was mentioned a few times, is that an X-ray hazard? Meaning, is it a dangerous source of X-rays since it attenuates X-rays worse than steel?

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Dennis P Brown
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Re: Plasma!

Post by Dennis P Brown »

Plastics are not an x-ray hazard - they absorb but don't emit x-rays. Steel attenuates x-rays as well as produces them (when bombarded by energetic enough electrons, of course.) It has to do with the inner electrons being knocked off the Iron (Fe) atoms by high energy electrons. Iron has many bound electrons so the binding energy of its inner electrons is huge. So when an inner electron is knocked off, an outer electron drops into that inner, "empty" orbit. This is done by the outer electron emitting a photon with a large energy - i.e. an x-ray. However, the Fe in this process also reduces the energy; so by a series of such absorptions this can significantly reduce the x-rays via that process (also, simple scatter/recoil during the process.)

Carbon based compounds have far fewer electrons so the binding energy of the inner electrons isn't very large. As such, they cannot produce x-rays under any circumstances by having an inner electron knocked off.

At high photon energies the specific process defines the difference between whether such a photon is an x-ray photon or a gamma ray photon - even for photons of identical energy. The process defines which name is used - an inner electron photon process is always an x-ray and any nuclear process that produces a high energy photon is a gamma ray photon.
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