Severe Outgassing Problems

Every fusor and fusion system seems to need a vacuum. This area is for detailed discussion of vacuum systems, materials, gauging, etc. related to fusor or fusion research.
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Erik smith
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Severe Outgassing Problems

Post by Erik smith » Sun Jan 07, 2018 2:46 am

When I first started using my vacuum chamber I noticed the "leak rate" would essentially vary when I isolated the chamber from my vacuum pumps using a right angle bellows valve. I figured it was outgassing when I noticed that my pressure increase would depend on how long I pumped my chamber for. With just a roughing pump the chamber can be pulled down to ~15 mTorr (8 mTorr after an intense bakeout). If I pump the chamber for 15 minutes, then close the valves the pressure will rise at ~2 mTorr/min. For 30 minutes of pumping the leak rate is around ~.7 mTorr/min. If I pump the chamber for an hour the leak rate is ~.4 mTorr/min. With this data, one can clearly see that the chamber is outgassing. To solve this problem I blasted the chamber with a heat gun for 2 1/2 hours trying to keep the entire chamber at 100C. During the bakeout the pressure rose from 11 mTorr to 23 mTorr. After the bakeout I let the chamber cool with the vacuum pumps running for an additional 2 hours then closed all valves and let the chamber lie dormant for 2 months hoping to let any other substances seep out.
main chamber.JPG
Main chamber
mastercool pressure gauge.JPG
Mastercool thermocouple gauge (pressure 8.8 mTorr)
When I came back after 2 months the chamber pressure had risen to 2 Torr. Seeing that the chamber still had outgassing problems I decided to let the chamber pumps for 3 days. On the first day of pumping I preformed 3 one hour bakeouts, blasting the cahmber with a heatgun. The second day I just let the chamber sit. the third day is when I decided to do plasma bakeouts. For this, I used my hippotronics 30C 30kV 10mA DC power supply. Using Richard Hull's FAQ on plasma baking, I used a needle valve to let air into the chamber, raising the pressure to 50 mTorr. I then isolated the pumps from the fusor chamber, then started to apply high voltage on the grid. The voltage applied was around -2kV at around 40mA of current. The grid started to glow red hot during this process and the pressure shot up to 3 Torr during the bakeout. This lasted for around 15 minutes. I did another plasma bakeout for 45 minutes using the same voltage and current but I had to stop due to my stainless steel grid melting.
plasma.JPG
Plasma (~2kV @40mA)
melted grid.JPG
Melted grid
After replacing my grid and cleaning the entire chamber with acetone and isopropyl alcohol, I preformed 3 more bakeouts. Much like the previous ones, I applied 2kV at 40ma to the grid at 50 mTorr and the pressure would shoot up to 3 Torr.

This is starting to becoming a large hassle since it appears that the plasma and external bakeouts are doing anything to clean the chamber of its volatile substances. I have tried replacing the chamber entirely with a new 2.75" conflat cross, but this does not seem to solve the problem as the pressure still increases dramatically when I strike a plasma inside the chamber. Is there anything I'm doing wrong with my current bakeout procedures or is there anything else I could do to rid my chamber of volatile substances?

Robert Dwyer
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Re: Severe Outgassing Problems

Post by Robert Dwyer » Sun Jan 07, 2018 4:18 am

Never use Isopropyl for vacuum cleaning! It contains lots of water that will embed itself into the chamber walls then outgass during pump down. Acetone is ok, but still has the an issue with leaving films and some some water that may outgass. At my work we primarily use methonal for cleaning vacuum components. The only time we use acetone is if we have a really dirty part that has greases or oils on it. Even then, we always make sure to do another wipe down with methonal to remove any water or film the acetone leaves behind. I recommend at least 99% purity methonal. I have used 95% purity methonal on my chamber and can pull down to 1mtorr with a roughing pump, but the less impurities the better.

I would try using methonal to clean your chamber, then doing another pump down, bakeout, and plasma cleaning.
If we throw more money at it, it will have to work... right?

Jerry Biehler
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Re: Severe Outgassing Problems

Post by Jerry Biehler » Sun Jan 07, 2018 6:20 am

You're chasing after ghosts for no reason. This thing will never be ran non-pumped so the amount of leakage is inconsequential, it wont matter in the end. Once you have to break down for one reason or another you are back to the start. This is why small stuff that needs to be kept under high vacuum often has a little ion pump attached to it.

ian_krase
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Re: Severe Outgassing Problems

Post by ian_krase » Sun Jan 07, 2018 11:35 am

Note that methanol is rather toxic, so some people avoid using it altogether.



When baking out, one problem is that evaporated vapor can then condense or adsorb onto a chamber wall that is at a lower temperature... where it can then outgas from.This is part of the reason for popularity of heating tape (or, for the moneybags set, custom heating mantles) -- it gives you uniform heating so everything gets outgassed and pumped away at the same time.

For this reason, I don't think that pumping down and then sealing off a chamber really helps for bulk outgassing.


I would not be surprised if you have both outgassing and a (very small, negligible for actual operation) leak.

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Richard Hull
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Re: Severe Outgassing Problems

Post by Richard Hull » Sun Jan 07, 2018 3:37 pm

I, long ago, gave up on my real leak and virtual ones related to it. I might see 4-5 month idle times for the fusor IV. Naturally, after a big group of runs for visitors, HEAS conferences, etc., I seal off the fusor chamber as an entity and the diff pump as an entity (two first rate bellows valves). Still, over that vast space of time the fusor goes to virual atmosphere via its leak and D2 out-gassing. The diff pump fares beter, usually being near 100 microns after months of shutdown.

An impromptu, force single day operation over a period of 4 hours will see the system pumped fully and well, but fusion is rotten at the end of 4 hours and maybe 125k n/s at 38kv, 5 mircons of D2 and 9ma. A shut down and, then, the second day, the system pumps and is fusing within 40 minutes.
However, about 250k n/s is the max. Shutdown again and on day three, the system is conditioned and enough D2 is soaked in to allow the mega mark to be reached if pushed. Days 4 and 5 are likewise a snap.

Those willing to search the operation forum, and old hands here, will remember my long, detailed crank up of fusor IV before the HEAS so that fusor IV will be super functional and hit the mega mark real easy. Like a classic steam engine that has her fires dropped for a week, it is a real job to bring her up to steam again.........A routine must be followed and time allowed for proper full power operational conditions.... I have said it before, it is an art.

As already advised. Seal your system and stop farting with it! Operate it for a few successive days. Day one is all frustration. Day 2 should show improvement and the next few days will improve your skill set and give the fusor time to find its running path. A good set of hands at the helm will prove to be the system. NOTE* you must have good control over voltage, current and pressure and full metering to know what the hell is going on in the fusor.

Check out the entire thread


viewtopic.php?f=6&t=3212

Richard Hull
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Fusion is the energy of the future....and it always will be
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Erik smith
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Re: Severe Outgassing Problems

Post by Erik smith » Sun Jan 07, 2018 4:33 pm

Robert,
I have seen several people (on this forum and in person) use acetone and IPA without any problems. The type of IPA i'm using is 99.9% pure anhydrous so I don't think water is the issue.

Ian,
Yes I do notice this problem when doing external bakeouts as the larger stainless steel pieces tend to increase the pressure inside the chamber when heated. I only seal my chamber when I'm fully convinced that all of the pieces have cooled off. I do this by measuring their temperature using an infrared thermometer.

Richard,
My voltage metering consists of a 100uA meter in series with a 500M Ohm resistor. since my power supply is a reverse polarity supply, I can simply ground the positive output through a 100 Ohm resistor and measure the voltage across it. All of this was built using the voltage and current metering details in the high voltage FAQ's.
The only reason im writing this post is that it seem that nothing has worked to clear my chamber of its contaminants. While I agree that i am playing with it too much, there seems to be very little improvements in the chamber despite the numerous bakeouts, plasma cleans, and user attempts to get a good "feel" for the system.

Bruce Meagher
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Re: Severe Outgassing Problems

Post by Bruce Meagher » Sun Jan 07, 2018 7:47 pm

Erik,

As Jerry and Richard mentioned for a fusor this level of outgassing isn’t an issue. However, I’m kinda like you in that I want to understand what’s happening and why. Here are a couple things you might consider. When plasma cleaning you need to be pumping, not valved off like you mention above. When baking you need to raise the entire vacuum system to the desired temp. As long as the pressure is high you need to keep baking and pumping. A higher bake temp is faster, but if you have any viton seals this will limit your max temp.

In my limited experience the gate valve seal is a likely outgassing candidate. Tough to get all those internal parts to a high temp (especially with a heat gun), and under compression those little voids can release trapped gasses. Have you dissembled your gate valve? Have you replaced the seal?

If you’re really curious I’d would divide and conquer. First remove the chamber and install your gauge to gate valve. Bake and see if you still have an issue. Then add chamber, but blank off everything. By adding incrementally and collecting data I think you’ll be able to identify the issue, and have some good fun in the process.

Bruce

Jerry Biehler
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Re: Severe Outgassing Problems

Post by Jerry Biehler » Mon Jan 08, 2018 5:20 am

99.9% anhydrous will just pull moisture from he air, it is hydroscopic. The reason that other have had good luck with it is they are not sealing off their system and expecting the pressure no to rise. If you want to be able to seal off the system and not have it rise you are going to have to have all metal seals and then pump down with a turbo and then ion or cryo and bake out the complete system at 200C for several days. A heat gun is not going to do it. Even then expect the pressure to creep up a little. That's why CRTs and other vacuum devices have active getters.

I could not pull an ultimate vacuum on one of my big systems (6" diff pump). Finally remember I had cleaned a valve with IPA (99%). Hooked up a RGA to the system and there it was, a nice ion response patter for IPA. It took days of pumping out at 10^-6 to get it out of the system.

Use methanol or mineral spirits to clean when all possible. You can clean with IPA but you need to wipe down with either of those two after to get most of the water gone. Just wear gloves with the methanol, its not a big deal.

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