Best gas for plasma cleaning?

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.
Post Reply
Jeroen Vriesman
Posts: 276
Joined: Mon Feb 07, 2011 9:08 pm
Real name: Jeroen Vriesman
Location: Netherlands
Contact:

Best gas for plasma cleaning?

Post by Jeroen Vriesman » Tue Feb 16, 2016 2:22 pm

I had a KF 100 cross, with a lot of sputtered copper and other contamination. Tried to fix that with iron chloride.
This worked for the copper, but now there was some other unknown black stuff, which was cleaned with hot nitric acid and other aggressive chemicals.

So far so good, but there was a lot of outgassing under vacuum, bake-out at 100 deg. C didn't help.

So I tried a combination of plasma cleaning and bake out (about 80 deg. C and 6kV 8mA AC power, with a pressure of about 0.3 mbar).
That worked, after 5 hours of bake-out and plasma cleaning the outgassing stopped.

I used normal air for the plasma cleaning, now this was just old stuff for my KF chamber, it gets down to 5E-6 mbar now.

Now I want to do plasma cleaning on a big new CF chamber, and I don't want to make any mistakes with cleaning, and it's more than 150kg steel so heating it for bake out can take a long time.

What would be the best gas to do the plasma cleaning with? Argon? Air? Dry air? Nitrogen? Oxygen? Other? The chamber is reasonably clean.

John Futter
Posts: 1604
Joined: Thu Apr 22, 2004 2:29 am
Real name: John Futter
Contact:

Re: Best gas for plasma cleaning?

Post by John Futter » Tue Feb 16, 2016 5:25 pm

Jeroen
I would use argon. If you can get the ion acceleration up to about 20kV this is the point of max sputtering so you are removing a small amount of surface .
It pays to put a sacrificial glass piece in front of view ports while doing this to stop sputtered material from plating out on the viewport

User avatar
Paul W Fontana
Posts: 38
Joined: Fri Mar 13, 2015 11:58 pm
Real name: Paul W. Fontana

Re: Best gas for plasma cleaning?

Post by Paul W Fontana » Tue Feb 16, 2016 10:38 pm

Back when I worked in toroidal magnetic confinement (not-quite-tokamak), we would make hydrogen plasmas during the day and use helium discharges for daily overnight conditioning. I'm not sure why, except that it was less prone to loading the walls than hydrogen and tended to produce a consistent starting point each day. There was also an intense UV lamp for use after bringing the vessel up to air to help dissociate water.


The main benefit of conditioning, though, is to heat the parts and surfaces to help liberate the adsorbed gases, so to lowest order unless you're shooting for very carefully controlled conditions the choice of gas probably doesn't matter much, though water-laden air may be the worst option.

-- pwf

John Futter
Posts: 1604
Joined: Thu Apr 22, 2004 2:29 am
Real name: John Futter
Contact:

Re: Best gas for plasma cleaning?

Post by John Futter » Tue Feb 16, 2016 10:43 pm

Jeroen

The heavier the gas molecule the better
cleaning = mass x acceleration.
Argon is the cheapest gas that is heavier than air /Water molecules
Krypton xenon would be better but eye wateringly expensive
its like playing snooker with peas (hydrogen ions) or bowling balls (argon ions) with O2 and N2 being about snooker ball mass

richnormand
Posts: 287
Joined: Sat Aug 27, 2005 12:30 pm
Real name: rich normand

Re: Best gas for plasma cleaning?

Post by richnormand » Wed Feb 17, 2016 12:51 am

With a large ion deposition system I used an argon discharge for several hours followed by an unfocussed deposition of titanium in such a way as it would cover most of the chamber.
The Ti would cover the walls and act as a getter for a while.

Jeroen Vriesman
Posts: 276
Joined: Mon Feb 07, 2011 9:08 pm
Real name: Jeroen Vriesman
Location: Netherlands
Contact:

Re: Best gas for plasma cleaning?

Post by Jeroen Vriesman » Wed Feb 17, 2016 10:09 am

Thanks for the comments. I will try with argon.

picture of the plasma cleaning with air of the KF chamber, and a picture of the CF chamber I want to clean. I cannot do a plasma cleaning on the rga and valves under the chamber left, so I will have to heat that part of the setup.
Attachments
cf_chamber.jpg
CF chamber
plasma_cleaning_kf_chamber.jpg
Plasma cleaning with air

User avatar
Dennis P Brown
Posts: 1854
Joined: Sun May 20, 2012 2:46 pm
Real name: Dennis P Brown
Location: Glen Arm, MD

Re: Best gas for plasma cleaning?

Post by Dennis P Brown » Wed Feb 17, 2016 1:53 pm

I agree with argon; also, this is generally used in the semiconductor trade for fast plasma cleaning. Besides argon being both cheap and safe, it does a good job removing many types of contaminates but does tend to depend on trace oxygen for carbon related contaminates.) A quick aside: generally, such plasma cleanings were under a few minutes - parts should be chemically clean before plasma cleaning - so long term plasma cleaning should not be needed.

Hydrogen is very aggressive (which can be a good thing if carbon is a problem) but can damage steel (infiltration; generally not an issue if only used under a hundred or so milli-torr and for short time periods) and would be rather harsh on seals, I would think.

User avatar
Richard Hull
Moderator
Posts: 12201
Joined: Fri Jun 15, 2001 1:44 pm
Real name: Richard Hull

Re: Best gas for plasma cleaning?

Post by Richard Hull » Wed Feb 17, 2016 6:53 pm

FYI..As noted before, hydrogen embedding in walls or anything in a fusor is a great boon......Providing it is the right kind of hydrogen.....Deterium.

There is just no better place for hydrogen (deuterium) to be lodged, wedged or hiding and leaking into a fusor than from the walls of the chamber. What a wonderous, continuous outgassing that will, hopefully, be with the chamber forever!

Hoping for much advanced hydrogen embrittlement of the materials in our fusors is a good thing.

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.

Post Reply