Gauge wrong after plasma cleaning?

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David Kunkle
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Gauge wrong after plasma cleaning?

Post by David Kunkle »

Did a plasma cleaning the other day. Started out following old posts using 1kV, 20-30 mA, @ 100 microns for 10 min. All pumps were off. Used a SS grid I made some time ago that didn't turn out quite like I wanted. Seemed to be going well and I saw the pressure rise on the TC. I thought as long as the pressure keeps rising, it must be knocking stuff off the chamber. So I kept it going for about an hour when the pressure seemed to stop rising at about 170 microns. I pushed it up to about 1200V and 40 mA toward the end. Then pumped it down with the turbo.

Problem is- it used to go down to about 5 E-8. Since the cleaning, it will only go down to about 5 E-7. ??? Could the plasma have done something to the ion gauge? Nothing else was changed or modified. FWIW, I can see thru the viewport there is some sputtering on the alumina insulation I slipped over the electrode support. Thanks for any input.
If your experiment needs statistics, you ought to have done a better experiment.

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Richard Hull
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Re: Gauge wrong after plasma cleaning?

Post by Richard Hull »

Is your ion gauge tube glass? If so, was their any plasma going into the gauge tube during glow cleaning? Is their any discoloration on or in the gauge tube or any of its elements? If not, something else is going on with the instrument.

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
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Jim Kovalchick
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Re: Gauge wrong after plasma cleaning?

Post by Jim Kovalchick »

Another thing to look at is the relationship between your ion gauge and thermocouple. Typically you turn on the ion gauge when the thermocouple is bottomed out. Have you noticed if the ion gauge reads differently than it used to when you first turn it on when the TC is bottomed? This couple be an indication of dirty or contaminated filaments in your ion gauge. The degas feature of most ion gauge controllers could help with this.

What kind of seals does your chamber have? Sometimes resilent seals like viton etc are impacted by exposure to plasma and crack causing minor leakage.
Jerry Biehler
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Re: Gauge wrong after plasma cleaning?

Post by Jerry Biehler »

You may have cleaned the gauge too. Impurities on the collector of the tube will cause it to show a better vacuum than it really is. It acts as an insulator and the lower current though the better the vacuum.
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Re: Gauge wrong after plasma cleaning?

Post by David Kunkle »

Hi guys,

Thanks for all 3 responses. First, I'd say there was no plasma going into the tube that I recall. There is no discoloration of the glass gauge tube or elements. Doesn't seem to be any sputtering on the viewport either. Now that I've removed the electrode, the sputtering on the electrode's 6" alumina insulator is dark closest to the electrode and fades to none the last 2" closest to the chamber. I have 2 Viton seals on the chamber- doubt any of the plasma got to them in light of the above, but I haven't examined them yet either.

The ion gauge reads about the same (about 5 E-3 micron) when the roughing pump has the TC bottomed out. I've also realized the degas does not work now that I have a manual. Upon switching on the degas with the gauge already running, the tube gets a little brighter for a few seconds and then the gauge shuts off altogether. Used to think that was normal. Turn off the degas, and the gauge fires up again OK.

Interesting theory about cleaning the gauge. Might explain why it barely gets to E-7 torr now instead of E-8, but the gauge was brand new when I first started measuring the pressure on this new chamber. Unless it was somehow dirty to start with.

What pressure would any of you expect a new, properly prepped/cleaned chamber to reach?
Also wondering about how the inside of the chamber smells. Burnt type/metallic/ welding smell- lot stronger since that long plasma cleaning. Had a bit of a smell when I'd open it up before.
If your experiment needs statistics, you ought to have done a better experiment.

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Richard Hull
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Re: Gauge wrong after plasma cleaning?

Post by Richard Hull »

The smell, if real, is a chemical reaction going on with any deposition on the walls and earth's atmosphere. There can be no smell in a vacuum as a smell is gaseous in nature. You will need to glow clean again now that the insides came up to air. Crap only smells due to atmospheric contact of something that is willing to react with it.

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: Gauge wrong after plasma cleaning?

Post by John Futter »

David
How many right angle bends are there between the ion gauge and the chamber??
At work with new beam line and or chambers they are solvent washed and then hot ultrasonically cleaned with Decon 90 then dried in a hot air oven for 4 hours @ 150 degrees celsius. We then assemble while still hot (all CF) and start pumping. After about two hours pumping we hit low ten to the minus 5 torr. After two weeks of pumping we get to low ten to minus 7 torr and after a month or two we get to mid ten to the minus eight torr of continuous pumping. If we come back up to atmosphere this is done by back filling with dry nitrogen, if we are quick and only open for an hour at most we go back down to low ten to the minus 7 torr in an hour or two and down to the mid ten to the minus 8 after 24 hours.

these measurements with hot ion gauges after degas- this is important the gauge gets reinfected with nasties as soon as you come back to atmosphere and we degas after we go into the high 10 to the minus 7 torr region.
Initial degas can take a good half hour there after a couple of minutes is enough
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Re: Gauge wrong after plasma cleaning?

Post by Jerry Biehler »

The reason the tube trips off when you try to degas is there is a heck of a lot of trapped gasses or stuff outgassing when you hit degas. The ion controller has a kill circuit that turns it off if the pressure gets too high to save the filament. It may take several degas attempts to get the majority of the crud out of the gauge tube. Even on a system that is alway at vacuum you can see a a pressure spike on a RGA when you degas the ion gauge.
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Richard Hull
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Re: Gauge wrong after plasma cleaning?

Post by Richard Hull »

Attempting to get as low as possible, such as 10e-7 or even deeper comes at a price, a big price. Hours and hours of work and vacuum tending will be demanded. Opening and closing a system can make this a life's work! Hitting 10e-5 is more than deep enough for a fusor...low 10e-4 is just fine too. You wind up squeezing until you scream. Your choice, of course.

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
David Kunkle
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Re: Gauge wrong after plasma cleaning?

Post by David Kunkle »

Responding to the last 4 posts:

The smell has gone away since I've closed it up and pumped it down. Seems it was caused by an arc thru a bit of epoxy.

There is 1 right angle fitting between the tube and chamber. Thanks for the rundown on what it takes to get to -8. That puts things in perspective. Obviously something is amiss with the ion gauge readings. Just got a 500MOhm resistor, so I should have time soon to see if I can calibrate the controller using the manual.

I've hit the degas at least 20 times. The gauge begins to rocket upward, and I realized it shuts itself off when it gets to the -2 range like it's supposed to. Also, the coil glows scary red hot by the time it shuts itself off. Even with all the times I've hit degas, the time it stays on does not seem to improve. Don't see how degas could be left on for 1/2 hour. Could it be getting that dirty?

FWIW, the setup will not support a plasma below 1 E-3- at least according to the gauge. At what torr will a visible plasma extinguish? Thought it might give me some idea of how far off my vacuum readings are.
If your experiment needs statistics, you ought to have done a better experiment.

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Re: Gauge wrong after plasma cleaning?

Post by John Futter »

David
yes it should glow scary red hot
no it should not shut down with high pressure-- this proves it is contaminated
our gauges at work jump up a couple of powers of ten from 10 to the minus 7 when we degas.

when you get rid of what is causing the big rise you will see a big rise after 5-10 mins as the hydrogen ( this assumes the 30 min degas previously)is expelled and after that short degas cycles when required
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Re: Gauge wrong after plasma cleaning?

Post by Richard Hull »

Hydrogen enters all metals. Good and not so good. Good when it lodges in the huge surface area of the fusor shell and bad for filaments of ion gauges and other items outside the fusor volume. This is why struggling for a 10 e- infinty vacuum is a fool's errand for the fusor.

Hydrogen (deuterium) is buried everywhere in everything! Be real happy if you can hit 10e-4...Your fusor will work just great! 10e-5 torr is a fantastically deep vacuum in the fusor world.

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
David Kunkle
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Real name: David Kunkle

Re: Gauge wrong after plasma cleaning?

Post by David Kunkle »

Looks like I stopped precisely one try too soon with the degas. Next time I put the system under vacuum and hit degas, the gauge actually didn't skyrocket and shut down. I left it on for 20 minutes. The gauge is reading consistently higher now- about 3 E-6 Torr after an hour of pumping. Still looks too low considering what John Futter wrote regarding the length of time it takes for a chamber to get to -7. Haven't used the 500MOhm resistors yet to check the calibration of the controller.
If your experiment needs statistics, you ought to have done a better experiment.

Ernest Rutherford
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