What causes these forms of instability and pulsation?

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ian_krase
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What causes these forms of instability and pulsation?

Post by ian_krase » Sun Oct 22, 2017 12:30 am

1. Instability of pressure in the high vacuum regime: periodic spikes of pressure on the ion gauge, of maybe 50%, that then resolve themselves

2. "sparkling" on electrode surfaces in a glow discharge -- is this indeed caused by contamination burning off?

3. "flickering" and wild changes in voltage/current in a glow discharge plasma

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Dennis P Brown
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Re: What causes these forms of instability and pulsation?

Post by Dennis P Brown » Sun Oct 22, 2017 2:12 am

Most such "variations" are indeed caused by trace contaminates; I was plagued by such until I had run my chamber a half dozen times - yes, once wasn't enough even after a high vac run (bottom 10^-5 torr) and then with a strong plasma at 5 microns. Keeping the chamber under vacuum helped, as well.

After a good cleaning, the chamber behaves even after I open it to the air for short periods of time. Part of this improvement may be from seasoning the DP oil for my system by numerous runs at high vac.

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Re: What causes these forms of instability and pulsation?

Post by ian_krase » Sun Oct 22, 2017 3:31 am

Huh. OK.

My chamber tends to be a bit on the dirty side. I am not very good at this and my homemade gear is hard to clean. I did just run it for a couple of hours today, baking down to 1.3E-5 torr. No luck getting to 1E-6.

Do you just valve off your chamber, or do you keep a pump running?

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Dennis P Brown
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Re: What causes these forms of instability and pulsation?

Post by Dennis P Brown » Sun Oct 22, 2017 12:30 pm

Well, your vacuum is just as good as mine was initially; however, it could get better with repeated plasma burning.Your current high vacuum base is certainly good enough for a fusor or that is, back filling with deuterium gas for a run after evacuating the fusor to that value.

My fore pump has an oil vapor anti-back streaming component (has a molecular sleeve.) Without that one MUST have some type of gate valve for the chamber or (better still) on the fore pump line to allow the system to remain under vacuum without the pump running continuously: i.e. if you turn off the pump and the system is under vacuum, then pump oil vapors flow back into the system and contaminate it.

Vacuum valves are expensive (even used and those can be defective) so I went with an anti-streaming Kf based component for my system (my DP came with a very nice butterfly manual gate valve.)

Yes, I keep my gate valve closed and the pump off (the electric bill would be a bit high - lol.)

Getting pump oil via vapor into the diffusion pump oil (if you have a DP) will cause issues when running a fusor.

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Richard Hull
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Re: What causes these forms of instability and pulsation?

Post by Richard Hull » Sun Oct 22, 2017 6:29 pm

As noted by Dennis, The dead giveaway is this is your first real serious run. The sparkling will go away. I prefer to pump only to about 100-50 microns and bombard the hell out of the system. This cleans it out real fast. This can readily be accomplished not with a fusor supply but a real stiff, variac controlled, 10KV supply. I tend to bombard, (neon workers term), at the pressure mentioned at 20-40ma, if the grid can take it. Do this for however long it takes to stop sparkling with the vacuum pump running. You should hold the pressure by valving with only the fore pump running. The fusor shell will get so hot you can't touch it. Never do this with a glass demo fusor!!!!!

A beefy 12-15kv neon sign transformer will serve to make a DC bombarder supply and it will serve to regulate your current. Bombarding is all abound current. Give it as much as it wants, but not so much you melt your grid. Here is where a view port and a video camera can be all important.

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.

ian_krase
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Re: What causes these forms of instability and pulsation?

Post by ian_krase » Wed Oct 25, 2017 9:35 am

Another excursion I'm wondering about:

- Have a system that has been pumped to (it seems like) the 10^-5 torr range for hours, turn on hot cathode ion gauge. Ion gauge reads pressure climbing all the way up to the high 10^-4 range and then drops.

Is this just gas desorbing from the ion gauge filament, or is it something worse? The gauge has already been run and it seems like it should be baked by this. Are ion gauge tubes just gas magnets or something?

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Dennis P Brown
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Re: What causes these forms of instability and pulsation?

Post by Dennis P Brown » Wed Oct 25, 2017 11:06 am

Since this is occurring only when you first start the ion gauge, I'd guess, yes. Most ion gauges have a "de-gas" option that heats the entire gauge to out gas so the gauge is more accurate at higher vacuums (10^-5 and "higher".) I always do that once I get below 10^-4 torr. That will, at first, raise the pressure in the tube but that quickly lowers. So, if you have that option, do that first (when you get into the 10^-4 torr range. It might cause it to turn off if it exceeds 10^-3 torr; repump and do it again.)

The "why" isn't exactly talked about (I guess because it is considered pretty obvious ...) but I think you are correct; the walls of the gauge tend to collect contaminates.

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Re: What causes these forms of instability and pulsation?

Post by ian_krase » Wed Oct 25, 2017 7:06 pm

Ok. The funny thing is that I had already done this since pumpdown.


Do I have a backstreaming problem?

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Dennis P Brown
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Re: What causes these forms of instability and pulsation?

Post by Dennis P Brown » Wed Oct 25, 2017 8:09 pm

Not sure what you mean by "have a problem"? - I have to out gas my ion gauge every time I go down below 10^-3 torr and I have an anti-back streaming device on my fore pump. Getting below 10^-3 torr is the start of high vacuum and many gases tend to line walls; if your fore pump is running, back flow will not occur. So if you turn off the pump and don't have a molecular sleeve (anti-back stream) or a fore line valve, yes, oil vapor will, in time, be an issue. You haven't let me know which case applies to your system.

In any case, what you described occurring for your high vac and ion gauge sounds pretty normal and mine operates in a similar manner; I get great stable plasma's and fusion. So, your high vac pump down sounds fine. To season a chamber, Richards advice is right on. I always run a strong plasma before trying a fusion run.

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