Chamber Pressure Drop

[Tom Dressel]

I have noticed something strange.

With the chamber pumped down to 10 microns, and valved off, I can turn on the power supply, and starting at 4 KeV and 10 Ma., the display will start in E beam mode and the chamber pressure will start to drop! The voltage starts to rise and the grid current starts to fall! The it will go through star mode at 8 KeV and 4 Ma. and eventually (~two minutes) to extinction at 10 Kev, 0 Ma. and 4 microns. This is with the chamber valved off!

How can this happen?

[guest]

I think you are seeing a sort of "gettering" activity. The residual gas molecules, a large protion of which are water, are being ionized to H,O, and OH and then chemically combining with sputtered metals from the interior (wire cage electrodes) and being gettered to somwhere out of the action.

That you see a repeatable, reduction of about 2.5x in pressure is a testimony to a tight, clean system.

You can often see something similar if a cold cathode Ionization gage in on the system. It acts like a small ion pump.

Looks like you're getting some free pumping action.

Dave Cooper

[Tom Dressel]

That may be what is going on. But what ever it is, I have to open the chamber valve to let air INTO the chamber from the foreline (the diff pump is cold) to restart grid current flow.

[Richard Hull]

In a full blown fusor, you would admit more deuterium to keep the glow going. The other option is to up the voltage. You can get glow discharge at 10e-4 torr with enough voltage.

The problem is the glow discharge region becomes extremely narrow and the operating finesse increases dramatically.

Where does the deuterium or air go when the pressure lowers? It gets buried and some small amounts get reacted chemically. If you take careful measurments, you should see the pressure rise slowly after shutdown as the excess buried gasses desorb to a limited degree.

This effect is used to rehydrogenate or repressurize larger, higher power hydrogen thyratrons (starting with the 5C22 tube). There is a separate heater associated with the "gas reservoir" this must be carefully exercised prior to the application of voltage to the tube. Hydrogen trapped in titanium, paladium or other material is chased out of the reservoir back into the tube envelope so that the tube's characteristics remain constant.

The same thing was common in the early cold cathode, gas x-ray tubes. A highly trained x-ray technician could tell when a tube was going "hard". He would go to the arm of the tube with the paladium metal in it and bombard it to regas the tube. This was a pure art form and there were no pressure gauges ever used. It was the eye and hand of the operator which restored the tube to functionality.

With filament electron sources in the coolidge tubes of the 1920's and hard vacuums, all this art went out the window and X-ray tubes could be relied upon for a stable operational characteristic and could be operated by trained monkeys.

The amateur fusor in its simplest form does require an expert operator trained in the art. The training comes from the demo fusor and Tom and others in the plasma club have had their time before the mast, each noted, in turn, the amazing characteristic of the discharge anomolies associated with plasmas near extinction. Go back far enough in the old songs posting and you will see my amazement, too!

Tom is observant and commented on the strange goings on. He has trained himself. For all the posting and reposting of this issue, nothing brings it "home" like th' doin'!!! Words pass before our eyes often un-appreciated. Pictures are worth a thousand words, to be sure., but the doin' conferes ownership of ideas and concepts.

Tom is poised to join the neutron club.

Richard Hull