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FAQ - Plasma cleaning & start up of long dormant systems

Posted: Thu Sep 25, 2014 6:35 pm
by Richard Hull
Long dormant amateur fusor systems and brand new "startup systems" can benefit from the following procedure which I have used for years. Plama cleaning is part of this effort.

When first running after a long period of dormancy or for that important first time evacuuation, I turn on the forepump, open the gas ballast wide open and wait until the foreline is near 15 microns, which is close to my forepumps bottom limit. (This normally takes about 20 minutes). This rids the forepump of most water in the oil. I then close the gas ballast and open all valves right up to and including the fusor. By now the forepump is quite warm if not hot. This is normal.

I then start a hard, hot plasma at about 50 microns. (Note: opening all the other valves causes the foreline TC gauge to rise to high pressures and the process starts anew for the diff pump and fusor) I force the plasma to draw about 20-30ma regardless of voltage. (To plasma clean you must have a "beefy" supply to allow current like this) I up the voltage as needed to maintain at least 20ma until the foreline TC gauge and the fusor chamber Barocell agree in the range of 10-15 microns which is the norm for a long running, bottom to my forepump. By this time, the fusor chamber is almost too hot to touch. (plasma dissapation and electron bombardment of the shell).

Note: In most systems, you rarely ever reach 10kv in plamsa cleaning mode as the pressures are rather high, 15-30 micorns. Often, when first starting to glow clean, 20 ma is drawn at only 2-3kv. You are, in effect, running a demo fusor at this point. Watch out for grid melting. Run at a point where your grid is not glowing more than a dull red, at most!

I next close the fusor chamber valve and start the diff pump heater and cooling fan. I have a temperature gauge on the boiler and as the temperature rises so does the foreline pressure due to water vapor and other volatiles leaving the diff pump oil. (This can be as high as 20+ microns!) As the temperature approaches the full operating temp of the oil to start through the jets, but before any significant pressure drop occurs below the normal foreline "hot" bottoming of about 12 microns, I quickly open the fusor chamber valve. Most of the time I open the valve just before the boiler hits 100 deg C., as my diff pump drops the 12 micron chamber pressure like a stone at about 110 deg. C. As this "jet functioning" temperature is reached, it is natural for all plasma to cease as it can't be maintained at any voltage. IMPORTANT!...Turn all power to the fusor off at this time.

This warrants that most of the water vapor is sent out of the system and that the water vapor and light volatiles don't back flow into the fusor.

There is an art to not fouling a long dormant fusor or new fusor and plasma cleaning is just part of it.

Richard Hull

Re: FAQ - Plasma cleaning & start up of long dormant systems

Posted: Fri Sep 26, 2014 12:37 am
by Bob Reite
Interesting. When I first built my system, I was thinking of various ways to bake it out and decided that the easiest way was just to run a plasma at a relatively high pressure and current, get it good and hot, then shut down the HV supply and open the exhaust valve all the way so that the turbo could remove any outgassing from the chamber.
Watch out for grid melting. Run at a point where your grid is not glowing more than a dull red, at most!
This is how I killed my first grid, by getting too 'piggy' even though it was made of tungsten. With a tungsten grid, I can safely go to orange red, but I now limit the grid dissipation to 400 watts and current to 25 mA.

Re: FAQ - Plasma cleaning & start up of long dormant systems

Posted: Fri Sep 26, 2014 2:59 am
by Richard Hull
We are all responsible for saving our grids. It is very important to monitor your actual fusor current. In this fashion you know when you are getting close to a meltdown and can avoid it.

Richard Hull