In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.Subject: vacuum info - Glow cleaning
Date: Dec 10, 3:10 pm
Poster: Richard HullOn Dec 10, 3:10 pm, Richard Hull wrote:
All,
I have mentioned this in passing over the past two years, but I have pulled data together which I will pass on in the interest of sharing some vacuum technique.
This is predicated with the usual provisos, gotchas and other catch 22's so often found to lie in the path between theory and th' doin'.
I have had fusor III sealed against the world for about 6 months now and have run it in this state with re-pumps a couple of times a week for an hour or two. So most of the tramp moisture and the worst of the nastey gases are fully desorbed and exhausted. The lesker micro maze has been used to fantastic advantage over this period also.
As most of you might know, I use only a mechanical pump for all my operations (albeit superbly conditioned and cared for). I feel that I have shepherded the system about as well as it can be done for what I have.
Last night I did a bit of data collection on vacuum in the system. Needless to say, I have no real leaks of any import in the fusor vessel itself. Thus, I can valve off the fusor and in about a week the pressure will rise to about 25-30 microns. Most of this is due to long term desorbtion of ion buried gases in the walls. These are, for the most part, deuterium, hydrogen, etc. There may also be micro leaks which would not impact the normal operation under active pumping.
I have been heavily involved with the lab infrastructure of late, creating new bench work, wiring more massive power circuts, etc. This means it has been about two weeks since I last worked the fusor.
The pressure was 35 microns as I started to power the system up. I always log this data and the trend is toward longer and longer time periods with less pressure rise. This is normal in all vacuum systems which are closely held under vacuum. Rather than start the valving needed to bring the micromaze on line and then the mechanical pump, I did nothing!
Instead I grabed the stop watch and monitored the pressure as I brought up the high voltage. I brought the chamber current up to 15 ma and a strong glow discharge. This occurs at 30 microns in the 1kv-2kv region. Immediately (few seconds), the pressure began to plunge. In one minute the current had backed off to 5 ma and the pressure had fallen to 15 microns. I jacked the current back up to 15ma. over the next two minutes the pressure fell to 4 microns and the voltage flew up to 7kv. When the current reached 5ma, (200 seconds into the test), I jacked the current back up to 15ma for a final time and the system fell to 6X10e-4 torr over the next 5 minutes. The voltage ending up at 12kv and the current at 3ma. This is a good starting point for rough fusor operations! Thus, in about 10 minutes I could bury the gases via glow cleaning and ion pumping by almost two decades!
In summary:
If your chamber is clean, If it is well sealed, If it is held under vacuum long enough and if you use the best vacuum technique and care in maintaining system components, only glow cleaning and a bit of ion pumping is needed between major runs to restore your vacuum to minimal operational levels for fusion. A mechanical pump at this level would offer little additional assistance as it is way beyond what is termed "transitional flow" and well into the purely "molecular flow" regime.
Without the micromaze, operation of the pump would actually increase the pressure the instant the valve is opened due to the backstreaming oil vapors which would foul the system until glow cleaning could catchup and reach an equilibrium.
Real "vac heads" know all this though, but now all of you do as well.
Richard Hull