After waiting nearly a year to run my fusor I noticed that its behavior is rather predicable and fairly steady after its initial cleaning. I was surprised by this since I really just keep it under a simple vacuum and rarely pump it out. Since I was running it today I noticed that after 30 minutes of very steady, predicable operation, my pressure started to tend upward - this caused by current to also slowly increase. This quickly created an unstable situation. That is, more current created more heat and that increased heat caused a pressure rise which increased the current. Very soon, a runaway situation quickly ensued.
Due to heat build up in my first series of fusion runs about a year ago, I decided to upgrade my chamber with water cooling. My electrode is forced air cooled by necessity (since it is isolated in a large glass plate so thermal contact alone will not effectively cool it.) These cooling systems do an excellent job on both the chamber main body and the isolated electrode. Both remain cool to the touch (of course, all HV is turned off, before I ever touch the electrode - it is fully insulated but I never trust depending on that safety measure alone!)
After turning off the HV I investigated various regions of the fusor. I noticed parts well away from my cooling coil system that had coupling systems involved for the fusor were warm, to even a bit hot. So I waited fifteen minutes for the system to cool - the running water works very well to cool even parts that are connected with o-rings assemblies.
I returned my fusor back to operation and again, it was extremely stable.
From this, I assume that the areas that are not effectively cooled in my fusor reached a temperature where contaminates started to evolve. Stopping operation allowed these places to quickly cool and stop this process.
Bottom-line: water cooling a fusor's main body has a possible up side for people who often encounter unstable operation; apparently, even with the plasma there are contaminates that still are more affected by the chamber walls getting above a specific temperature (likely water in the metal surfaces.) Wrapping my main chamber body with copper coils and using high temp tolerant epoxy enabled my chamber to remain cool even when I am pushing 1000 watts for a half hour. A very simple adaption for those that have a water cooled diffusion pump (I cool the pump first and use that water to then cool the fusor body.) I think this is well worth considering for people that have water available. Yes, baking can avoid this but not all of us have heating tape and this avoids the wait for that process. One could heat the system using the HV but that does not always work effectively (as I discovered in the past before I water cooled my fusor.)
Finally, I also discovered a year ago before I water cooled my chamber that teflon piping (used for my feed gas) could soften since my chamber would get very hot after a 20 - 30 minute run. This also made me worry about my o-rings seals. The water cooling has eliminated that issue and allowed my fusor to run rather stable.
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