With the recent discussion in the vacuum column about controling the fusor gas flow and chamber pressure in a feedback loop, I thought I would pass on the way It actually is obtained via manual operation.
1. Pump system to base pressure (10 minutes to 1 hour) a. D2 gas valves shut b. All main vacuum line valves wide open.
2. Glow clean once below 100 microns. a. to glow clean, run the current and voltage to whatever point the grid just starts to glow dull red. This will vary with dropping pressure, but make sure to keep raising the voltage until it approaches 20kv or more. By this time the current is limited to about 10 ma or less. b. Don't move to the next step until you are below 5 microns! (preferably lower)
3. Admit D2 by bleeding it in until you are holding against the pump at 20 microns. ( a minute or two) a. Now throttle back on both the d2 valve and the main pump valve until the main chamber vacuum valve is almost shut off. You are now only slightly pumping the chamber and the at a minimal usage of valuable D2.
4. Start applying voltage and attain glow mode at a bleeding D2 pressure of 8-15 microns. Once glow mode is achieved, you should see the star and the pressure will drop. (ion pumping) a. carefully re-adjust the main vacuum valve and the d2 bleed as the voltage is raised to maintain the fusion D2 pressure of 8-15 microns. b. The art is in nursing the unit to ever higher voltage and holding the current down to a level where the grid doesn't melt or vaporize. (~6-8ma @28kv). This voltage is voltage across the fusor chamber and not voltage out of the supply. (You should have a limiter/dropping resistor between the fusor inner grid and the power supply minus hot lead. About 500-1000 volts is lost in this resistor).
It is this delicate dance near the extinction of glow mode while maintaining enough current to do fusion, and at a high enough voltage to be at a useful part of the d-d crossection curve that will make you go nuts. It begs for closed loop control, all interlinked to a sliding scale of current vs. voltage (data lookup table??) Remember, your computer has no way of knowing the grid is white hot and moving to the vapor phase! The horrible symptom here is that current runs away as the inner grid emits electrons as a hot cathode creating more ions and allowing more current to pass, etc. in a viscious cycle. Once over this knee, rapid reduction in applied voltage is the only solution!
Probably a nice IR led looking in the port would be of help here. Just set its differential amp for orange hot in the IR and use that as a limit detect on the power application.
Richard Hull
Created on Tuesday, March 6, 2001 3:45 PM EDT by Richard Hull