I been figuring out how to use my camera, these are better pics. The luminous intensity gets lower, the higher the voltage I go, because the vacuum has to be lower to run at the higher voltages.
This is at 30kV @ 15mA, the pressue is ~2x10-4 Torr.
I'm just setting the gate valve slightly open, and then adjusting the D2 flow with a presicion needle valve until I get the plasma to be stable at the desired voltage. I've also gone back and enhanced the 20 and 25kV pics so that you can see the structure of the plasma. As I'm increasing the operating voltage, I'm dropping the pressure, and this can be seen in the size of the center poisser.
Mike Amann