Fascinating stuff, Ed. Some random thoughts and semi-random questions:* What polarity are you using for the carbon electrode?
* Possibly a second carbon electrode would make it easier to avoid oxidizing copper, in the absence of water cooling but in the presence of breathable (~20% O2) atmosphere.
* Do you have a larger inductor that could let you attempt oscillation at lower frequencies? (It's harder to achieve stable oscillation at higher frequencies, especially without a volatile hydrocarbon atmosphere.)
* Higher voltage might indeed permit operation in a steeper portion of the negative resistance slope, but it is also possible to go too far the other way, so some power supply adjustability could be desirable. Also, bear in mind that your ballast will dissipate more heat, too.
Keep us posted. Thanks.