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[Doug Coulter]

Nope, it was real local heating. The reason the one in the picture was full length is the fat wire and I was trying to cram turns on there. The ones that got hot were #24 (the picture is #16) and only an inch long winding, well insulated by an inch or two of glass from the end of the feedthrough. I use an IR thermometer for that, and to watch the tube temperatures. I've wound around 5 of these, testing various inductance and frequency ranges. The one you see appears to be the keeper. It's been an issue of the faster I want to go, the higher volts I seem to require, and both are more difficult...
In fact, the evidence is that having enough ions in there to do meaningful fusion increases the voltage difference between the applied and net field, and not in our favor. At DC the numbers tend towards 1/10th or worse - putting 50kv on there, I seem to measure transit times that would say "stuff has 5kv energy on it".

No matter. Thing is, that FT end does get quite warm and since I've been suspending things off that socket end + setscrews, they have to be pretty stiff at ~ 100c or there are other problems. I might not do that forever, but for now I'm trying to retain the option to put that whole mess back into the 8" pvc pipe with screen on the outside to keep RF and corona in the shack down.
FWIW, 8" pipe with just a little internal sanding is a tight fit over a 6" CF flange...it only takes one screw to hold that 3 foot pipe on there...and ground the copper screen on the outside of it.
But when I'm fiddling a lot right there, I'm running with it off - it's not that easy to R&R the pipe over fragile stuff hanging off the FT (2 person job, one to hold it aligned and put in the screw, other to shove it home).