Richard, when you point out
Townsend arc breakdown in a partial vacuum will be a great noise source across a rather broad spectrum
All I can say is, and this is from recent experience, I've learned this to be an understatement compared to the reality! Also, when the star ground configuration is used (and always a good idea to use this for safety
!), one is also "piping in" huge voltage
surges into the ground system which all your electronics can easily "see". My poor counter ran at the 120 Hz from the fusor!
I am slowly gathering together and assembling the equipment to have a battery supplied 2 kV DC and 120 AC power supply (for my counter) to operate/read my fully shielded Russian neutron counter. I will use a shielded cable/detector and locate the HV & counter some distance away (alas, shielding the car battery and HV x-former & variac is not practical as of yet.)
I do not think this subject in the detector FAQ's section is sufficiently emphasized as it so richly deserves.
My fusor - which easily achieves fusion conditions and (finally!) is highly stable - is, currently, as you correctly point out is just a very complex noise machine. Further, at least this aspect of a real fusor is its easiest property to demonstrate - LOL!