The 55kv anode is seperated by >62mm. That should be plenty "even" with an air gap at ~3kv/mm for air. However on glas even slight contamination leads to creepage/breakdown along the gas.
Okay, that's plenty of separation. I was thinking you might have to pot a Cockcroft-Walton multiplier or transformer secondary or the like where there might not be much separation between extreme potential differences.
Regarding potting: Even the "worst" compounds I have used for work had at dielectric strength in the order of kv/mm.
For example the first compound I picked randomly on digikey claims up to 20kv/mm
Uh-huh... The problem here is the high electric field strength causes chemical reactions to occur that otherwise wouldn't (or only at an infinitesimal rate). Just because a material is rated to withstand 20kV/mm doesn't mean it can withstand it
indefinitely.
That said, properly cured silicone - usually requiring addition of a curing agent or catalyst in potting applications - has a good combination of high dielectric strength and chemical inertness. But, you have to vacuum degas it otherwise the dielectric strength in the void(s) is merely that of air...
Any thoughts on how paraffin would behave. I mean it would start to melt around the tube - which is good since that would provide some cooling - But also the pressure of the molten wax inside the solid paraffin block would rise. My guess is that the glass tube is much stronger than the paraffin block which would break first but...
No idea. Paraffin isn't particularly rigid or strong even when solid, but your concern is valid. FWIW, low to moderate power commercial x-ray tubes are usually potted in silicone while higher power tubes are almost always immersion-cooled in a high dielectric strength oil.