Just another point.... with DC, do all you can to cover metal surfaces with a dielectric. This creates a blocking electrode condition... cutting off the supply of electrons. The air can still ionize, but it will do so much less vigorously, without a free supply of electrons from an exposed metal surface.
Shielding your wires with Tygon as you describe is an excellent way to accomplish this. Generally, a snug fit is better than a loose fit. Also the vinyl will give you at least 3 times the breakdown strength of open air, so your clearances may prove to be adequate. If you could find some polypropylene, polyethylene or PTFE (Teflon) tubing this would be still better, as the dielectric strength of these materials is superior to that of vinyl.
As Richard notes, any corona or glow discharges around the elctrodes will create very aggressive chemistry inside your enclosure, NOX,O3,various acids and what not.
For what it's worth... where such conditions do exist, my experience has been that polycarbonate (LEXAN)does remarkably well. Slight discoloration, even in direct 60 - 90 kV E-beam exposure for hundreds of hrs. So that would be my first choice as a building material. It IS pricey though, and operating temps are below 100C.
Dave Cooper
Created on Monday, April 23, 2001 8:07 PM EDT by David Cooper