Sometime if I need a high voltage capacitor of somewhat larger capacitance I'm gonna try an "array" of pickle jars filled with aqueous ammonia and wrapped in aluminum foil, immersed in a mineral oil bath.
Depends on the risetime. Say 30pf is 1 ft. A 100ps risetime pulse would ring like crazy. During the initial portion, the coax would not look capacitive. It would look like the characteristic impedance of the coax. I think JL wants a lumped capacitance.
There are ex-USSR capacitors on ebay claiming to have PTFE dielectric, which might be cheaper and thinner than the Rogers stuff, though some pictures of disassembled ones on the internet suggest that the capacitors as sold may not have extended foil nor connections with high enough current rating. Disassembling them for the PTFE film might be reasonable value though, if you don't have another source of it.
IIRC, 100ps is just possible in the lowish kV, with a liquid filled spark gap in a coaxial trigger setup.
I'm not sure what Steve is thinking about lines as capacitors, though -- as soon as you've got a time spec, if you want the line to look like a capacitor, the line length must be significantly shorter than that. At
100ps, your "line" has to be smaller than a 1206 chip cap. Otherwise, a line is a line, and... what's wrong with that? Transmission lines are only slightly more work than capacitors.
Nobody is insisting that you use just one chunk of coax. A nice radial arra ngement of shorter bits of coax would present a lower characteristic impeda nce, and shorter propagation times. Using different lengths for the various chunks of coax could smear out the voltage-doubling reflections from the u nterminated open ends.
Plenty of room for ingenuity, of the kind John Larkin seems to think he off ers.
Steve Wilson wrote in news:XnsA8B33082B814Eidtokenpost@220.127.116.11:
I see what you are saying. The end of the segment gets insulated. That is what makes a small HV capacitor out of it. So you are saying the the shape of the "plates" makes a difference and that his FR4 spaced copper plates have no such characteristics?