wound, 23-turn coil of small-diameter coax. An academically
> interesting modification would be to replace the foam with a
> non-flammable, thermally-conducting, dielectric liquid, perhaps a
> Freon, or uranium hexafluoride (which is used in some
> extremely-high-voltage electric power cables). Chemical
> compatibility with the coax-jacket material and the PVC shell would
> be an issue.
>
UF6 is hideous corrosive stuff ( Teflon was invented as a pump seal for UF6)
used in, primarily, isotopic enrichment processes.. You're thinking Sulfur
Hexafluoride (SF6), very dense (1/2 pound/cu ft!), inert, good insulator for
HV, but sadly, somewhat expensive and hard to get a hold of. They use it in
Gas Insulated Switchgear (GIS) where lack of flammability and packing
density is needed (underground substations in cities, for instance). They
also use it in lab HV gear where close distances and HV are needed at the
same time, and they don't want to fool with oil. I don't think it's much
used for long distance cables, though.
The Freons used to get used in things like HV probes, but are essentially
unavailable now, even on the black/gray market.
Good old mineral oil is probably the best bet... Shell Diala AX transformer
oil is about $4-5/gallon, low viscosity, good electrical properties, etc.
and has reasonably thermal transfer properties, and comes in handy 5 gallon
pails. Make sure you keep it dry!
The problem with ALL fluid dielectrics is leaks... It's insanely difficult
to make a good seal with inexpensive and casual equipment and materials like
PVC pipe and glue, and adequately handle things like temperature and
pressure changes.
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