Re: secondary core other than air

["Mark Fergerson" <mfergerson1-at-home-dot-com> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

  I thought the problem with ferrite/whatever cores was increasing the
internal arcing (coil-to-core) likelyhood. Since materials with mu>1
tend to be more or less conductive, the idea sounds counterproductive
in principle.

  I^2R heating of the core looks likely (a Bad Thing), too.

  Not to mention screwing up the "isolated coil" assumption in various
equations for predicting L, Q, distributed C, M, and so on. I'm trying
to design an enclosure (for gas pressurization/vacuum tests) for a
miniature coil, and it's giving me fits. Conductive shell means
non-isolated. Non-conductive is a relative term at these voltages, and
burn tracks are no fun.

  I'm starting to think up silly stuff like a non-conductive inner
pressure/vacuum shell wrapped with mumetal or copper tape, to prevent
unwanted arcing while keeping the field from "reaching through" the
enclosure. You can probably tell I'm getting desperate here...

  Anybody got any data (or even good guesses) on how large relative to
coil dimensions a conductive shield should be to avoid "shorted-turn"
trouble (besides too far for easy arcing)? RF coil shields in radios
don't look like a good model due to the modest ampere-turns in that
application and the skin effect difference at lower (TC) frequencies.

  While I'm at it, can a magnifier be wound on the same form as the
secondary? Well, of course it can, but I s'pose what I really mean is
what separation is required to prevent secondary/magnifier coupling? I
get the idea that they shouldn't be (or more properly don't need to
be) magnetically coupled from looking at A. C. M. de Querioz's papers
on the subject. I want to put the whole shebang into the enclosure.

  I'm willing to live with, or even use, the coil-to-shell capacitance
as part of the final load. I just need some general guidance on the
spacing to shoot for.

  Mark L. Fergerson