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Re: A maximum acceleration ?



On Sun, 30 Nov 1997, brian whatcott wrote:

You are indeed wrong to think that electric breakdown does not occur in the
vacuum. It is intuitively plausible but in fact, the breakdown voltage
is smaller for a given vacuum gap than for an air gap. This was a matter of
considerable practical importance when high power radars were carried to
great heights, where they proceeded to misbehave until their waveguides
were pressurized.

Well, I think this is another case of the conflict between the ideal,
which is what Ludwik is interested in, and the "merely" practical--that
which brian is so adept at reminding us all of.

Clearly, to the extent that "electric breakdown" requires the presence of
a gas, it simply cannot happen in a vacuum. On the other hand, vacuums
are not so easy to come by (except for in the region between the molecules
of a gas ;-) ) and the breakdown voltage of typical gases at moderately
low pressures is far lower than at atmospheric pressure. This is the
result of the increased mean free path length over which ionized particles
can be accelerated and the resulting higher probability that they will
induce additional ionization. As the pressure is further reduced,
however, the relative paucity of targets becomes more important and the
breakdown voltage begins to rise again.

As I recall, low voltage breakdown is not a problem at the pressures found
at typical spacecraft altitudes, but high voltage power supplies are
either kept turned off on the way up or fully "potted" with an epoxy-like
electric insulator to avoid breakdown problems at lower altitudes.

John
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