Jon & the group,
The breakdown potential of a gaseous dielectric (air included) is
directly proportional to the gas pressure EXCEPT at high (>100 PSI) and
very low pressures (approaching tube or vacuum capacitor quality).
Dielectric breakdown in this middle range is facilitated by ionization
of the gas molecules due to collision, environmental radiation, and
cosmic radiation. Once breakdown is initiated, if the field strength is
high enough, free electrons may collide with neutral atoms producing more
ions and more free electrons... zap!
A number of gases have better dielectric characteristics than air.
Sulfur Hexafloride with a nominal breakdown rating about 2.5 times that
of air, and often employed under pressure for even greater effect, is a
favorite of the electric power industry.
When very few molecules are present, ionization by collision is
severely limited allowing the characteristic high breakdown of a vacuum
tube (or capacitor). In this "dielectricless" case, an arc is
initiated in small gaps (less than 1 mm or so) by field emission from the
surface of the electrodes and the breakover voltage is highly dependent
on the shapes and the constituent materials of the electrodes.
Curiously, copper has one on the worst characteristics in this regard
(steel is 3 times as good!) but, is used for vacuum capacitors because
of its other electrical properties.
One theory covering larger vacuum gaps states that if the applied
voltage is great enough sporadic free electrons emitted from the
"cathode" are driven into the "anode" with enough energy to liberate more
ions & electrons. The liberated positive ions transit back to the
cathode creating more free particles and both elements thereby contribute
to the mix until a conduction path is established... zap, again.
73,
Marv WC6W
*
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