Re: Electric Breakdown

[David Bowman <dbowman@tiger.gtc.georgetown.ky.us>]

David Dockstader wrote:
> Earlier in this discussion the comment was made that there is an optimum
> pressure for breakdown.  More or less pressure requires higher voltage.
> Can anyone elaborate on why this is true?  The increase in voltage at the
> low pressure end seems reasonable enough but why an increase at the high
> pressure end?  I always wondered why an engine that runs perfectly at mid
> throttle would miss at full throttle.  This seems to offer a potential
> explaination, but before I jump to this conclusion I'd like to understand
> it better.

If the pressure is so low that the particle mean free path length is much
longer than the distance between electrodes at different potentials then
the electric field between the electrodes cannot do much to ionize the
intervening gas.  At most the gas particles may polarize somewhat and drift
in response to any inhomogeneity in the field.  If they strike one of the
electrodes with much kinetic energy they might liberate an electron on
impact which may provide a very weak background current.  If a hapless ion
drifts into the intervening region it will quicky be accelerated to the
appropriate electrode and contribute to a very tiny background current.  The
gas particles themselves do not ionize and full breakdown cannot occur
(mostly due to a carrier shortage).  If OTOH, the pressure is very high and
the interparticle mean free path is a tiny microscopic distance then the
breakdown field will be very high since in order for breakdown to occur a
hapless ion needs to be accelerated to a sufficient kinetic energy to cause
further ionization of a neighboring gas particle when the ion collides with
the neutral particle in its path.  Therefore the breakdown field will be of
the order of the ionization energy of the gas particles per mean free path
length.  When the pressure is high the mean free path length is low and the
breakdown field strength is therefore high.  When the pressure is lower the
mean free path length is longer and the breakdown field strength is then
lower since in this case the mean free path length is longer and any ions
are free to pick up sufficient kinetic energy over a longer distance before
their next collision to be able to initiate the breakdown chain reaction.

David Bowman
dbowman@gtc.georgetown.ky.us