Re: dV/dT

To: tesla-at-pupman-dot-com
Subject: Re: dV/dT
From: dll-at-egg-rb-dot-com
Date: Tue, 17 Sep 1996 08:25:23 -0400 (EDT)
dV/dt definitely effects the breakdown voltage of a gap.
The ratio of Dynamic Breakdown Voltage to Static Breakdown 
Voltage is called the Surge Ratio. 

The physics can be modelled quite simply. 
At atmospheric pressure, the electrons leaving the negative
electrode make multiple collisions with gas molecules
while traversing the gap. As a result, they rapidly achieve
terminal velocity. Breakdown is assumed to occur when
the initial electron cloud including electrons created by 
collision with gas molecules reach the opposite electrode.
Since the electrons are moving at a fixed velocity while 
the applied voltage is increasing, the value of voltage at 
which breakdown occurs will be a function of the rate of 
rise of the applied voltage. 

If you compute this effect, you will find that the Surge Ratio
for frequencies below 1MHz will be less than two.  The effect
doesn't become pronounced until the rise time of the applied
voltage is in the 100 nanosecond range.

It is assumed in this analysis that the gap is fully deionized
before reapplication of the voltage.  If this is not the case,
the breakdown voltage will be substantially reduced.  I believe
that this is the main reason for rotary and/or gas blast gap
configurations.