Triggered Gap

From:  Jim Lux [SMTP:jimlux-at-earthlink-dot-net]
Sent:  Saturday, January 31, 1998 10:39 AM
To:  Tesla List
Subject:  Re: Triggered Gap

> I am building a small Oudin coil driver and was thinking about trying a 
> triggered gap, input voltage is low at 200-600 VDC so the only other
> is a Triac or SCR, I will try a three terminal gap with the center 
> electrode used as the trigger, EHT pulse from an 8kV xenon flash Tx.  The

> coil wattage is low at < 100W.
> I think a triggered gap may quench better as the gap spacing is much
> than normal for the high current path.  I assume the gap spacing is
> dependant on the triggering voltage?

Typically, the gap is set so the static breakdown voltage (i.e. w/no
trigger) at 2-3 times the operating voltage. Make sure you have enough
energy in the trigger pulse.
There are several general configurations for the triggered gap.
One scheme has a small trigger electrode in the center of one of electrodes
in sphere gap (or really a hemisphere gap). Automotive spark plugs are
popular (grind the ground electrode off and the center electrode flush with
the insulator and the threaded part of the plug. A standard auto spark coil
provides the trigger pulse.

Another has the third electrode midway between the two main electrodes,
either 1/3 or 1/2 of the way between. Overvolting half the gap (by applying
HV to the trigger) breaks it down, which then makes the remaining gap
overvolted, which then breaks down (called a swinging cascade). 

yet another uses a third electrode between the main electrodes to distort
the field causing the local gradient to exceed the 30 kV/cm breakdown.

Check out EG&G's web page on triggered spark gaps for some more information
(http://www.egginc-dot-com). Maxwell Labs (http://www.maxwell-dot-com) also has a
nice brochure on triggered gaps. Finally, you might check out a paper by J.
Pasley (http://home.earthlink-dot-net/~jimlux/hv/pasley1.htm) on pulse
switching techniques.