Re: Corrected di-el strength of gas

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Bert Hickman <bert.hickman-at-aquila-dot-net> 

Hi John,

Obviously the spark gap must be completely immersed in a pressurized pure 
hydrogen atmosphere to avoid other "interesting" effects. The combustion 
hazard is eliminated by thoroughly purging the chamber of residual oxygen 
prior to firing (possibly via an N2 purge). While typical recovery times 
for common dielectric gases such as air, nitrogen, argon, and SF6 is of the 
order of 10 milliseconds, hydrogen recovers about one order of magnitude 
more quickly - about 1 millisecond, and under the right circumstances, 
recovery times can be pushed down to as little as 100 usec even when 
switching multi-kilojoule energy levels.

For example, in the April, 1991 issue (volume 38, Issue 4) of IEEE 
Transactions on Electron Devices, Stuart Moran and Leonard W. Hardesty from 
the Naval Surface Warfare Center reviewed their work using a pressurized, 
unblown hydrogen triggered spark gap (trigatron) with copper-tungsten 
(Elkonite) main and trigger electrodes. They were easily able to achieve 
recovery times of ~1 millisecond, and were able to reduce this to 100 usec 
without forcing gas through the gap. Best results were obtained by 
undervolting the gap (using ~50% of the self-breakdown voltage) combined 
with high trigger voltages. Pressures were varied from atmospheric up to 7 
MPa (~1000 psig). Although the power levels were MUCH higher than those 
used in TC's - they were able to achieve 100 usec recovery in a trigatron 
that handled a 50 kV 170 kA current pulse (12.5 kJ!) by using a trigatron 
pressurized to 400 psig of H2 - without using flowing gas.

More recent work by a team at the Prather Air Force Research Laboratory 
(IEEE 12th Pulsed Power Conference, 1999) confirms similar results using a 
sealed pressurized H2 trigatron. The device can switched 70 Joule pulses at 
600 Hz (>40 kW!) with no gas replenishment over a 1 year period.

Looks like hydrogen (or deuterium) is the king for rapid quenching... :^)

Best regards,

-- Bert --
-- 
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Tesla list wrote:

>Original poster: "john cooper" <tesla-at-tesla-coil-dot-com>
>Hi Bert:
>Maybe I'm not following this too well but how in the world is a flammable 
>gas, i.e., hydrogen, used in a spark gap?  Totally contained?
>Under pressure?  I suppose it would need oxygen to burn.  When I picture 
>blasting a stream of hydrogen into an uncontained spark gap I also see a 
>flame thrower, maybe it's just me but I've no exposure or experience with 
>the devices you've mentioned so please excuse my naivete'.  They do sound 
>strangely compelling though.
>John
>
>---------- Original Message ----------------------------------
>From: "Tesla list" <tesla-at-pupman-dot-com>
>Date:  Tue, 04 May 2004 12:59:31 -0600
>  >Original poster: Bert Hickman <bert.hickman-at-aquila-dot-net>
>  >
>  >Hi John,
>  >
>  >Thanks for the table. However, there's apparently more to quenching than
>  >can be gleaned by comparing relative dielectric strengths. Although
>  >dielectric strength will provide an indication of the voltage standoff,
>  >this parameter does not indicate how quickly a previously conducting spark
>  >gap will recover its dielectric strength (i.e., how well it will quench).
>  >
>  >For example, although hydrogen has only about half the dielectric strength
>  >of nitrogen, it recovers more quickly from a plasma state to a
>  >non-conductive state, giving it superior quenching ability. Hydrogen's
>  >small molecules have a higher molecular speed so that heat can be removed
>  >more quickly from a recovering gap. Hydrogen was used for high performance
>  >multiple-gap switches for spark radio, and it's sometimes even used today
>  >for special high-power high reprate spark gaps. Hydrogen and deuterium are
>  >also the fastest "fill" gases available for high speed thyratron switches,
>  >the low pressure evolutionary cousins of spark gaps.
>  >
>  >Best regards,
>  >
>  >-- Bert --
>  >--
>
>
>
>.