RE: water as spark gap dielectric

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

Original poster: "Lau, Gary by way of Terry Fritz <teslalist-at-qwest-dot-net>" <Gary.Lau-at-hp-dot-com>

Intriguing, but my understanding of using DI water as a dielectric is that 
the duration of the pulse must be brief, or else the water will break 
down.  In a TC gap, the gap voltage ramps from zero to the breakdown value 
at a lazy 60 Hz rate.  Does my understanding require revision?  I hope so, 
this sounds like a great way to improve gap efficiency!

Gary Lau
MA, USA

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Original poster: "Jim Lux by way of Terry Fritz <teslalist-at-qwest-dot-net>" 
<jimlux-at-earthlink-dot-net>

I just ran across an interesting paper by Xiao, et.al., titled "Recovery of
Water Switches" where they look at using DI water as the dielectric in a
spark gap (and as the dielectric in a Blumlein pulse generator).  The
interesting thing from a TC standpoint is this:

1) DI water has good dielectric properties in short gaps. They reference
1MV/cm, so a TC gap at 20kV would be on the order of 0.5 mm (0.02"). Since
gap length is the big factor in gap loss, this might greatly reduce the
losses in the spark gap, compared to more conventional air spark gaps, which
are 10-20 times bigger.

2) The recovery time for their gap was greatly improved by moderate water
flows through the gap. They used flows on the order of 1 m/s, which is not
much flow through a tiny gap.

Their results showed 1 kHz rep rates at 30kV kinds of levels, and energy
deposition into the gap of 1.8Joule/cm for a 0.3mm gap  At 4J/cm, they got
600 Hz reprates.  The typical TC might be somewhat higher energy into the
gap, in air, but it's possible that with the much shorter gap, you might get
to these kinds of levels.

They got best results with an annular gap (where you feed the water in
through the middle of the electrode) in a hemisphere against plane kind of
gap.  For a TC, maybe something like a piece of copper pipe with a very
carefully trimmed end against a flat copper plate with a 20 mil gap all the
way around. This would give you a lot of area for the spark to distribute
the energy around.