Re: water as spark gap dielectric

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

Original poster: "Bert Hickman by way of Terry Fritz <teslalist-at-qwest-dot-net>" <bert.hickman-at-aquila-dot-net>

Hi Jim,

Interesting article! BTW, those who are interested can view a copy at:
http://www.eece.unm.edu/cp3/Publications/Shu.pdf

An external spark gap is still necessary to isolate the water gap while 
energy is being replenished in the energy storage element. For TC use, this 
means that we can't dispense with the air gap in series with the water gap, 
so instantaneous gap losses will be higher. However, if the water gap 
provides superior quenching ability, the additional gap losses may still be 
worth it if we can achieve "one notch" quenching at high coupling coefficients.

The water gap recovery curves don't look very promising for single P-S 
energy transfers. The recovery chart for reasonable bang sizes (Figure 9 in 
the paper) implies that the gap will only recover ~5% of its standoff 
capability after ~250 uSec pretty much independent of power level. If we 
use a relatively low frequency TC (say, 50 kHz with a coupling coefficient 
of 0.22), the time to the first notch is ~2.5 cycles, or ~50 uSec. This 
implies relatively poor quenching...

Best regards,

-- Bert --
-- 
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Tesla list wrote:
>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.
>
>.