Re: Improving spark gaps

Hi Jim, All,
    I checked the ieee web site under opera.  They don't have anything
that far back.  It would be very interesting indeed to find where you
found this article.  There might be others there also?

----- Original Message -----
From: Tesla List <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Saturday, May 15, 1999 12:10 AM
Subject: Improving spark gaps

Original Poster: "Jim Lux" <jimlux-at-jpl.nasa.gov>

A useful article that I ran across recently:

MacGregor, S.J., et.al, "Methods of Improving the Pulse Repetition
of High Pressure Gas Switches" in an IEEE publication of some kind
my computer died, taking my bookmark with it, but I'll try to find it
again). They are at the Univ. of Strathclyde, so you Glaswegan coilers
there might want to track these folks down.

They're talking PRFs of 100's to 5 kHz, tens of kV, etc. which is
right up
the TC alley. High pressure means 1 bar (1 atmosphere) The paper talks
the variations in breakdown recovery for both pulsed and DC gaps as
change pressures and gases. All of this is for static (sealed gaps),
although they talk about applicability to flowing gas gaps.

Interesting stuff:
1) They look at the recovery of a gap in Air and in SF6. There is sort
of a
plateau effect. After the gap fires, the breakdown voltage starts to
back up and is at about 50% after 1 mSec, then holds there until about
200-300 mSec, when it comes back to 100%. As you might expect SF6 is a
better than air, but not much (5-10%, reading from the graph in the

2) Changing the pressure changes the height of the plateau, but not
time. At 1.5 bar, the plateau is at 90% of eventual gap strength, but
takes more than a mSec to get there.

3) They did some experiments with mixes of He and SF6, because they
that high thermal conductivity of the He would help, and apparently
blends have been used in flowing gaps. It didn't. It's actually worse
in the
static gap.

The most important thing they found is that putting  a field on the
after discharge helps to "sweep" the ions out (as would fast (i.e.
supersonic) gas flow, like in a Fruengel argon gap). Then, they go
and talk about how you can stabilize a gap, increasing its recovery
rate by
adding corona in a SF6 gap. The corona "inhibits the breakdown in the
until the voltage reaches the required value". They have a histogram
of gap
breakdown times where the corona stabilized version is tightly packed
1 mSec and the unstabilized one is all over from 0 to 2 mSec, with a
hump at <100 uSec. They used a rod/plane gap, which has a lot of
corona, as
opposed to a uniform field gap. Their gap was 20 mm and the rod was 2
mm in
diameter in SF6 at 0.5 bar. Finally, they show a graph of a relaxation
oscillation where the period is 200 uSec (5 kHz) and the voltage on
the gap
is 36kV. I assume from the waveform shown that it is a resonant
charging of
a capacitor (at least that's what the shape of the rise looks like).