The interior of the
gunsling relay and
consists of either a vacuum or a helium gas at a very low pressure.
If these contraptions are surrounded by dense atmosphere, they must
be enclosed in a vacuum chamber. When the projectile leaves the
chamber via an explosively driven valve, a large volume of air leaks
in before the valve is closed. To reduce the leak, plasma is placed
between the chamber and the valve. The low density and high viscosity
of the plasma reduce the leak by 3 orders of magnitude. The plasma
window lasts a fraction of a second -- long enough to open and close
the valve and yet short enough to draw power from a large
electromagnet. Plasmatron and shaped charge detonation may prove
to be less expensive methods of making plasma than electrodes used
by Ady Hershcovitch.
"High-pressure arcs as vacuum-atmosphere interface and plasma lens
for nonvacuum electron beam welding machines, electron beam melting,
and nonvacuum ion material modification," Journal of Applied
Physics, Vol. 78, No. 9, November 1 1995, pp. 5283-5288.
Web page at Brookhaven National Laboratory.
Discover Magazine, February 1997, p. 20 (Second half of the paper
issue is missing.)
Plasmatron at von Karman Institute in Belgium.