The Townsend theory of dielectric breakdown of gases does not apply at low gas density, where the mean free path of electrons maybe of the order of the dimensions of the discharge chamber. An alternative model of breakdown for this region between true gas breakdown and vacuum was presented recently, and shown to apply in the case of mercury vapour. The mathematical model has been extended to lower pressures than previously, and now includes an analysis of possible secondary mechanisms. The theoretical section is complemented by experimental work, again at lower pressures and for a different gas, namely argon. It is shown that the alternative model of breakdown still applies at the lower gas pressures.

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We describe a flexible 4.5 MJ pulse power supply (PPS) for use as a driver for experimental electrothermal guns. The PPS consists of 18 identical pulse forming modules which can be set to discharge independently at predetermined intervals. The system features 1 % capacitor bank charge repeatability from shot to shot and an ability to generate short and long pulses for each of the modules using a combination of pulse shaping inductors, crowbar resistors and diodes. Fiberoptic isolation for the controls and diagnostics provides a high level of noise immunity from the main gun circuit. Critical components were selected with ample safety margin, to allow for reliable operations and ability to fie-through a gun breech short circuit fault.

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