A New Pre-Ionization Technique for the HJ1 Coaxial Plasma Gun for PJMIF*

We are exploring a new technique for preparing and pre-ionizing the initial gas slab before turning on the main discharge in a coaxial plasma gun for driving PJMIF. The present pre-ionization scheme used an annular array of capillary discharges driven by a capacitor storing up to 1.5kJ of energy and tend to produce long jets. In the new scheme, a capacitor storing about 50 J of energy applies a voltage across the gap between the electrodes. As the working gas (e.g. Ar) is introduced into the gap, a sharp trigger voltage is given to an annular array of 20 tungsten pins positioned in the gap to emit electrons by field-enhanced emission, in order to facilitate the Townsend avalanche. The Townsend discharge is appropriately limited by external circuit impedance to control its growth rate to allow time for the avalanching electrons to diffuse annularly. When the annular spread of the glow is complete the main discharge to the gun is turned on.