Numerical description of pulsed breakdown between parabolic electrodes

A fluid code and a hybrid fluid-particle code are applied to an experimental setup used for breakdown research. The setup consists of two cylindrically symmetric parabolic electrodes, with their tips separated by 3.3 mm in a 3.5-torr argon environment. A voltage pulse was applied with a rise time of 30 mu s, followed by a period at a constant voltage of 350 V. Experimental observations of the light emitted by the discharge showed a stratified prebreakdown light flash in a run-up period. After the applied voltage had risen above the 300-V minimum operating voltage, a light front was observed to appear near the anode, to cross the discharge gap toward the cathode, and to spread over its surface. The prebreakdown flash is modeled with the hybrid model. It is caused by the charges that remained in the volume from previous pulses. The model results reproduce striations in the electron energy and density, which are found to occur due to the specific electric field configuration of the electrodes in the discharge chamber. The crossing of the light front is described with the fluid model. Ionization avalanches that start at the cathode due to secondary electrons cause a space charge that is largest near the anode and starts to affect the electric field there first. This extends the anode potential toward the cathode and is observed as a moving front. The results of both models agree qualitatively with experimental observations.