Application of a particle simulation to modeling commutation in a linear thyratron

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Abstract
Commutation is the period during which a thyratron makes the transition between an open state and a conducting state. During commutation the voltage across the thyratron decreases from the holdoff voltage to the conduction value; 10’s of kV to <100 V. The time for commutation, as measured by the anode voltage fall time, is usually 10–100 ns, a value which depends on the holdoff voltage, internal gas pressure, and grid geometry. In this paper, a model for commutation in a thyratron is described and its results are compared to experiment for a thyratron having a linear geometry. The model uses a Monte Carlo particle simulation for electrons and a continuum fluid representation for ions. A particle multiplication and renormalization scheme is used in the model to simulate electron avalanche so that only a moderate number of particles (4000–12000) need to be used. A modified null cross-section technique is used to account for large changes in the density of electron collision partners as a function of position or time. A model for the external circuit enables simulation of current and voltage. Results from the model for these quantities agree well with experiment, and indicate that commutation occurs in two stages. A survey study contrasts the tradeoff between high-voltage holdoff and switching speed.