Electromagnetic Simulations of the ZR Gas Switch

Summary form only given. Electromagnetic simulations of the gas switch proposed for the new ZR accelerator have been revisited. The switch is designed for operation at 6 MV. The original simulations incorporated a model for cascade-gap arc formation that allowed the full cascade to make the transition from insulator to electrical conductor (cascade "run time") in about 9 ns from the time of the trigger-gap breakdown. Recent streak camera data from the cascade section of the ZR gas switch as well as careful analysis of laser-trigger-gap timing and downstream electrical measurements from recent gas switch test shots have indicated a much longer cascade run time of about 35 nanoseconds. This difference was expected to impact the electric field strength that develops on the gas-housing surface at switch closing, affecting switch reliability and lifetime. Two components of the individual gap breakdown time (and therefore of full cascade run time) are incorporated into the prescription for cascade-gap conductance evolution to enable the study of the effect of each: aft, streamer production time, or arc formation time, and cgt, conductivity growth time. Different combinations of aft and cgt that produce cascade run times (14-37 ns) covering the range seen experimentally were investigated. We find that for a given run time, longer cgt lessens the electric field on the inside of the gas housing after switch closing.