Dynamics and physics considerations of extended duration rep-rate operation of laser triggered gas switches

Summary form only given. The Electra pulse power system at the U.S. Naval Research Laboratory (NRL) uses laser triggered SF₆ gas switches to transfer energy to a high power electron beam diode. These 1 MV switches have a charge transfer of 10 mC per shot and have been well characterized at rep-rates up to 5 pps with 2σ switch jitters of less than ± 2.5 ns, and a switch electrode durability of 200,000 pulses.¹ During long repetitive switch operation, the causes of intermittent misfires are not well understood. The temporal and spatial evolution of the switch dynamics have been examined. A streak camera with a 10 ns sweep records the process from the initial laser triggered breakdown to the onset of switch closure. The captured images indicate fast streamer propagation from the laser breakdown site(s) at switch center towards the cathode. Once that channel is fonned, the streak camera images then show very fast streamer propagation from the switch center towards the anode, which closes the switch. Our observations differ from the dynamics described in a recent model for a PIC code.2 The streak camera images indicate that the switch dynamics are dependent on the repetition rate. For single shots and 1 pps operation (9,000 continuous shots), no misfires (mistimings or nonfIres) are observed in the switch. This observation was concomitant with a consistent and reproducible breakdown position and breakdown dynamics. At 5 pps, variations in the laser breakdown position (preferentially toward cathode) and the laser breakdown emission intensity was observed. After several thousands of continuous shots, misfIres are detected in small groups, before normal operation resumes. We will examine the possible cause(s) of misfIre shots during 5 pps operation and offer mitigation techniques.