Dynamics of Laser Triggered, Gas-Insulated Spark Gaps During Repetitive Operation

The Electra pulsed power system at the U.S. Naval Research Laboratory uses laser triggered SF6 gas-insulated spark gaps to transfer energy to a high power electron beam diode. These 1-MV spark gaps have a charge transfer of 10 mC per shot operating at 5 pules per second (pps) utilizing electrodes with the potential durability of 300 000 pulses. During continuous repetitive switch operation, after a few thousand shots intermittent nonfire events occur with greater frequency approaching 100 000 shots. The nonfire events and the lack of reproducibility and reliability reduce the potential applications for laser triggered spark gaps. Therefore, the temporal evolution and the spatial evolution of the switch dynamics have been examined. Gated camera images, streak camera images, and interferometry are used to characterize the dynamics of single shots to directly compare to the dynamics during continuous operation. The streak camera images indicate that the switch dynamics are dependent on the repetition rate. For single shots and 1 pps operation (9000 continuous shots), no misfires (mistimings or nonfires) are observed in the switch. This observation is concomitant with a consistent and reproducible breakdown position and breakdown dynamics. At 5 pps, the variations in the laser breakdown position (preferentially toward cathode) and the laser breakdown emission intensity are observed. After several thousands of continuous shots, nonfire events are detected in small groups, before normal operation resumes. A temperature differential of at least 22.2 K (40 F) between the SF6 gas at the top of the housing and the bottom of the spark gap housing is observed. The onset of nonfire events is correlated with the hydrodynamic changes, from increased temperature, of the SF6 gas flow within the spark gap. A redesign of the spark gap along with adequate cooling to reduce the hydrodynamic flow and forces of the insulating SF6 gas would mitigate if not eliminate nonfire events.