Hydrogen spark switches for rep-rated accelerators

The Pulsed Power Technology Branch at NAVSWC is investigating high-power switch technologies for use in rep-rated, high-current accelerators. Switches are needed that can handle 10 kJ of energy, 500 kV, 100 kA, while operating with jitter less than 10 ns at repetition rates up to 10 kHz. In-house efforts have concentrated on spark-gap switches because of their high-voltage and high-current capabilities in single-shot devices and because of their simplicity and low cost. We have shown that hydrogen gas, with its high thermal diffusivity, allows an order-of-magnitude improvement in the recovery time (and, therefore, repetition rate) of an unblown spark-gap switch. Recovery of the switch can be made even faster by triggering the switch well below its self-break voltage, allowing voltage to be reapplied while the gas is still hot. Tests have shown that recovery times (to the operating voltage) can be reduced an order-of-magnitude when the gap is undervolted by approximately 50%. The combination of high-pressure hydrogen gas and undervolted triggering provide a factor of 100 improvement over typical air spark gaps. Recent tests have demonstrated 100-μs recovery of an undervolted hydrogen spark gap without gas flow. High energy tests have been performed at 50 kV, 170 kA and 12 kJ with 100 μs recovery times. High voltage tests with a 5-pulse burst at 500 kV are presently underway. Recovery of the switch appears to be largely independent of voltage and energy transferred.