Material testing on high voltage laser triggered gas switches for ZR

Abstract

The Z machine at Sandia National Laboratories is a thirty six module pulsed power driver utilized for the study of inertial confinement fusion, isentropic compression experiments, and high density physics. Currently it is undergoing an upgrade, called Z-Refurbishment (ZR). The upgraded Z pulsed power driver requires thirty six gas switches to be capable of low jitter high voltage switching, to deliver energy to the load. The switches must remain open as voltage rises in ∼one microsecond, then close with a few nanosecond jitter upon arrival of the laser pulse. Switch performance is directly related to component materials since switches must routinely withstand a 6.25 MV, 750 kA pulsed power environment and perform reliably upon each command fire. Switch lifetime is primarily influenced by insulator flashover and electrode degradation. Early in the program the most high profile problem was random flashing of the insulator housing. Triple point shielding, cleaning procedures and an isolation window that separated the gas switch volume from the laser can volume were implemented which reduced housing flashes, to problems attributed to material debris. Electrode materials were studied in an attempt to optimize switch lifetime with respect to erosion rate, housing flashes associated with material debris and to reduce degradation of laser optics that are in close proximity to the switch. Theories on electrode ablation have contributed it to enhancing fields in the trigger section and flashing of the cascade housing. Electrode materials investigated included, tungsten-copper, stainless steel, molybdenum, tantalum and brass. SEM imaging was utilized to examine effects of arc damage for different materials. SEM imaging is also being used in attempts to understand preconditioning of electrodes and early shot switch performance.