Operation of parallel rail-gap switches in a high-current, low-inductance crowbar switch

The Field-Reversed Configuration Heating Experiment (FRCHX) was designed to form closed-field-line magnetized target plasmas for magneto-inertial fusion and other high energy density plasma research. These plasmas are in a field-reversed configuration (FRC) and are formed via a reversed-field theta pinch on an already-magnetized background plasma. To extend the duration and uniformity of the pinch, the capacitor bank driving the reversed-field discharge is crowbarred near the current peak. Four parallel rail-gap switches are used on FRCHX for this application to ensure a low-inductance crowbar discharge path and to accommodate the large magnitude of the discharge current (often greater than 1 MA). Parallel operation of spark gap switches in a crowbarring arrangement, however, has often proved to be difficult due to the very low voltage present on the bank and across the switches at the time of peak current [1]. This paper reports on the successful efforts made to develop a low-inductance crowbar switch for FRCHX and to ultimately enable successful triggering and operation of the four parallel rail-gap switches used in the crowbar. The design of the parallel switch assembly is presented first, followed by a description of the triggering scheme employed to ensure conduction of all four switches.