PFMA-1: a 1-Hz, 150-kJ pulsed power system for plasma focus generation

PFMA-1 is a 150 kJ, 40 nH Mather-type plasma focus designed for repetition frequencies up to 1 Hz and dedicated to the neutron-free endogenous production of F-18. PFMA-1 utilizes 32 capacitors, 11.1 µF each, charged in parallel at 30 kV. Snowplow calculations predict a 1.5 MA peak current and a 2.5 µs quarter-period for this device. To achieve low inductance and high peak current, one spark gap and four coaxial cables are employed per capacitor. Field-distortion, swing-cascade spark gaps were developed by R. E. Beverly III & Associates for this application (model SG-183). These switches mount directly onto the capacitor's modified Scyllac-type bushing and are enclosed by a low-inductance grounded shield. The rated peak current is 160 kA, the maximum charge transferred is ca. 0.4 C/shot, the self-inductance is 26 nH, and the breakdown jitter is ca. 1 ns at 30 kV and 60 kPa dry air. The switches are cooled by a combination of deionized water, SF6 and dry air. The estimated electrode lifetime is ca. 0.1 Mshots. Special B-dot probes within each switch canister monitor the discharge of every capacitor. Simultaneous triggering of all capacitors is accomplished by a 32-channel PFL generator. Some experimental results and waveforms obtained during preliminary testing of the facility are presented and compared with design values.