High Current, Low Jitter, Explosive Closing Switches

Isentropic compression experiments (ICE) using a high explosive pulsed power (HEPP) system have been developed to obtain isentropic equation of state data for metals at megabar pressures [1][2]. The HEPP system comprises a magnetic flux compressor, an explosively-driven opening switch and a series of closing switches; fast rising current pulses are produced, with rise times of ~500 ns at current densities exceeding many MA/cm. These currents create continuous magnetic loading of the metals under study. The success of these experiments depends on the precise control of the current profile, and that in turn depends on the precise timing of the closing switches to within 50 ns at currents of the order 10 MA and voltages of ~150 kV. We first used Procyon closing switches [3] but found their timing to be unacceptably imprecise for ICE with a jitter of typically 600 ns. We suspected that the switch timing was sensitive to applied voltage; this was subsequently confirmed by experiment, as we will show. A simple shock model was developed to explain the voltage sensitivity of closure time, dt/dV, and from the model we designed a low jitter switch that uses the shock-induced electrical conduction of polyimide. The predicted dt/dV was exactly equal to the measured value, thus confirming the model. This new switch design proved successful and met the 50 ns criterion; it is now used routinely in HEPP-ICE experiments.