Materials and Experimental Discharge Generation

A schematic of the electrical circuit used to generate the discharge pulses is shown in Figure S1. A pulsed power supply was constructed using spark gaps in a configuration similar to those used in other pulse generator systems (1). A direct current (DC) power supply (Bertran Model 205A-50R) capable of 50 kV output and reversible polarity was connected to a RC charging circuit. The DC circuit charges the capacitor until the voltage is sufficient to cause breakdown across the primary spark gap, SG1. The length of SG1 determines the voltage applied to the load. A current transformer (Pearsons model 2877) and voltage probe divider (North Star PVM-4) were used to estimate the current and voltage to the electrode. The voltages and currents measured by the probes may not precisely be that experienced by the electrode due to complications regarding the high frequency components of the pulses, bandwidth of the probes, parasitic losses, and displacement currents. The R, C, and power supply voltage, Vps, determine the repetition rate of the discharge which was typically between 1 and 50 Hz. A second adjustable spark gap, SG2, acts to remove the voltage from the load. Variation of the length of SG2 adjusts the duration of the pulse. If SG2 is longer than SG1, the 2nd spark gap does not fire and the circuit operates in a stepped voltage rather than pulsed voltage mode. The operation of the pulsing circuit is largely independent of the load as only a small fraction of the power goe