Experimental investigations of subnanosecond gas breakdown

The recentiy developed concept of plasma limiters requires breakdown in gas with subnanosecond delays. If subnanosecond breakdown can be achieved in a transmission line or waveguide, most incoming power could be reflected, protecting any circuitry on the other side. This experiment consisted of a study of breakdown in gas using a needle tip to plane geometry with high voltage pulsers, ranging from 1 to 7.5 kV, with a typical risetime of 500 ps. The setup consists of a pulser feeding a 50 U transmission line to a needle-plane test gap inside a vacuum chamber, terminated in a 50Q load line. The needles used have a tip radius of less than 1|xm to produce a high electric field enhancement at the test gap. Measurements were taken from current and voltage sensors on the input transmission line, and the luminosity of discharge was measured with a photomultiplier tube. For voltages of 1.7 kV and below, a minimum breakdown delay of 12 ns was measured. At 15 kV, achieved with a 7.5 kV pulser with the voltage doubling at an open circuit at the test gap, subnanosecond breakdown delays were observed. The pressure was varied from 10^-6 Torr to 600 Torr to obtain breakdown delay characteristics according to pressure. The results of the fastest breakdown observed point toward a vacuum type discharge with field emitted electrons as the dominant breakdown mechanism, with a certain pressure of gas required to sustain the discharge across the test gap.