Sub-nanosecond point-plane gas breakdown in a conical-shaped spark gap

Breakdown with sub-nanosecond delay time is of interest for high-speed low rep-rated switching and for plasma limiters to protect sensitive RADAR equipment from EM bursts. In order to apply fast high-voltage pulses to a test spark-gap without major pulse distortion, we use a coaxial transmission-line connected via a conical section to the spark-gap. Conventional gaps integrated into the inner conductor of a coaxial transmission-line with point-plane geometry suffer from major impedance mismatches as well as lumped capacitances. This limits the pulse risetime at the gap to at least one nanosecond. A gradual reduction of the inner and outer-conductor diameters was used in the conical section. This resulted in a voltage risetime across the gap of about 500 ps, where an original pulser risetime of 400 ps was used. Measured breakdown delay times, as a function of pressure in argon, show delay-time minima of less than 600 ps at a pressure of several torr and applied voltage amplitudes of 10 kV, for radii of curvature < 0.5/spl mu/m. Typical current amplification times, I/(dI/dt), are on the order of several 10/sup -10/ sec. Experiments with repetition rates of up to 1 kHz do not show any differences to single shot discharges at atmospheric pressures. At larger radii of curvature (10 /spl mu/m), corona-type discharges are observed with both point polarities, with currents smaller than 10/sup -4/ A, also without any differences between single shot and 1 kHz rep-rate.