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Subnanosecond point-plane gas breakdown in a conical-shaped spark gap

Subnanosecond point-plane gas breakdown in a conical-shaped spark gap,10.1109/PPC.2003.1278064,J. W. Spears,H. Krompholtz,L. L. Hatfield

Subnanosecond point-plane gas breakdown in a conical-shaped spark gap  
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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 < 0.5μm. Typical current amplification times, I/(dI/dt), are on the order of several 10-10 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 μm), corona-type discharges are observed with both point polarities, with currents smaller than 10-4 A, also without any differences between single shot and 1 kHz rep-rate.
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