Scaling laws for sub-nanosecond breakdown In gases with pressures below one atmosphere

With a RADAN 303 A pulser (risetime 150 ps, maximum voltage 150 kV into matched load), fast breakdown in argon and air is investigated. An oil-filled coaxial transmission line is coupled with a lens to a biconical section and a radial millimeter size gap operated at subatmospheric pressure. On the other side of the gap, the arrangement is symmetrically continued to represent a matched load. Pulse risetime at the gap is increased to about 180 ps. With capacitive dividers the voltage across the transmission line separating incident and reflected pulses is measured, which allows to determine voltage across and current through the gap. Temporal resolution is defined by the digitizer (20 Gs/s, 6 GHz). Breakdown usually happens during the rising part of the applied voltage pulse. Breakdown curves, i.e. breakdown voltage or time-to-breakdown vs. pressure, have been measured for different applied dV/dt's (from 2x1014V/s to 8x1014 V/s) and they resemble Paschen curves with a steep increase toward low pressure, and a slow increase toward high pressure. the major findings, such as shifts of the minimum formative time toward increasing pressure with increasing dV/dt, are discussed in terms of similarity laws. Discharges for this case are characterized by runaway electrons over much of the pressure range, with a strong excitation and ionization layer at the cathode surface, and "free-fall" conditions with negligible gaseous ionization for the rest of the gap. Monte-Carlo simulations for the initial stage of the discharge are expected to confirm and quantify the experimental findings