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Gas breakdown in quasi homogeneous electric fields with amplitudes of up to 3 MV/cm is investigated. The setup consists of a RADAN 303 A pulser and pulse slicer SN 4, an impedance-matched oil-filled coaxial line with a lens-transition to a biconical line in vacuum or gas, and an axial or radial gap with a width on the order of mm, with a symmetrical arrangement on the other side of the gap. Capacitive voltage dividers allow to determine voltage across as well as conduction current through the gap, with a temporal resolution determined by the oscilloscope sampling rate of 20 GS/s and an analog bandwidth of 6 GHz. The gap capacitance charging time and voltage risetime across the gap is less than 250 ps. Previous experiments at TTU with a slightly larger risetime have shown that breakdown is governed by runaway electrons, with multi-channel formation and high ionization and light emission in a thin cathode layer only. In argon and air, time constants for the discharge development have been observed to have a minimum of around 100 ps at several 10 torr. A qualitative understanding of the observed phenomena and their dependence on gas pressure is based on explosive field emission and gaseous ionization for electron runaway conditions.

The x-ray emission of highly overvolted spark gaps under electron runaway conditions is investigated. The pulse source, a RADAN 303 A, is connected to a test chamber through an oil-filled coaxial line, a coupling lens, and a biconical transmission line section, with a symmetrical arrangement attached on the opposite side of the chamber with a matching load. The test chamber allows pressure variation from 10-6-670 torr with argon or dry air used as a background gas. Voltage pulses with amplitudes of 40-150 kV, risetimes less than 200 ps, and FWHM less then 300 ps are applied across hemispherical electrodes with 1 mm spacing. A scintillatorphotomultiplier combination with a temporal resolution of 2 ns is used as X-ray detector. Metallic absorber foils of different thicknesses are used to obtain a rough energy spectrum of the x-rays and electrons in the range of about 10 to 150 keV. Results show a high electron-energy component (>60 keV) existing up to atmospheric pressure, and an intense soft component (5 to 20 keV) at pressures around 100 torr. The observations are compatible with gaseous ionization and runaway conditions for extremely high E/p.

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