Ultrafast gas breakdown at pressures below one atmosphere

Abstract

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.

Subnanosecond Breakdown of Insulating Media

“…incident and reflected voltage on the input side, and transmitted voltage at the output side, allow the determination of gap voltage and gap conduction current, and especially their temporal correlation [23,24]. The accuracy of this determination is limited by noise following the main pulse from the RADAN, and by reflections on the transmission lines, especially at feedthroughs, used in the second-phase setup with standard transmission lines.…”

“…The decay time constant of this system is about 2 ns, and the x-ray measurements have to be considered time integrated. Details are described in the pertinent publications [23,24].…”

“…They show that relatively hard x-rays (E> 30 keV) are emitted for pressures up to 150 torr. An improved setup with largely reduced noise level was used to measure the x-ray emission for setup # 3, for axial discharges, with results shown in [24]. These measurements show the extension of hard x-ray emission (> 50 keV) to almost atmospheric pressure.…”

“…This setup represents the achievable optimum with respect to minimizing pulse distortion and still providing accessibility for a variety of diagnostics methods. [24] …”