New features of the generation of runaway electrons in nanosecond discharges in different Gases

This paper presents new experimental data on the generation of runaway electron beams and X-rays in repetitively pulsed discharges in helium and air with an inhomogeneous electric field. Both negative and positive voltage pulses of duration 1-5 ns and amplitude in the incident wave up to 18 kV were applied to a 'tube-to-plane' gap at a pulse repetition frequency of up to 1 kHz. It is shown that at a helium pressure of several and tens of torrs, whatever the polarity of the main voltage pulse, an electron beam is generated on arrival of reflected negative voltage pulses in the gap. X-rays were detected over a wide pressure range of helium and air, including atmospheric pressure. The breakdown of atmospheric-pressure air gaps with a special cathode design at a rate of voltage rise of 10¹⁴ Vs was studied with subnanosecond and picosecond time resolution. At a voltage amplitude higher than 100 kV, a fast electron beam in atmospheric-pressure air was detected in the space downstream of the cathode, which was made of thin wires arranged parallel to a thin flat foil. The current of the fast electron beam downstream of the cathode depended strongly on the anode material.