Modes of Generation of Runaway Electron Beams in He, $\hbox {H}_{2}$ , Ne, and $\hbox {N}_{2}$

In this paper, the characteristics of runaway electron beams downstream of a foil anode were studied at a pressure of helium, hydrogen, neon, and nitrogen of 1-760 torr. High-voltage pulses (~150 and ~250 kV) with pulse rise times of ~300 and ~500 ps were applied to the tubular cathode-plane anode gap. It is shown that the highest amplitudes of a supershort avalanche electron beam (SAEB) of 100-ps pulse duration are attained in helium, hydrogen, and nitrogen at pressures of ~60, ~30, and ~10 torr, respectively. It is demonstrated that further decreasing the pressure changes the mode of generation of runaway electron beam and increases the beam current amplitude and the voltage pulse duration across the gap. It is found that increasing the pressure of helium, hydrogen, and nitrogen to hundreds of torr decreases the delay time between the instants the voltage pulse is applied to the gap and the SAEB is generated, as well as the maximum voltage across the gap.