Modes of generation of runaway electron beams in gases at a pressure of 1–760 Torr

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 were applied to the tubular cathode - plane anode gap. The SLEP-150 generator, with no transmission line, produced voltage pulses of amplitude ~160 kV across a high-resistance load; the incident wave amplitude in the transmission line was ~150 kV. The pulse rise time was ~250 ps at a level of 0.1-0.9. It is shown that the highest amplitudes of a supershort avalanche electron beam (SAEB) of pulse duration about 100 ps are attained in helium, hydrogen, and nitrogen at a pressure of ~60, ~30, and ~10 Torr, respectively. Decreasing the pressure 1.5-2 times of helium (p <; 60 Torr), hydrogen (p <; 30 Torr) and nitrogen (p <; 10 Torr) changes the mode of the e-beam generation. It is demonstrated that decreasing the pressure 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.