Modes of generation of runaway electron beams in gasis at high pressure

Summary form only given. The characteristics of runaway electron beams downstream of a foil anode were studied at high pressures of helium, hydrogen, neon, and nitrogen. 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 ~50 kV The pulse rise time was ~250 ps at a level of 0.1-0.9 and the FWHM of the voltage pulse with a matched load was ~1 ns. The use of the transmission line in the SLEP-150 generator allowed recording the incident and reflected voltage waves and to recover the voltage across the gap during the generation of the electron beam. The RADAN-220 generator produced voltage pulses of amplitude 250 kV, FWHM with a matched load 2 ns, and rise time 0.5 ns. It is shown that the highest amplitudes of a supershort avalanche electron beam (SAEB [1]) 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. Using the extracted data, estimates show that with the generators used, optimization of all parameters of the gas diode makes possible a SAEB current amplitude of about 500 A at a helium pressure of 60 torr. As the mode of the beam generation is changed due to the decrease in pressure from the above values, one can gradually control the - WHM of the beam current pulse between 100 and 500 ps in all gases studied. In this case, the maximum amplitude of the beam current is about 1 kA and is greater than the maximum SAEB amplitudes. At low pressures of helium, hydrogen and nitrogen in the gas diode, the amplitude of the beam current decreases and its duration continues to increase to over the duration of the generator pulse with the matched load.