Runaway electron beam generation and disruption at pulsed gas discharge

Summary form only given. Runaway electron beam generation arises at pulsed electrical discharge in gases with a strong enough overvoltage. Recent experiments with fast sub-ns table-top power supply with voltage rise rate of about 2 MV/ns, have demonstrated production of fast picosecond electron beams accelerated in runaway mode. A two-step model has been proposed to describe the picosecond runaway electron beam generation and disruption at such a fast device with strong overvoltage. First step is the charged particle generation during the low-voltage ns prepulse, which results in the streamers formation near the cathode edge. Second step is the acceleration of the electrons by the front of the main high-voltage pulse. One can find that there is an increase in the ionization frequency for the decreasing of gas pressure from 1 atm to 0.1 atm. This results in an enhancement in the pre-pulse streamer growth, thence one can expect increase in the picosecond runaway electron beam current at a reduced pressure. The runaway electron beam disruption within ps duration has been interpreted by a beam instability developing with growing plasma oscillations in a dense plasma. It has been experimentally found that there is a fast (tens ps) termination in the `primary' runaway electrons beam and there is a wide consequent runaway electron avalanche, having several times lower energy. This likely confirms our proposition on the disruption mechanism. One would expect a decreasing in the duration of the runaway electron beam for a reduced pressure, as both plasma formation time and instability growth time reduces.