Nanosecond volume discharge in air initiated by a picosecond runaway electron beam

A voltage pulse with an amplitude of 250 kV and duration of 1 ns was used to study discharge in atmospheric air. The discharge commences with the emergence of a field emission current from a cathode field enhancer. Next, a beam of runaway electrons with an amplitude of 0.5–1 A and duration of $10^{-11}$ s appears. Interaction between the beam and the voltage pulse was investigated using the reflectometry method. The discharge event was identified by the reversal of the reflected pulse polarity. If the emergence of the runaway electron beam is delayed by a time interval $\Delta t_1$, polarity inversion is delayed by the time $\Delta t_2$=$\Delta t_1$=200 ps. This is due to the small discharge formation time, 33 ps, as a result of the large number (about $10^8$) of runaway electrons that initiate the discharge. The time interval between the beginning of the discharge and the voltage inversion is no longer than 100 ps. This process is theoretically estimated based on the concept of multielectron initiation of discharge. It is shown that what is observed in the experiment is the nanosecond multielectron-initiation discharge produced by runaway electrons of the discharge itself.