Runaway electrons at the formation of a positive ionization wave in nitrogen and air

The generation of runaway electrons (REs) at a subnanosecond breakdown of a “plane–needle” gap caused by the development of a positive ionization wave starting from a grounded needle electrode (anode) has been studied experimentally for the first time. Using a four-channel ICCD camera, a Hamamatsu streak camera, and an original method for measuring the displacement current generated by an appearing and propagating ionization wave, the generation of REs has been studied together with the dynamics ionization waves in air and nitrogen at pressures from 20 to 100 kPa. Current pulses of REs shorter than 100 ps have been observed in air at pressures of 60 kPa and below and in nitrogen in the entire pressure range. Double current pulses of REs have been observed in both gases at pressures below 50 kPa. It has been established that the generation of REs occurs after the arrival of the ionization wave at the planar cathode rather than at its start near the needle electrode, as could be expected. The energy of REs under these conditions is lower than the breakdown voltage by a factor of 4 or more. The data obtained indicate that REs are generated in the cathode layer.