Positive and negative streamers in air and nitrogen in a sharply inhomogeneous electric field under conditions of runaway electron generation

The development of positive and negative streamers in a point-to-plane gap filled with air and nitrogen at various pressures(50-200 kPa)and voltages(8-25 kV)was studied.A four-channel intensified charge-coupled device(ICCD)and a streak-camera were used.Electrical parameters were measured with high resolution(10 GHz).An original method of measuring a displacement current caused by a streamer was applied.As was expected,positive streamer branches in nitrogen at low voltages and/or elevated pressures while a large-diameter streamer is formed in air.However,at high voltages or negative polarity,the large-diameter streamer is formed both in nitrogen and air in the entire pressure range.It was found that runaway electrons(REs)are generated in the very first picoseconds of gas ionisation near the pointed cathode.It was assumed that REs can be generated near the pointed anode and produce bremsstrahlung radiation due to a sharply inhomogeneous distribution of electrical potential.It was found that in the final stage of negative streamer development in air and nitrogen,the gas between the streamer front and the opposite electrode is ionised almost simultaneously in the entire volume when the streamer diameter is almost equal to the interelectrode gap.