Inflections of spark leaders in elevated-pressure nanosecond gas discharges

The formation of spark leaders and a diffuse discharge due to the breakdown of gaps with a non-uniform electric field is studied in different elevated-pressure gases under the conditions of runaway electron beam and X-ray generation. Negative voltage pulses are applied to an electrode with a small radius of curvature. For pulse rise times of 0.5 and 1.5 ns, inflections on the spark leaders were observed. In a number of cases, two inflections appear making a right angle to the longitudinal axes of the leaders. It is shown that the formation of spark leaders is preceded by the appearance of diffuse tracks, which bridge the gap for no longer than 1 ns.