Fast electrons downstream of a plane grid cathode in a nanosecond discharge in atmospheric pressure air

In this work, an experiment to examine the conditions under which a runaway electron beam moves not only toward the anode but also backward (to the space downstream of the cathode) was performed. For recording a backward runaway electron beam, a positive voltage pulse was applied to the high-voltage electrode of the gas diode. The breakdown of atmospheric pressure air gaps with a special cathode design at a rate of voltage rise of 10¹⁴ V/s was studied with subnanosecond and picosecond time resolution. In the space downstream of the cathode, which was made of thin wires arranged parallel to a thin flat foil, a runaway electron beam (fast electrons) was detected. The current of the fast electron beam downstream of the cathode depended strongly on the anode material. With the Ta anode, the number of electrons in the backward beam increased 4 times compared to that with the Al anode. Measurements of the X-ray exposure dose downstream of the cathode show that for the Ta anode, it was also 4 times higher.