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

Summary form only given. In our work, we performed experiments 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). In the experiments, a SLEP-150 generator was used which allowed us to obtain 6·10¹⁰ electrons in atmospheric pressure air downstream of an Al foil anode 10 m thick [1]. The design of the SLEP-150 generator is described in [1, 2]. For recording a backward runaway electron beam, a positive voltage pulse was applied to the high-voltage electrode of the gas diode. The amplitude of the voltage pulse in the incident wave was about 150 kV and its FWHM was about 1 ns. The voltage pulse rise time was determined by a peaking switch and was 0.3 ns.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 fast 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. The conducted experiments on the SLEP-150 generator with positive voltage polarity allow the basic conclusion that the fast electrons recorded downstream of the cathode are due to the generation of a certain amount of electrons (10% by estimate) whose energy under these conditions is markedly higher than eUm. Here, Um is the maximum voltage on the gap. These results agree well with the data reported in [3].