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

In this 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). For recording a backward runaway electron beam, a positive voltage pulse was applied to the highvoltage electrode of the gas diode. The breakdown of atmospheric pressure air gaps with a special cathode design at a rate of voltage rise up to 1 MV/ns 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, the fast electrons were 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.