Electron beam generation in nitrogen and helium at a low voltage on a gas diode

Abstract

In [1][2][3][4][5][6] (see also reviews [7][8][9] and the references therein) it was reported that electron beams were generated behind the foil of gas diodes filled with gases under increased pressure. In these studies, voltage pulses with amplitudes greater than 100 kV were applied to the gap. The electron beam was generated both in the leading and trailing edges of the voltage pulse when the gap conductivity increased [7][8][9]. The last measurements have demonstrated that the duration at half maximum of the runaway electron beam current was ~0.1 ns under a pressure of various gases of 1 atm [4][5][6]9].Investigations of x-ray radiation of gas diodes [10,11] and electron beams behind the grid anode [12,13] at small amplitudes of voltage pulses ≤20 kV are also known. However, oscillographs with comparatively low time resolution were used in [10][11][12][13]. In addition, the region of electron beam generation and the collector were divided by a grid at low voltages on the gas diode, while it is well known that it is better to use a thin foil to extract the runaway electron beam. The last circumstance influenced the accuracy of measuring the electron beam parameters.The purpose of the present study is to investigate the parameters of a runaway electron beam in nitrogen and helium at a low gap voltage (~25 kV) and to determine the beam generation delay time and the beam duration under these conditions. During investigations, the electron beam parameters were measured behind a metallized film, and the time resolution of the collector and oscillograph was no worse than 0.1 ns.The experimental setup comprised a discharge chamber and a cable pulse generator [14]. The design of the discharge chamber was described in [15]. The inside diameter of the chamber was ~50 mm. Two electrode configurations were used. The first electrode configuration was formed by the flat anode and the convex cathode made from stainless steel with a deposited magnesium coating. The cathode diameter was 20 mm, and its radius of curvature was 12 mm. The second electrode configuration was formed by the flat anode and the cathode with small radius of curvature, which resulted in the electric field gain near the cathode. The cathode from a titanic foil 50 µm thick had a small radius of curvature and was shaped as a tube ~5 mm in diameter. To measure the discharge current, the flat anode was connected to the chamber housing through the shunt consisting of 60 film chips-resistors with resistance of 3.3 Ω each. To register a runaway electron beam, the anode was formed by two metal grids with a transparency of 64% and a metallized film (with a deposited aluminium layer 0.2 µm thick) made from kimfol (C 16 H 14 O 3 ) 2 µm thick or an aluminum foil 3 µm thick. The distance between the cathode and the anode was 6 mm (the first electrode configuration) or 12 mm (the second electrode configuration). The collector with time resolution no worse than 0.1 ns was placed at a distance of 9 mm behind the anode formed by the grids and foil.Some experiments were car...