Electron Kinetics in the Hot-Cathode Negative Glow of a Helium Discharge

Abstract

The electron kinetics in the negative glow of a thermionic-cathode discharge in helium is discussed based on the results of Langmuir and emissive probe measurements performed in the axial direction of a 5-cm-long discharge at 0.65 Torr helium pressure and low discharge current. The electron energy distribution function shows two groups of electrons: the primary ones, which are emitted by the cathode and accelerated into the cathode fall, and the secondary ones, which are produced by inelastic collisions between primary electrons and neutral atoms. The cathode fall voltage and primary-electron energy show a strong dependence on thermionic current intensity. Based on the experimental results obtained for a thermionic current corresponding to a cathode fall voltage slightly above the helium ionization potential, a simplified electron kinetic model using the Monte Carlo flux simulation is proposed to obtain the secondary-electron energy distribution. Since the axial profile of the measured plasma potential shows a potential well, which corresponds to a maximum in the negative glow plasma density, the simulated secondary-electron energy distribution can be obtained mainly as a result of the accumulation of low-energy electrons yielded from inelastic collisions.