Electron kinetics in the hot-cathode negative glow of a helium discharge

The electron kinetics in the negative glow of a thermionic-cathode discharg e in helium is discussed based on the results of Langmuir and emissive prob e 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 d istribution 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 t he experimental results obtained for a thermionic current corresponding to a cathode fall voltage slightly above the helium ionization potential, a si mplified electron kinetic model using the Monte Carlo flux simulation is pr oposed to obtain the secondary-electron energy distribution. Since the axia l profile of the measured plasma potential shows a potential well, which co rresponds to a maximum in the negative glow plasma density, the simulated s econdary-electron energy distribution can be obtained mainly as a result of the accumulation of low-energy electrons yielded from inelastic collisions .