Electron energy probability function in plasma controlled by high-energy run-away electrons

The electron energy probability function (EEPF) probe measurements in cold plasma controlled by high-energy (0.5–1 keV) run-away electrons generated in an 'open-discharge' configuration are presented in this paper. High plasma stability along with the second harmonic lock-in measurement method provide a sufficiently high accuracy of the EEPF measurements which makes it possible to thoroughly study the features of both cold plasma and the probe method itself. The experiments have been carried out in pure gases: Ar, He, O₂ and H₂. The run-away electron beam is revealed to produce a lot of cold electrons in each gas and the EEPF is Maxwellian with T _e about few tens of meV (which is slightly higher than the gas temperature). It is shown that an EEPF can be correctly measured under these conditions if the modulation amplitude and, accordingly, the energy resolution of the method is less than the electron temperature, T _e. At the same time, integration of the measured EEPF always underestimates the electron density, n _e, due to the 'depletion effect' for low-energy electrons, regardless of the energy resolution of the measurements. It is demonstrated that the open discharge allows obtaining a sufficiently dense and cold, in fact thermal, plasma, which may be of interest for research and various applications. (paper)