Spatial Variations of Plasma Parameters in a Hollow Cathode Discharge
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Spatial Variations of Plasma Parameters in a Hollow Cathode Discharge S. N. Andreev1 · A. V. Bernatskiy1 · V. N. Ochkin1 Received: 4 June 2020 / Accepted: 23 October 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract The transformations of the electron energy distribution (EEDF), their concentration, and plasma space potential along the discharge gap between the hollow rectangular cathode and the mesh anode were experimentally studied. The discharge was 3 cm long, with a cross-section of 10 cm2. A new version of measurements with several single probes with the formation of current-voltage characteristics (IVC) in the probe circuit was proposed with the simultaneous application of voltages in the form of a combination of periodic and noise signals. The proportions of the signals were varied for different sections of the current-voltage characteristics. Measurements along the central axis of the discharge were taken. The dynamic range of the EEDF was not less than 4 orders of magnitude at the electron concentrations of 2–13 × 1010 cm−3, which exceeds the best known achievements. Measurements for discharge in helium at reduced pressures of 1–1.2 mbar and currents of 150–400 mA showed that the EEDFs differ from Maxwell ones, with an excess of fast electrons in the region of 10–20 eV at medium energies 4–6 eV. The fraction of fast electrons decreased in regions closer to the anode, which is associated with the nonlocality of the mechanism of the spectrum formation of free electrons. EEDFs transformations led to the space dependence of electron drift velocities on the plasma area. The dependence of the voltage drop across the cathode on the gas pressure and discharge current was noted. Keywords Low-temperature plasmas · Langmuir probes · Probe voltage waveforms · Dynamic range at non-equilibrium · Hollow cathode · Near-wall reactor plasma · Electron parameters in space · Non-locality
* A. V. Bernatskiy [email protected] S. N. Andreev [email protected] V. N. Ochkin [email protected] 1
P.N. Lebedev Physical Institute of the Russian Academy of Sciences, 53 Leninskiy Prospekt, Moscow 119991, Russia
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Plasma Chemistry and Plasma Processing
Introduction Nowadays, the problem of monitoring the chemical composition of the near-wall plasma of fusion reactors is of great interest [1, 2]. One of the promising approaches is associated with the development of new methods of quantitative emission spectroscopy [3–5]. To obtain absolute particle concentrations in a nonequilibrium low-pressure plasma, calibrations using a combination of spectral and probe measurements are proposed [6, 7]. The physical and methodological problem that arises, in this case, is associated with the fact that the emitting states of particles are excited by electrons with energies of 10–30 eV, and their average energies under such conditions are 4–8 eV. At the same time, the analysis of plasma-chemical processes in which the complex composition of small components is established re
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