Artiushenko K. Physical processes in near-electrode sheaths and plasma of glow and radio frequency capacitive discharges.

The thesis is devoted to studying the processes in the sheaths adjoining the electrodes (the cathode sheath of the dc glow discharge and near-electrode sheaths of the RF capacitive glow discharge), as well as within the plasma region (in the positive column of the dc glow discharge and in the quasi-neutral plasma of the RF discharge). For N2O it has been shown that the collision–related Child–Langmuir law version for constant ion mobility may be applied for the description of the cathode sheath only at low pressure (up to 0.3 Torr). In the transient pressure range (between 0.3 and 0.75 Torr) none of the Child–Langmuir law versions can be correctly applied for describing the cathode sheath of the glow discharge. At higher N2O pressure (above 0.75 Torr) one has to apply the law version for the constant ion mean free path. Only the Child–Langmuir law version for the constant ion mobility mi is demonstrated to be applicable throughout the total hydrogen pressure range studied (0.07 – 2 Torr). The law version associated with the constant ion mobility is analytically shown to be applicable even in a high electric field of the cathode sheath. Ratio J/p2 values of the normal current density to the gas pressure squared in N2O, argon, hydrogen and oxygen within the gas pressure range from 0.1 to 10 Torr have been determined. The ratio J/p2 has been found to be constant only at the gas pressure above 1 Torr (J/p2 = 0.44 ± 0.03 mA/(cmTorr)2 for N2O, J/p2 = 0.072 ±0.02 mA/(cmTorr)2 for H2, J/p2 = 0.33 ±0.05 mA/(cm Torr)2 for O2 and J/p2 = 0.092 ±0.02 mA/(cm Torr)2 for Ar). However lowering the pressure (below 1 Torr) produces the fast growth of the J/p2 ratio which may be tens and hundreds times higher than one at the gas pressure above 1 Torr. A mechanism responsible for this phenomenon is suggested. The measurements with the Langmuir probe have demonstrated that at the fixed discharge current the plasma concentration in the negative glow in long inter-electrode gaps is larger than in short ones. This thesis suggests a mechanism how a distance between the cathode and the anode affects the voltage drop across the electrodes and the cathode sheath thickness. An analytical model for the reduced electric field E/p in molecular gases in the ambipolar regime is presented when the direct ionization of gas molecules via electron impact has to cancel the loss of charged particles due to their escape because of ambipolar diffusion. Simple formulas have been obtained for E/p furnishing a good description of experimental data. Two analytical models for the reduced electric field E/p in noble gases have been developed. The first of them deals with the uniform positive column in the ambipolar regime. The second model takes into account not only the direct ionization of argon atoms via electron impact but also such processes with the participation of metastable atoms as the step-wise ionization, their loss under collisions with electrons (under second-kind collisions with electrons and excitation transitions from the metastable level to the radiative ones and to the resonant levels) and under their binary collisions with one another, as well as the diffusion escape of metastable atoms to the walls of the discharge tube. Calculation data obtained within the framework of these models are in good agreement with our experimental data. There are presented the results of the experimental study into the modes of the RF capacitive discharge in hydrogen, nitrogen and N2O with the different values of the inter-electrode gap and pressure. An analytical model of the weak-current a-mode of the RF capacitive discharge has been developed for the cases of the constant mean free path and the constant mobility of positive ions. It follows from the model data and experimental ones that the normal current density in the RF capacitive discharge is usually directly proportional to the gas pressure e.g. Jn ~ p.