In high-temperature plasma research aimed at solving the problems of controlled thermonuclear fusion, one of the key problems remains the ingress of light and heavy impurities into the plasma. This is due to contamination of the inner surfaces of vacuum chambers, which leads to a deterioration in plasma parameters, increased energy losses, radiation collapse and disruptions in tokamak-type devices. Therefore, effective preparation and cleaning of vacuum surfaces is an essential part of the functional cycle of thermonuclear installations. The most common cleaning method is the use of glow discharge plasma, although there is still a need for more detailed research of its parameters. However, this approach has a significant drawback – the sputtering of surface materials due to the high energy of ions, which exceeds the sputtering threshold and depends on the discharge voltage. Reducing the anode voltage to a safe level is extremely difficult to implement in practice. Instead, the use of a combined glow-microwave discharge allows significantly reduce the anode voltage, which makes it promising for cleaning toroidal chambers. However, the research in this area is insufficient and the physical mechanisms of the discharge remain poorly understood.
At the same time, it is important to improve plasma diagnostic methods, which play a key role in understanding the physical processes in gas discharge systems. In particular, the use of microwave refraction makes it possible to detect local inhomogeneities in electron density, determine azimuthal shifts in plasma structures, and estimate its rotation frequency. Further progress in plasma physics largely depends on the level of development of diagnostic tools, which necessitates their improvement.
Therefore, this work was aimed at expanding the physical picture in gas discharge plasma of glow discharge and combined glow-microwave discharge, which are implemented in toroidal chambers of thermonuclear devices through a complex study of its parameters and the implementation of the results on thermonuclear devices. For this purpose, the parameters of glow discharge plasma in various gas atmospheres were studied, the current-voltage characteristics and plasma composition were determined, the parameters of glow discharge plasma were calculated using computational modelling, combined glow – microwave glow discharge, was investigated, and the method for determining local plasma inhomogeneities using microwave refraction was improved.
During the experimental studies, the dependence of the breakdown voltage on pressure was measured for Ar, He and H2 gases when using spherical calotte anodes. It was found that the breakdown voltage depends to a large extent on both the type of gas and the shape of the anodes, and the obtained experimental dependencies of the breakdown voltage correspond to Paschen's law. The highest breakdown voltage values were observed in a helium atmosphere, while the lowest were in an argon atmosphere. A similar pattern was observed for anodes of curved cylinder shape. A comparative analysis of the two types of anodes (curved cylinder and spherical calotte) under the same conditions showed a significant difference in breakdown voltages. This difference in breakdown voltage may be due to differences in the geometry of the anodes, which, in turn, affects the nature of the electric field distribution in the vacuum volume, and calculations of the electric field using the FEMM code confirm its complex inhomogeneity structure.
Current-voltage characteristics were measured for different gases (Ar, He, H2, N2) using two types of anodes. The obtained dependencies are typical for hollow cathode discharges and demonstrate an increase in current with increasing voltage regardless of pressure or gas type. The intensity of optical emission also increases with current, indicating an increase in electron density and temperature. A voltage difference observed at the electrodes is probably due to the inhomogeneity of plasma parameters in a large discharge volume. Spherical calotte anodes provide higher discharge voltage than curved cylindrical anodes. The highest voltage was observed in hydrogen plasma, the lowest in helium plasma. All dependencies are non-linear: the current increases sharply with a slight increase in voltage.
The parameters of the glow discharge plasma were investigated using a movable triple probe. Measurements were also performed for different gases (argon, helium, hydrogen) and two anode systems. In all cases, typical plasma parameters for glow discharge were recorded, and the radial profile showed that the maximum values of plasma density, electron temperature and floating potential were observed in the centre of the plasma column, while these parameters gradually decreased towards the chamber wall. Under practically identical conditions, the highest plasma density is observed in argon, up to ≈ 8,3∙1014 m-3, than in helium, up to ≈ 4∙1014 m-3.
