Derevianko V. Structure and properties of direct current gas discharge in long tubes

The thesis paper outlines the study of glow discharge ignition in short and long tubes, of the discharge axial structure together with processes in the cathode sheath and the positive column. The study demonstrates that the conventional Paschen's law is valid only for short discharge tubes for which the inter-electrode distance is below the tube radius L/R < 1. Under the opposite condition L/R > 1 increasing the inter-electrode distance L shifts the breakdown curves Udc(p) to the region of higher breakdown voltage and lower gas pressure values. It was found for the first time that for large ratio values L/R > 10 20 the breakdown curves with the distance L growing were shifted to the region of higher breakdown voltage values with practically unchanged gas pressure at their minimum. Asymptotic treatment of the breakdown criterion was performed and the formula for the gas pressure at the breakdown curve minimum in a long tube was found furnishing a good description of the results of this thesis paper. Probe technique was employed to study the axial profiles of plasma parameters (electron temperature, electric field strength, plasma potential and plasma concentration) of the direct current glow discharge in nitrogen for certain gas pressure values. In a long tube the plasma concentration maxima are observed in the negative glow where the electric field strength is small, and the minima are located at the start of the dark Faraday space. The conditions were established for the stratification of the glow discharge positive column in the constant electric field in nitrogen in the tubes of different radii. The strata are observed in the limited range of gas pressure in closed ranges of current and voltage applied. The stratification of the positive column is found to satisfy the similarity law. The presented strata thickness values d/R were shown to obey Goldstein-Wehner law d/R = C/(pR)^m, where the constants for nitrogen have the values C = 1.05 and m = 0.32. The diffuse and contracted modes of glow discharge in nitrogen and CF4 were studied in experiment. The existence region of the contracted mode (at the gas pressure fixed) in narrow inter-electrode gaps was found to possess an ambivalent pattern. It was established which of Child-Langmuir law versions one has to apply for describing the cathode sheath of the glow discharge in the nitrogen pressure range from 0.04 Torr to 2.5 Torr. It was demonstrated that for the nitrogen pressure of p < 1 Torr, as well as for p > 1.5 Torr the characteristics of the cathode sheath were well described by the collision Child-Langmuir law version with the constant ion mobility.