In the present work, we experimentally study the formation dynamics of positive column constriction in neon glow discharge at intermediate pressures (pR 50 500 Torr cm = ¸) with the simultaneous development of longitudinal instabilities in the form of moving striations. Discharge current, which exceeded the critical value for the transition to the constricted regime, was modulated by short rectangular pulses in order to avoid the inhomogeneous heating of the gas. During the pulse, the ionization balance was establishing and then discharge was switching to the constricted regime. The temperature field of neutral atoms was determined using interferometric methods based on a scheme of the Michelson interferometer. The heat equation was solved for the stationary and pulsed regimes. It is shown that under the described discharge conditions, in spite of the absence of inhomogeneous gas heating, discharge switches to the constricted regime with the appearance of moving striations. On the basis of the performed experiments, it can be concluded that inhomogeneous gas heating in neon is not the main cause of discharge constriction. A nonlinear dependence of the ionization rate on the electron density related to the Maxwellization of the electron energy distribution function should be considered as the basic mechanism of constriction.

We present a detailed self-consistent model of a positive column in argon glow discharge at moderate pressures and currents. This model describes the discharge transition between diffuse and constricted states. The model includes an extensive set of plasma chemical reactions and equation for inhomogeneous gas heating. The nonequilibrium behavior of an electron distribution function is also considered. One of the main features of the model is an accurate treatment of radiation trapping by solving the Holstein-Biberman equation directly. Influence of the radiation trapping on macroscopic parameters of the constricted positive column is studied. We propose a method for solving a boundary-value problem, including particle and energy balance equations for electrons, ground state atoms, atomic and molecular ions, and excited species. Unlike traditional solution approaches for similar systems, the method provides continuous Z- and S-shaped characteristics of discharge parameters, describing hysteresis in transition between diffuse and constricted discharge regimes. Performed experiments include measurements of volt-ampere characteristics and spectroscopic study of radial density profiles of excited atoms by measuring line emission and absorption, and electrons by measuring bremsstrahlung intensity. The role of resonance radiation trapping in spatial redistribution of 1s and 2pstates of argon is demonstrated. Results of modeling are compared to the experimental data.

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The magnetic field of a magnetron serves to increase the residence time of electrons in the ionization region and thereby enables the discharge to be sustained at low working gas pressures. This hinders the electrons to reach the anode which is necessary to close the electrical circuit. At high atom densities in the ionization region, and in the presence of an electric field, collisions of electrons with heavy species consecutively push electrons across the magnetic field lines, which is known as the classical cross-field transport mechanism. At low atom densities in the ionization region, collisions are rare and the classical cross-field transport mechanism is insufficient to carry the discharge current. This gives rise to plasma instabilities, called spokes, that locally provide pathways for electrons to escape from the near-target region and across the magnetic field lines. Here, we show experimentally, for the case of a high power impulse magnetron sputtering discharge with an aluminum target, how spokes gradually disappear with the increase in local gas density. We present an analytical model that shows that under these high gas density conditions, the classical electron transport mechanism is indeed strong enough to solely carry the discharge current. This highlights the importance of the local gas density in the ionization region for the intensity of spokes in a magnetron sputtering discharge and suggests ways for process optimization.