Experimental and Numerical Study of the Structure of a Low-Pressure Surface-Wave-Sustained Discharge

Abstract

An experimental and numerical study of a low-pressure discharge sustained by a surface electromagnetic wave (SEW) in argon (p ≈ 1−2 Torr) has been performed. The discharge has been excited by a high-frequency (HF) field (445 MHz, 45 W) in a quartz tube (24 mm in diameter) using a surfatron. The self-consistent simulation of the discharge has been performed using the KARAT PiC code, which takes into account the dynamics of the electromagnetic field and the spatiotemporal evolution of the plasma. The longitudinal and radial profiles of the plasma density and the distribution of the SEW field of a stationary discharge, the propagation velocity of the ionization front, and the time it takes for the discharge to reach a stationary state has been experimentally measured. The numerical simulation shows good agreement with the experimental data and has allowed a detailed study of the discharge formation dynamics, including the evolution of the electromagnetic field and the spatial distribution of the plasma density. It is shown that under the experimental conditions (p ≈ 1−2 Torr), the initial ionization occurs near the tube wall, forming a tubular plasma structure. At times determined by ambipolar diffusion, the density profile transforms into a distribution close to the Bessel function. A numerical model has demonstrated that, as the pressure and tube radius vary, a transition from a tubular structure to the plateau-shaped profile and plasma density distribution with a maximum on the axis is observed. The structure of the discharge ionization front has been studied. In the critical density region, an enhancement of the HF field is observed, accompanied by nonlocal heating of electrons and the formation of an ambipolar field. As the pressure increases to 3 Torr, when the collision frequency becomes close to the cyclic frequency of the field, the field enhancement and the increase in electron energy at the front are hardly observed, the ambipolar diffusion component weakens, and the propagation velocity of the ionization front decreases significantly.