AXIAL STRUCTURE OF LOW-PRESSURE HIGH-FREQUENCY DISCHARGES SUSTAINED BY TRAVELING ELECTROMAGNETIC SURFACE-WAVES

For the last two decades, the discharges sustained by RF and microwave surface waves (SW) have been a subject of both experimental and theor etical studies. These plasmas are created by the electric field of an electromagnetic wave travelling along the discharge vessel. The RF or microwave power flow is supplied by a high-frequency generator coupled to the discharge through various matched wave launching structures (s urfatron, surfaguide, waveguide surfatron, or Re-box) the individual o perating frequency band of which overlaps to cover the domain from les s than one MHz up to 10 GHz. As a rule, these discharges are sustained in a cylindrical dielectric tube. The plasma column surrounded by its dielectric discharge tube is a self supporting waveguide or an integr al part of the waveguiding structure when the tube is within a metalli c enclosure. The length of the plasma column is generally much larger than its diameter. The whole system extends axially and, under travell ing wave conditions, it is axially nonuniform since plasma density dec reases away from the launcher. Our concern is the theoretical descript ion of these discharges, The electric field maintaining the discharge is assumed small enough for the wave propagation across the plasma med ium to be described by relations for the linear regime. The local elec tromagnetic properties of the plasma are univocally determined by the local value of its relative permittivity epsilon(p) = 1 - omega(p)(2)/ omega(omega + i nu), where omega and omega(p) are the wave and plasma angular frequencies, respectively, and nu is the effective electron-ne utral collision frequency for momentum transfer. A steady state discha rge exists when the production of charged particles is balanced by the ir recombination loss. With reduced pressure plasmas depending on spec ific conditions, one can observe either the diffusion or the recombina tion gas-discharge regime. In a diffusion controlled discharge, the ch arged particles generated in the discharge volume are all lost at the walls, while in a recombination controlled discharge, the particle los s occurs through recombination in the bulk of the plasma. The balance between the power input from the electromagnetic field to the plasma a nd the power loss due to collisions of electrons with the gas atoms go verns the steady state of any RF or microwave discharge. The model of a plasma column sustained by a travelling electromagnetic wave consist s of at least three basic equations: (i) a local wave dispersion relat ion; (ii) a wave to electron power balance equation, and (iii) a relat ion between the absorbed power per unit length and the electron densit y, which depends on the gas-discharge regime. Such a model generally y ields the conditions for a steady-state discharge to exist and also al lows one to calculate the axial distribution of the electron density n (z), wave power S(z), wave number k(z) and the wave field components. Only the quantities which can be externally controlled when running th e discharge are given a priori in the calculations. These are the quan tities describing the discharge vessel dimensions and configuration, t he waveguiding structure if any, the nature and pressure of the gas, t he operating angular frequency omega and the input microwave power S-e xc. Note that the plasma densities n(exc) and n(end) at the origin (ne ar the wave exciter) and at the end of the column are not set external ly and have to be determined from calculations. Our theoretical review of SW plasmas includes models of plasmas sustained by surface waves i n various electromagnetic modes (azimuthally symmetric and dipolar) an d considers the influence of an axially directed static magnetic field . The basic results of these theoretical studies are in a good agreeme nt with the available experimental data.