A complete theoretical description of a low-pressure, cylindrical plasma column sustained by a weakly damped surface wave is derived in which the equations describing the steady-state plasma maintenance conditions are coupled with the surface wave propagation and attenuation characteristics. The present formulation extends a radial theory of the maintenance of the plasma column previously reported to include the axial variation in the radially averaged plasma properties. Simple similarity laws for this type of discharge are proved to exist under quite general conditions. Analytic expressions are derived for the variation of the electron density along the discharge axis and for the length of the plasma column as a function of the operating parameters. The calculation are shown to agree well with available experimental data.

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Modelling of a low-pressure plasma column sustained by a surface wave

This paper presents a theoretical analysis of high-frequency, diffusion-controlled discharges in which the ionization is provided by direct electron impact with gas atoms within the discharge volume. The theoretical formulation uses the electron Boltzmann equation along with two-moment, hydrodynamic-type equations for the electrons and the ions. The treatment followed covers a wide range of applied field frequencies, ω/2π, from ω νc to ω νc, where νc is the electron-neutral collision frequency for momentum transfer; it is, however, limited to situations for which ω τ−1, where τ is the characteristic electron energy relaxation time by collisions. It is shown that, under steady-state conditions, the maintenance field and the average absorbed power per electron are practically independent of the field configuration and essentially determined by the gas pressure, the configuration and the dimensions of the discharge vessel, and ω, and not by the applied HF power level. Both the maintenance field and the average absorbed power per electron are shown to obey similarity laws as a function of the discharge operating parameters.

The similarity laws for the maintenance field and the absorbed power per electron in low-pressure surface wave produced plasmas and their extension to HF plasmas in general

Long plasma columns of high electron densities, in many instances preferable to the positive column plasmas of dc discharges, can be sustained by means of an electromagnetic surface wave that propagates along the column at microwave frequencies. This paper presents a waveguide-based launcher (surfaguide) that can efficiently generate such plasma columns. It is a broadband device that can be operated with microwave powers of the order of kilowatts. The surfaguide is described and analyzed in terms of equivalent circuit theory. Guidelines for its design and operation are given.

A Waveguide-Based Launcher to Sustain Long Plasma Columns through the Propagation of an Electromagnetic Surface Wave

Axial structure of low-pressure high-frequency discharges sustained by travelling electromagnetic surface waves

The surface wave produced plasma belongs to a class of RF and microwave induced plasmas. It results from the propagation of an electromagnetic wave which uses the plasma column it sustains and the plasma tube as its sole propagating media. This type of plasma offers several advantages compared to the positive column plasma of dc discharges or to other RF and microwave produced plasmas. Surface wave plasmas require no internal electrodes, and they can be applied over an extremely broad range of wave frequencies (27 MHz to 10 GHz demonstrated) and gas pressures (about 10−4 Torr to a few times the atmospheric pressures). Using the surface wave plasma technique, a large variety of plasma column diameters have been created (0.5–150 mm demonstrated) and no limitation on plasma column length (column up to 6 m long demonstrated) has been found. The surface wave produced plasma is used in elemental analysis and to sustain emission in lasing media. This article is intended as a guide for potential users of surface wave plasmas in the field of plasma processing and plasma chemistry.

Microwave and RF surface wave sustained discharges as plasma sources for plasma chemistry and plasma processing

On the non-linear theory of a long gas discharge produced by an ionizing slow electromagnetic wave

The problem of the propagation of a slow high-frequency ionising electromagnetic wave along a thin plasma cylinder surrounded by a dielectric tube is considered, assuming that the characteristic diffusion and thermal conductivity lengths are small in comparison with the discharge length. The wave phase characteristics as well as the radial and axial profiles of the wave field and of the plasma electron temperature and density are determined. The obtained results suggest an approach for explanation of the experimental data.

http://iopscience.iop.org/article/10.1088/0022-3727/17/11/010/pdf

A.G. Boev

Papers

On the non-linear theory of a long gas discharge produced by an ionizing slow electromagnetic wave

Slow ionising high-frequency electromagnetic wave along a thin plasma column