Surface wave discharge

Discharge on a surface wave , English Surface-wave-sustained plasmas (SWP) is a form of gas discharge excited by surface electromagnetic waves . Surface electromagnetic waves propagating along the plasma boundary can be effectively absorbed by it, thus supporting the discharge. A discharge on a surface wave makes it possible to obtain a uniform plasma in volume, the transverse dimensions of which exceed several wavelengths of the exciting radiation. A discharge on a surface wave should not be confused with a microwave discharge on the surface of a dielectric .

Content

Study History

Surface electromagnetic waves having strong fields only near the plasma boundary were theoretically described in 1958 ^[1] and 1959 ^[2] . Moisen and his team from the University of Montreal studied ^[3] various configurations of the discharge system at high power in a wide frequency range (from 1 MHz to 10 GHz), the diameter of the discharge volume is up to 150 mm, although the size range from 30 to 100 mm was most often used . The simplest of the sources worked without an external magnetic field.

Physical Principles

For a long time, plasma sources based on a microwave discharge without a magnetic field were considered unsuitable for creating high-density plasma. Volume electromagnetic waves cannot propagate in a plasma with a density greater than critical. The wave is reflected on the plasma surface due to the skin effect and becomes damped. Penetration depth corresponds to the depth of the skin layer $\delta$ ${\ displaystyle \ delta}$ , which can be roughly written as

\delta \simeq c\,{\big /}{\sqrt {\omega _{p_{e}}^{2}-\omega ^{2}}}.

{\ displaystyle \ delta \ simeq c \, {\ big /} {\ sqrt {\ omega _ {p_ {e}} ^ {2} - \ omega ^ {2}}}.

However, despite the fact that the skin effect impedes attempts to transfer energy into the plasma “across”, a nonzero depth of the skin layer allows the plasma conductivity to be used to propagate the wave “along” its boundary. The wave energy in this case is transferred to the plasma due to the damping surface wave, which exponentially attenuates in the direction perpendicular to its surface. Such a mechanism allows the creation of a plasma of supercritical density. Moreover, for the propagation of a surface wave, it is fundamentally necessary that the plasma density be higher than the critical one defined by the expression:

n_{c}={\frac {\varepsilon _{o}\,m_{e}}{e^{2}}}\,\omega ^{2}

{\ displaystyle n_ {c} = {\ frac {\ varepsilon _ {o} \, m_ {e}} {e ^ {2}}} \, \ omega ^ {2}}

.

Practical Implementation

For the practical implementation of this type of discharge, a plasma-resistant dielectric (also called a dielectric antenna) is placed in the discharge volume, from one end of which there is a waveguide through which microwave power is supplied. A microwave wave, leaving the waveguide in the discharge volume, causes a microwave breakdown in it, leading to the formation of a plasma. When the plasma density reaches a critical density for a given frequency, conditions are created for the propagation of a surface wave, which transfers energy along the dielectric, providing ionization. A self-sustaining plasma waveguide arises, the role of the conducting walls, which the plasma plays. Due to the fact that the plasma conductivity is significantly less than the metal conductivity, these "walls" have a relatively high resistance, and the current induced in them transfers the power of the electromagnetic wave to the plasma.

Industrial Application

Currently, there are no technological installations on the market that use plasma sources for a discharge on surface waves. Sources of this type are inferior to those with inductively coupled plasma in such fundamental parameters as the practically attainable plasma density and the uniformity of its distribution over the treatment zone. To obtain high density sources, it is necessary to use frequencies of the microwave range 1..10 GHz. For practical applications, the most theoretically and experimentally studied cylindrical configuration of the discharge is in most cases unsuitable due to the fundamental need to fulfill the condition $l>>R$ ${\ displaystyle l >> R}$ , which makes it impossible to achieve the required uniformity of the plasma density ^[4] . In this regard, special interest is also shown in systems with plane geometry ^[5] .

Notes

↑ Smullin, Chorney, 1958 .
↑ Trivelpiece, Gould, 1959 .
↑ Moisan et al., 1986 .
↑ Lieberman, Lichtenberg, 2005 .
↑ Komachi, 1993 .

Literature

Smullin LD , Chorney P. Properties of Ion Filled Waveguides // Proc. IRE. - 1958.- T. 46 . - S. 360 .
Trivelpiece AW , Gould RW Space Charge Waves in Cylindrical Plasma Columns (English) // J. Appl. Phys. : magazine. - 1959. - Vol. 30 , iss. 11 . - P. 1784 . - ISSN 00218979 . - DOI : 10.1063 / 1.1735056 .
Moisan M. , Zakrzewski Z. , Lawrence H. Luessen . Radiative Processes in Discharge Plasmas / Joseph M. Proud . - Springer US, 1986. - P. 381-430. - (NATO ASI Series). - ISBN 978-1-4684-5307-2 , 978-1-4684-5305-8.
Komachi K. Affecting factors on surface wave produced plasma // Journal of Vacuum Science Technology A: Vacuum, Surfaces, and Films. - 1993. - T. 11 , no. 1 . - P. 164-167. - ISSN 0734-2101 . - DOI : 10.1116 / 1.578284 .
Lieberman MA , Lichtenberg AJ Principles of Plasma Discharges and Materials Processing. - John Wiley & Sons, 2005 .-- ISBN 0-471-72001-1 .

[_ed78a0e7d1b0194a-1] Smullin, Chorney, 1958 .

[_11030dd572a6939a-2] Trivelpiece, Gould, 1959 .

[_8fa3cf4e2c276426-3] Moisan et al., 1986 .

[_ddab8a9f102e944c-4] Lieberman, Lichtenberg, 2005 .

[_e62998b6068ca567-5] Komachi, 1993 .