A surface-wave-sustained discharge is a
plasma that is excited by propagation of electromagnetic
surface wave
In physics, a surface wave is a mechanical wave that propagates along the Interface (chemistry), interface between differing media. A common example is gravity waves along the surface of liquids, such as ocean waves. Gravity waves can also occu ...
s.
Surface wave plasma sources can be divided into two groups depending upon whether the plasma generates part of its own
waveguide
A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction. Without the physical constraint of a waveguide, wave intensities de ...
by
ionisation or not. The former is called a
self-guided plasma. The surface wave mode allows the generation of uniform high-frequency-excited plasmas in volumes whose lateral dimensions extend over several wavelengths of the electromagnetic wave, e.g. for microwaves of 2.45 GHz in
vacuum
A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often di ...
the
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...
amounts to 12.2 cm.
Theory
For a long time,
microwave
Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequency, frequencies between 300 MHz and 300 GHz respectively. Different sources define different fre ...
s without a magnetic field were not considered suitable for the generation of high density plasmas.
Electromagnetic wave
In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visible ...
s cannot propagate in
over-dense plasmas. The wave is reflected at the plasma surface due to the
skin effect
Skin effect is the tendency of an alternating electric current (AC) to become distributed within a conductor such that the current density is largest near the surface of the conductor and decreases exponentially with greater depths in the cond ...
and becomes an
evanescent wave
In electromagnetics, an evanescent field, or evanescent wave, is an oscillating electric and/or magnetic field that does not propagate as an electromagnetic wave but whose energy is spatially concentrated in the vicinity of the source (oscillati ...
. Its penetration depth corresponds to the
skin depth
Skin effect is the tendency of an alternating electric current (AC) to become distributed within a conductor such that the current density is largest near the surface of the conductor and decreases exponentially with greater depths in the cond ...
,
which can be approximated by
:
The non-vanishing penetration depth of an evanescent wave opens an alternative way of heating a plasma: Instead of ''traversing'' the
plasma, the conductivity of the plasma enables the wave to propagate ''along'' the plasma surface. The wave energy is then transferred to the plasma by an
evanescent wave
In electromagnetics, an evanescent field, or evanescent wave, is an oscillating electric and/or magnetic field that does not propagate as an electromagnetic wave but whose energy is spatially concentrated in the vicinity of the source (oscillati ...
which enters the plasma perpendicular to its surface and
decays exponentially
A quantity is subject to exponential decay if it decreases at a rate proportional to its current value. Symbolically, this process can be expressed by the following differential equation, where is the quantity and ( lambda) is a positive rat ...
with the
skin depth
Skin effect is the tendency of an alternating electric current (AC) to become distributed within a conductor such that the current density is largest near the surface of the conductor and decreases exponentially with greater depths in the cond ...
. Transfer mechanism allows to generate
over-dense plasmas with electron densities beyond the critical density.
Design
Surface-wave-sustained plasmas (SWP) can be operated in a large variety of recipient geometries. The pressure range accessible for surface-wave-excited plasmas depends on the process gas and the diameter of the recipient. The larger the chamber diameter, the lower the minimal pressure necessary for the SWP mode. Analogously, the maximal pressure where a stable SWP can be operated decreases with increasing diameter.
The
numerical modelling of SWPs is quite involved. The plasma is created by the
electromagnetic wave
In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visible ...
, but it also reflects and guides this same wave. Therefore, a truly
self-consistent
In classical deductive logic, a consistent theory is one that does not lead to a logical contradiction. The lack of contradiction can be defined in either semantic or syntactic terms. The semantic definition states that a theory is consistent ...
description is necessary.
References
Waves in plasmas
Surface waves
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