Surface-wave-sustained mode
Encyclopedia
Plasmas that are excited by propagation of electromagnetic surface waves are called surface-wave-sustained. Surface wave
plasma sources can be divided into two groups depending upon whether the plasma generates part of its own waveguide
by ionisation or not. The former is called a self-guided plasma. The surface wave mode allows to generate uniform high-frequency-excited plasmas in volumes whose lateral dimensions extends over several wavelengths of the electromagnetic wave, e.g. for microwaves of 2.45 GHz in vacuum
the wavelength
amounts to 12.2 cm.
For a long time, microwave
plasma source
s without magnetic field have not been considered suitable for the generation of high density plasmas. Electromagnetic waves cannot propagate in over-dense plasmas. The wave is reflected at the plasma surface due to the skin effect
and becomes an evanescent wave
. Its penetration depth corresponds to the
skin depth
,
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
which enters the plasma perpendicular to its surface and decays exponentially with the skin depth. 'Transfer mechanism allows to generate over-dense plasmas with electron densities beyond the critical density.
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, but it also reflects and guides this same wave. Therefore, a truly self-consistent description is necessary.
Surface wave
In physics, a surface wave is a mechanical wave that propagates along the interface between differing media, usually two fluids with different densities. A surface wave can also be an electromagnetic wave guided by a refractive index gradient...
plasma sources can be divided into two groups depending upon whether the plasma generates part of its own waveguide
Waveguide
A waveguide is a structure which guides waves, such as electromagnetic waves or sound waves. There are different types of waveguides for each type of wave...
by ionisation or not. The former is called a self-guided plasma. The surface wave mode allows to generate uniform high-frequency-excited plasmas in volumes whose lateral dimensions extends over several wavelengths of the electromagnetic wave, e.g. for microwaves of 2.45 GHz in vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
the wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
amounts to 12.2 cm.
For a long time, microwave
Microwave plasma
Microwave plasma is a type of plasma, that has high frequency electromagnetic radiation in the GHz range. It is capable of exciting electrodeless gas discharges.-Properties of microwave-excited plasma:...
plasma source
Plasma source
Plasma sources generate plasmas.Excitation of a plasma requires partial ionization of neutral atoms and/or molecules of a medium.There are several ways to cause ionization:collisions of energetic particles, strong electric fields acting on bond...
s without magnetic field have not been considered suitable for the generation of high density plasmas. Electromagnetic waves cannot propagate in over-dense plasmas. The wave is reflected at the plasma surface due to the skin effect
Skin effect
Skin effect is the tendency of an alternating electric current to distribute itself within a conductor with the current density being largest near the surface of the conductor, decreasing at greater depths. In other words, the electric current flows mainly at the "skin" of the conductor, at an...
and becomes an evanescent wave
Evanescent wave
An evanescent wave is a nearfield standing wave with an intensity that exhibits exponential decay with distance from the boundary at which the wave was formed. Evanescent waves are a general property of wave-equations, and can in principle occur in any context to which a wave-equation applies...
. Its penetration depth corresponds to the
skin depth
,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
Evanescent wave
An evanescent wave is a nearfield standing wave with an intensity that exhibits exponential decay with distance from the boundary at which the wave was formed. Evanescent waves are a general property of wave-equations, and can in principle occur in any context to which a wave-equation applies...
which enters the plasma perpendicular to its surface and decays exponentially with the skin depth. 'Transfer mechanism allows to generate over-dense plasmas with electron densities beyond the critical density.
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, but it also reflects and guides this same wave. Therefore, a truly self-consistent description is necessary.