Editing Surface-wave-sustained discharge (section)

==Theory==
For a long time,{{when|date=April 2019}} [[microwave plasma|microwave]] [[plasma source]]s without a magnetic field were not considered suitable for the generation of high density plasmas. [[Electromagnetic wave]]s cannot propagate in [[over-dense plasma]]s.  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]] <math>\delta</math>, 
which can be approximated by 

:<math>\delta  \simeq
c\,\big/\sqrt{\omega_{p_e}^2 - \omega^2}. </math>

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 [[exponential decay|decays exponentially]] with the [[skin depth]].  Transfer mechanism allows to generate [[over-dense plasma]]s with electron densities  beyond the critical density.