# Surface-wave-sustained mode

For a long time, microwave plasma sources 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 ${\displaystyle \delta }$, which can be approximated by
${\displaystyle \delta \simeq c\,{\big /}{\sqrt {\omega _{p_{e}}^{2}-\omega ^{2}}}.}$