Abstract
Some general features of long plasma columns sustained by various kinds of electromagnetic waves are described. A steady-state discharge can be maintained only by a limited category of waves as defined in the paper. Both the possibility of existence and the axial structure of the produced plasma column depend mainly on the attenuation characteristics of the wave. The expressions for the axial profiles of electron density and wave power are given. A typical illustration of application of the general results obtained is shown. It is concluded that the discharges sustained by travelling waves constitute a distinct class of microwave discharges.