A low-pressure high-density plasma is excited in a plasma chamber surrounded by a cylindrical inner wall of a ring-shaped waveguide resonator to which the microwave energy is fed by a coupling from a microwave generator. The output coupling of the microwave energy from the standing wave maintained in the waveguide resonator to the plasma chamber is effected through a multiplicity of equispaced slits whose spacing is one half or one waveguide wavelength and which extend parallel to the generatrices of the cylindrical inner wall of the ring-shaped waveguide resonator.