Abstract: A plasma discharge tube (5) having a diameter that corresponds to a quarter wavelength of the standing wave is selected and the waveguide system (2) is dimensioned and tuned such that the standing wave forms a first voltage maximum at a first side of the plasma discharge tube (5) and the standing wave is also supplied reflected, so that it forms a second, anti-phase voltage maximum at a second side of the plasma discharge tube (5) that lies opposite the first side and faces toward an end termination (12) of the waveguide system (2). A controlled magnetic field is applied in order to achieve an especially low working pressure.