Abstract: A plasma reactor includes a pair of parallel plate electrodes (1,2) one of which is grounded and the other driven, mounted within a chamber (8) which is connected via a port (8a) to a control device (16) for controlling the pressure of the gas within the chamber (8). The driven electrode (1) is connected to a voltage supply which has a driving frequency of 13.56 MHz, via an amplifier (9) and a superposed higher resonance frequency via a variable frequency power generator (11). With the plasma reactor sheath resonance in the glow discharge between the electrodes (1,2) can be generated to have a standing wave and thereby ensuring a greater coupling of the power in the system.