In plasma activated chemical vapour deposition a plasma decomposition unit (1) is used that is arranged in or connected to a vacuum vessel having a relatively low pressure or vacuum, to which an operating gas is provided. Periodically- repeated voltage pulses are applied between the anode (15a: 15b) and the cathode (7) of the plasma decomposition unit in such a manner that pulsed electric discharges are produced between the cathode and the surrounding anode of the plasma decomposition unit. The anode is arranged in a special way so that at least a portion thereof will obtain only an electrically conductive coating or substantially no coating when operating the unit. For that purpose, the anode includes a portion (15a) located in the direct vicinity of the free surface (5) of the cathode. The portion is a flange or edge portion which is located or extends over margins of the free surface of the cathode. In that way, the anode will include a portion (17) that is shielded for direct coating with particles from the plasma formed and that hence will obtain e.g. substantially no dielectric coating at all.