A hollow cathode apparatus, the apparatus comprising: a cathode tube composed of an electrically conductive material which is a refractory metal or a refractory metal alloy, the cathode tube having an input end for receiving an input gas, an output end, opposite to the input end, for outputting a plasma formed from the input gas, the cathode tube defining a central hollow cavity for forming the plasma, and an end wall at the input end of the cathode tube, the end wall closing the hollow cavity and functioning as a mounting base for the hollow cathode apparatus; a supply conduit for inputting the input gas into the hollow cavity, the supply conduit having an outlet opening located in the end wall; a tubular emitter positioned internally within the cathode tube and circumferentially surrounding an end portion of the central hollow cavity which is adjacent to the output end, wherein an outer tubular surface of the tubular emitter is electrically connected to an inner tubular surface of the cathode tube, and the tubular emitter is composed of a material for emitting electrons into the input gas to form the plasma in the end portion of the central hollow cavity when an electrical potential, above a particular potential threshold, is applied to the material of the tubular emitter; wherein the hollow cathode apparatus is heaterless and is adapted so that a passage of electrical current through the emitter is sufficient to emit electrons from the emitter to form the plasma from the input gas, characterised by an outer tubular dielectric barrier circumferentially surrounding an outer tubular surface of the cathode tube, the outer tubular dielectric barrier being composed of a barrier material which is electrically non-conductive, wherein the outer tubular dielectric barrier includes a flanged end region which surrounds an end surface of the cathode tube at the output end; and an inner tubular dielectric barrier circumferentially surrounding a surface portion of the inner tubular surface of the cathode tube, which surface portion is adjacent to the tubular emitter, and the inner tubular dielectric barrier is composed of a barrier material which is electrically non-conductive. a cathode tube composed of an electrically conductive material which is a refractory metal or a refractory metal alloy, the cathode tube having an input end for receiving an input gas, an output end, opposite to the input end, for outputting a plasma formed from the input gas, the cathode tube defining a central hollow cavity for forming the plasma, and an end wall at the input end of the cathode tube, the end wall closing the hollow cavity and functioning as a mounting base for the hollow cathode apparatus; a supply conduit for inputting the input gas into the hollow cavity, the supply conduit having an outlet opening located in the end wall; a tubular emitter positioned internally within the cathode tube and circumferentially surrounding an end portion of the central hollow cavity which is adjacent to the output end, wherein an outer tubular surface of the tubular emitter is electrically connected to an inner tubular surface of the cathode tube, and the tubular emitter is composed of a material for emitting electrons into the input gas to form the plasma in the end portion of the central hollow cavity when an electrical potential, above a particular potential threshold, is applied to the material of the tubular emitter; wherein the hollow cathode apparatus is heaterless and is adapted so that a passage of electrical current through the emitter is sufficient to emit electrons from the emitter to form the plasma from the input gas, characterised by an outer tubular dielectric barrier circumferentially surrounding an outer tubular surface of the cathode tube, the outer tubular dielectric barrier being composed of a barrier material which is electrically non-conductive, wherein the outer tubular dielectric barrier includes a flanged end region which surrounds an end surface of the cathode tube at the output end; and an inner tubular dielectric barrier circumferentially surrounding a surface portion of the inner tubular surface of the cathode tube, which surface portion is adjacent to the tubular emitter, and the inner tubular dielectric barrier is composed of a barrier material which is electrically non-conductive.