Plasma enhanced gas reactor

A reaction chamber is adapted to be coupled to a source of microwave radiation. A pair of opposed field enhancing electrodes concentrate microwave energy so as to form plasma in a localised region between the electrodes. Gas passages are arranged for passing a gaseous medium into and out of the chamber so that the gaseous medium passes through the said localised region of plasma. The electrodes comprise electrically conducting tubes held in electrode holders located in the chamber wall, the electrode tubes being removable and replaceable from outside of the apparatus.