Restorable cold cathode in a gas discharge electron gun

A gas discharge electron gun for generating an electron beam by means of a glow discharge comprising an envelope, means to maintain a gaseous ionizable medium inside said envelope, at least an anode and a cathode between which the said glow discharge takes place, from which cathode electrons are released by secondary emission in such a quantity that the said electron beam consists mainly of said electrons and which cathode has a shape that reduces the erosion occurring at the cathode surface, and wire means for restoring the eroded material in the active part of the cathode surface at least partly during operation of the gas discharge electron gun, said wire means comprising a plurality of slideable concentric cylinders extending through an aperture in the active surface of the cathode.