Abstract: An electron gun which is rugged, provides good thermal decoupling of the hode, permits precise alignment of the electrodes to be achieved easily and allows geometric changes to be carried out with ease is described. In the cathode assembly three set screws and a split ring of insulating material are used to secure the cathode. Alignment is achieved by suitably adjusting the set screws and because contact can be limited to the three set screws thermal coupling is reduced. Similarly a Whenelt electrode is secured and aligned by a further group of set screws. Finally, the anode is secured to replaceable stand-offs by means of screw passing through oversized holes in an anode plate. The oversized holes permit easy alignment and the use of stand-offs permits the spacing between anode and cathode to be changed as desired by replacing the stand-offs.