Abstract: Dimensional stability of an electron discharge heating device having electrodes for focussing the discharge current is improved by providing one or more heat pipes for transferring heat from an electrode to cooling fins outside the vacuum chamber. In one embodiment, a second electrode is water-cooled. The improved cooling not only permits of use of a structure which takes a short time to reach thermally stable conditions due to the small heat capacity of the solid material but also permits of different parts of the gun assembly being operated at different temperatures.