an ion plasma electron gun for the generation of electron beams which exhibits electron beam dose uniformity and which is capable of varying the dose received by a material to be irradiated . positive ions generated by a wire in a plasma discharge chamber are accelerated through an extraction grid onto a second chamber containing a high voltage cold cathode . these positive ions bombard a surface of the cathode , causing the cathode to emit secondary electrons which form an electron beam . after passing through the extraction grid in the plasma discharge chamber , the electron beam exits from the gun by way of a second grid and a foil window supported on the second grid . the gun is constructed so that the electron beam passing through the foil window has a relatively large area and uniform electron distribution which is subsantially the same as the ion distribution of the ion beam impinging upon the cathode . means are provided for creating a pulse of secondary electrons by varying the period of time in which the secondary electrons are transmitted through the foil .