Production of Tightly Focused E-Beams with High-Current Accelerators*

Using numerical modeling we study several approaches to the problem of designing an injector to produce a 3-30 kA, 2-4 mm diameter electron beam in the energy range IO-20 MeV. The cathode may be small in diameter and immersed in a strong magnetic field. producing an equilibrium beam for transport to a target (the “immersed ” case). This approach appears to be the most promising for applications such as radio-graphy, and we shall emphasize it in this paper. The alternative is the conventional “non-immersed ” cathode. in which the beam from a larger-radius, cold-beam cathode is focused with magnetic lenses to a small spot on the target. Because the non-immersed case has been extensively studied. and because it has disadvantages for our purposes. we shall only discuss a few of our non-immersed-cathode injector studies, primarily for purposes of comparison. Either type of diode is to be powered by an inductive voltage adder based on the successful SABRE/Hemles III/RADLAC (SMILE) magnetically-insulated-transmission-line design concepts. l. * A possible variation uses a re-entrant geometry with low electric stresses so that only the cathode face emits. We discuss issues such as dumping excess current and voltage dependence of the focus. I