Controlled production of warm electron beams

A warm beam is produced by passing a cold electron beam through a system of three closely spaced parallel grids. The beam and the grid structure are immersed in a magnetic field. The middle grid is biased at a large positive potential, and the resulting electric field near the grid wires scatters part of the electrons axial energy into transverse energy. Thus, although the beam remains monoenergetic, a controllable spread in both the transverse and axial energy distribution is obtained.