Beam conditioning for electron energy recovery systems in devices employing axis-encircling beams

Abstract

Depressed collectors are extensively used in linear-beam devices. In spiraling beam devices, the motion of the spent beam needs to be converted into axial motion before it can deliver energy to the retarding axial electrostatic field produced by depressed collectors. A method of doing so is presented here, along with simulation results. A large-orbit gyrotron is chosen as an illustrative example in which the axis-encircling beam is generated by passing an axially streaming beam through a magnetic cusp. After rf interaction, an 'unwinding' of the beam is done by a second magnetic cusp with a reversed magnetic field. The trajectories of electrons going through the two cusps have been simulated in order to evaluate the effects of finite cusp width and finite width of emitting surface. Also studied are the effects of variations of the magnetic field, the accelerating voltage of the beam, and the distance between cusps. These simulations show that the rotational energy of electrons is largely converted into axial energy after passage through the second cusp. An initial geometry has been chosen for three collectors, of which two are at depressed potentials. Changes in the energies of the particles are introduced between the two cusps as an approximation of the effect of rf interaction. Simulation of trajectories in the region of the depressed collectors has demonstrated energy sorting of the beamlets to different depressed collectors.