Design considerations for a sheet beam electron gun for the quasi-optical gyrotron

The quasi-optical gyrotron is a possible rf source for electron cyclotron resonances heating of large fusion reactors. To fully exploit its potential, a sheet beam electron gun should be utilized. A schematic and the parameters of the sheet beam gun are shown in Fig. 1. The gun shown is a magnetron injection gun modified for planar geometry. It actually produces a double sheet beam since up-down symmetry is assumed. In terms of the parameters of the beam in the gun, the gun parameters1 are derived for the parameters shown by conservation of canonical momentum, adiabatic compression in the presence of a voltage drop Vo-Va. The initial transverse energy comes from E × B motion in the voltage drop Va. Limiting currents are determined by the current being a small fraction of the space charge limiting current, and also by the current being able to propagate through the interaction region. Both of these limiting currents turn out to be expressed in terms of a current per unit length rather than total current. Thus, by making the emitting region long enough, the gun is capable of putting out arbitrary current. The focusing electrode shapes can be determined by electrode synthesis, modified to a planar configuration.2 The remaining issue is of edge losses. We show that the electrode synthesis technique can be modified to shape edge electrodes in such a way as to minimize edge effects. Results for such an electron gun design will be shown. This work was supported by the Department of Energy.