Theoretical evaluation of the angular spread of the electron beam generated in a guiding magnetic field

Electron beams generated in high-current diodes with a guiding magnetic field are of interest for a number of applications and, in particular, for plasma heating in solenoids, for microwave generation, and for further acceleration in inductive linacs. The paper considers possible sources of a finite angular spread of the beam for the case when electron gyroradius is much smaller than all the characteristic dimensions of the diode. Under such conditions, there are three major sources of the beam angular spread (besides the scattering in the anode foil, if such a foil is present): i)non-adiabatic effects near the cathode surface, especially if there is some ripple on the emitting surface; ii)non-adiabatic effects near the anode foil (if it is present); iii)a possible presence of irregular electric and magnetic fields produced by beamlets emitted by microspikes at the cathode surface. Efficient analytic formulas are presented that allow to evaluate the resulting beam angular spread both for purely electron and bipolar flows.