Numerical Study of Circulating Electrons in a Cylindrical Accelerator With Converging Electron Beam

The cylindrical triode-type accelerator GESA IV-m was built for the application of a radial converging pulsed electron beam to modify the surface of an anode tube. The experimental studies showed an anode current of only 80% of the emission current although the control grid has a transparency of more than 96%. Responsible are beam electrons that miss the anode and circulate in the accelerator volume until striking the control grid. The numerical simulations of the beam performance generally confirm the theoretical predictions based on the conservation of the angular momentum: the amount of circulating electrons depends mainly on the azimuthal velocity spread of the electrons at emission, the accelerating voltage, and the radii of cathode and anode. Minor influence of the beam current density and azimuthal space-charge inhomogeneity on the portion of electrons passing by the anode is observed. Circulating electrons have a strong influence on the formation of a virtual cathode between the control grid and the anode. The reduction of the respective critical currents is quantified as a function of the electrons' initial velocity spread.