Numerical investigations of radially converging electron beam generated in cylindrical GESA IV facility

The cylindrical triode-type electron accelerator GESA IV was developed for treatment of metallic rods, specifically cladding tubes for nuclear reactors. The target (anode) diameter is therefore fixed at about 10 mm by the application, which leads to problems of homogeneity and stability of the radially converging beam. Due to the large difference between cathode diameter (about 150 mm) and anode diameter, a virtual cathode may form between grid and anode, electrons may miss the target and start to circulate around the anode, and the self-induced magnetic field may lead to large distortion of the electron trajectories. In this study, we investigate the influence of various crucial effects on the beam performance by PIC code simulations using the software package MAGIC. In particular, we consider monopolar and bipolar flow (i.e., the influence of ions generated at the target and moving towards the cathode), the effects of scattering at the grid and of backscattering at the target, the angular velocity spread of the electrons at emission, and the influence of the grid potential. The numerical results are compared with experiments performed at the GESA IV facility, where the influence of the target material and of the self-induced magnetic field on the beam performance are investigated.