Improvement of the spatial current density distribution of intense pulsed electron beams

Abstract

Summary form only given, as follows. Multifold measurements of the radial electron beam current density distribution, performed at the GESA facility, demonstrate that it commonly has a maximum near the beam axis in contradiction to calculations, which predict that it should be homogeneous. The reason for this is related to the influence of target ions on the distribution of the emission current density of the cathode operating in a space charge limited mode. Ion flux arises due to ionization of absorbed atoms and plasma formation on the target surface. Ions, emitted from the target plasma under the action of the electron beam space charge, move to the electron source and disturb the electric field distribution in the region of electron beam formation. Since ions are not magnetized their trajectories do not coincide with the electron trajectories. There for the distortion of the electric field takes place mainly near the central part of the source. This results in a change of the electron beam current density distribution. We present the results from numerical and experimental investigations to suppress the ion flux towards the cathode. The investigations are performed for the pulsed electron beam facility GESA 1 (electron beam parameters. 15 kV) ion trap consisting of a thin disk placed in front of the target and for negatively biased (>-15) target itself. For both cases the ion flux towards the cathode is eliminated.