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

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. The 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. Therefore, 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. In this paper we present the results of numerical and experimental investigations with the aim to suppress the ion flux towards the cathode. The investigations are performed for the pulsed electron beam facility GESA 1 (electron beam parameters <150 keV, 300 A). Different electrostatic ion traps are considered. Best results are achieved using a positively biased (>15 kV) ion trap consisting of a thin disk placed in front of the target and for a negatively biased (>-15) target itself. For both cases the ion flux towards the cathode is eliminated.