Ion flux impingement on the cathode surface in an axisymmetric electron gun

The flow of the residual gas ion generated by a collision with high-energy electron in an axisymmetric gun was numerically investigated. Since the lifetime of the gun is dominated by the cathode damage induced by the ion bombardment, it is of industrial importance to reveal how the ions impinge on the cathode and how the inflow can be repelled. Under the normal operating condition of the electron gun (40 kV, 6 A), we determined the inflow basin, i.e. the region where the gas ions were born there and finally reached to the cathode. The basin consisted of four separated zones characterized by the potential distribution on the beam axis. To eliminate the downstream zones, it was useful to append a positive voltage imposed electrode behind the anode. The voltage necessary for repelling the ion inflow was found to be 300 V being less than 1% of the electron acceleration voltage.