Speaker
Description
The operating lifetime of GaAs-based photocathodes in DC high-voltage electron photo-guns is limited by ion back-bombardment, where ions created within the photo-gun potential are accelerated toward the photocathode. This leads to an undesired reduction of the photocathode quantum efficiency (QE). At the Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Lab, experiments were performed to test the effectiveness of biasing the anode to mitigate ion back-bombardment. The experiments demonstrated marked improvement in photocathode operating lifetime when the anode was biased. The custom element IONization And Tracking Of Residual gas (IONATOR) has been developed using the framework of General Particle Tracer (GPT) to simulate electron impact ionization in real time. While IONATOR was originally developed for simulations of the CEBAF photo-gun, it is readily extensible to other applications. The custom element uses Monte Carlo routines to calculate the ion production rate and the secondary electron kinetic energy based on user-defined gas pressures, the ionization cross section, and the secondary electron differential cross section. It then uses relativistic kinematics to track the secondary electron, the scattered electron, and the ion after ionization. The ion production rate and the secondary electron energy distribution determined by the custom element have been benchmarked against theoretical calculations. The IONATOR custom element will be described in detail and its application in GPT simulations of the biased anode experiments to improve photocathode lifetime at the CEBAF accelerator will be presented.
