Increasing charge lifetime in dc polarized electron guns by offsetting the anode

Charge lifetime of strained superlattice Gallium Arsenide photocathodes in dc guns is limited by ion back bombardment. In this work, we propose and present simulation results for an offset anode scheme to increase charge lifetime in dc guns. In this scheme, the axial symmetry of the cathode-anode configuration is broken by offsetting the anode transversely, while keeping the laser spot on the electrostatic center. This eliminates the bombardment of high energy ions on the cathode and enables maximum usage of the available cathode area. Depending on the size of the available cathode area, this method can increase the charge lifetime by an order of magnitude compared to the current best alternative method. An anode assembly, capable of in-vacuum movement, is required for this method, which has been designed and fabricated at Brookhaven National Laboratory.