Abstract
Charge lifetime of strained superlattice GaAs photocathodes in DC guns is limited by ion back bombardment. It needs to be improved at least an order of magnitude to meet the requirements for future colliders such as Electron-Ion Collider (EIC). In this work, we propose and present simulation results for an offset anode scheme to increase charge lifetime in DC guns. This scheme 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.
Highlights
Strained superlattice GaAs photocathode based high voltage dc (HV-dc) guns have been used as sources of polarized electron beam in various facilities around the world [1,2,3,4]
If ion back bombardment is the dominant mechanism of cathode degradation and there is no significant beam loss, the charge lifetime of a gun is directly proportional to the laser spot area
The negative electron affinity (NEA) layer deteriorates due to ion back bombardment and part of the laser illuminated area becomes incapable of electron emission
Summary
Strained superlattice GaAs photocathode based high voltage dc (HV-dc) guns have been used as sources of polarized electron beam in various facilities around the world [1,2,3,4]. Apart from improving the vacuum levels in a gun, multiple operational techniques have been explored to increase the charge lifetime of a polarized gun These techniques include increasing the laser spot size, operating the laser off center, limiting the cathode active area and biasing the anode to block ions coming from downstream of the dc gap [8,10,18]. Offsetting the laser for higher charge lifetime has been used at MAMI Mainz and Jefferson lab, which is currently the standard mode of operation for polarized electron guns [4,19] In this scheme, beam is extracted from a spot radially offset from the electrostatic center (EC) of the cathode. There we deal with the ability to transport the electron beam with no loss in the gun, and keeping a good emittance in this nontrivial 3-D transport problem
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