Recent advances in polarized electron sources

Current experimental physics programs at a number of electron accelerator laboratories worldwide require the delivery of high average current highly polarized electron beams for long periods of time. The polarized electrons are produced by near bandgap photoemission from certain semiconductor photocathodes. The authors observe the quantum efficiency of these cathodes to be inversely related to the total charge they have delivered. Recent developments in ultrahigh vacuum technology, electron trajectory control, photocathode preparation, and lasers have led to operationally reliable delivery of many hundreds of coulombs of polarized electrons, at rates as high as 8 coulombs/day. Currently, their photocathode operational lifetime is almost completely dominated by ion back bombardment. Further gains in the high average current lifetime of these cathodes may be expected, which will allow photoemission electron guns to be used for accelerator applications other than polarized sources