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Description
Nuclear physics experiments at Jefferson Lab (JLab) require spin-polarized electron beams generated from delicate semiconductor photocathodes in a photoemission electron gun (photogun). The JLab photogun operates at 130 kV using an inverted geometry alumina insulator (feedthrough) as a holding structure to the highly polished stainless steel cathode electrode inside an ultra-high vacuum chamber. The alumina feedthrough is conical in shape for connecting the photogun to the power supply using commercial high voltage cable connectors. The use of compact inverted feedthroughs in high voltage dc photoguns was implemented by Jefferson Lab in 2010 and it is now a well-established technology in all high voltage photoguns which have since been built. Although the state of the art photoguns with inverted feedthroughs are capable of operating at 200-300 kV, proposed polarized high intensity electron beams as a driver in the production of polarized positrons, requires photoguns operating at higher voltage to generate the electron beam at ten times the intensity in the present JLab photogun. This will require emission from larger areas in the photocathode to spread out ion damage for sustaining photocathode lifetime. The present feedthroughs are limited to ~ 300 kV, but higher voltage is required to condition the electrodes for field emission free operation at the desired voltage. This contribution describes the development of an unprecedented feedthrough tested to 500 kV. The alumina feedthrough was connected to a high voltage power supply using a commercial cable connector via a modified epoxy receptacle with intervening SF6 layer and a test electrode in a vacuum vessel.
