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We have been developing a compact-sized cartridge-type cathode exchanging system installed in BNL-type IV photocathode RF gun. We can replace a cathode without breaking the vacuum of RF gun, so that a high quantum efficiency photocathode is not surrounded by oxygen or moisture. The advantage of this system can be controlled the quality of the each cathode by making cathode plugs in a factory. Moreover we can easily change a cathode material, such as visible light driven cathode (AgOCs NaK2Sb) the high QE cathode(Cs2Te) for high brightness beam, metal cathode(Mg) for ultra-fast phenomena. Therefore we can investigate characterization of variable cathode materials in high gradient electric field of ~100MV/m. The cavity with the exchanging port and the beam trajectory is calculated by superfish and GPT, respectively. We found that the parameters of the cavity with a plug is almost same compared with normal back plate. Using this system, we can investigate the cathode material and deliver the stable electron beam by one RF gun.
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