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At FLASH, high radiation dose at the undulator is crucial problem in the long pulse operation of the SASE. Dark current generated at the rf gun is the most responsible source of the high dose. The L-band gun is operated with a maximum rf field of 42 MV/m at the cathode. At the rf gun of the European X-ray FEL, an rf field up to 60 MV/m at the cathode will be applied to decrease 1 nC beam emittance below 1 mm·mrad. According to the Fowler-Nordheim equation, much higher dark current is estimated at the XFEL gun. In this article, ideas to reduce the dark current at the gun and a collimator design are presented.
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