Abstract
The proposed FELs at CEBAF require an electron source delivering 120 pC bunches at a repetition rate of 7.485 MHz, corresponding to an average current of 0.9 mA. To meet this requirement we will employ a 500 kV DC photoemission electron gun to produce nominal 100 psec bunches of modest peak current. The subsequent injector system will bunch and accelerate this beam, producing 60 A, 2 psec bunches for the FELs. The photoemission gun will use a negative electron affinity GaAs photocathode, which provides good quantum efficiency and an adequate temporal response. The optical beam will be provided by a frequency doubled Nd:YLF laser system, actively mode locked to a subharmonic of the fundamental accelerator frequency. The principal technical difficulties associated with an electron source of this type involve the operating lifetime of the photocathode, and the operation of a high voltage gun in the presence of the alkali metals necessary to produce the photocathode. Various design a
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