Previously we had shown transverse slice and longitudinal emittance measurements using a 1.6 cell S-band rf gun with a Cu cathode [1]. Slice emittances at low charge (15 pC) set an upper limit of 0.6 microns per mm radius. Previous measurements were limited to a maximum of 600 pC total charge due to the relatively low quantum efficiency (5 10-5) of the copper cathodes. In order to increase the available charge to the nC level, we have installed a Mg cathode. The lower work function of Mg compared to Cu increases the quantum efficiency for a fixed laser wavelength but also potentially increases the thermal emittance. Emittance measurements with the Mg cathode will be presented and compared with previous Cu cathode measurements.

The Linac Coherent Light Source (LCLS) will be the world’s first free electron laser, and the successful operation of this very short-wavelength FEL will require excellent beam quality from its electron source. Therefore a critical component is the RF photocathode injector. This paper describes the design issues of the LCLS RF gun and accelerator structures. The injector consists of a 1.6 cell s-band gun followed by two 3-meter SLAC sections. The gun and the first RF section will have dual RF feeds both to eliminate transverse RF kicks and to reduce the pulsed heating of the coupling ports. In addition, the input coupler cavity of the first accelerator section will be specially shaped to greatly reduce the RF quadrupole fields. The design for the accelerator section is now complete, and the RF design of the gun’s dual coupler and the full cell shape is in progress. These and other aspects of the gun and structure designs will be discussed.