RF coupler for high-power CWFEL photoinjector

A high-current emittance-compensated RF photo-injector is a key enabling technology for a high-power CW FEL. The design presently under way is a 100-mA, 2.5-cell pi-mode, 700-MHz, normal conducting demonstration CW RF photoinjector. This photoinjector will be capable of accelerating 3 nC per bunch with an emittance at the wiggler less than 10 mm-mrad. The paper presents results for the RF coupling from ridged waveguides to the photoinjector RF cavity. The LEDA and SNS couplers inspired this "dog-bone" design. Electromagnetic modeling of the coupler-cavity system has been performed using both 2-D and 3-D frequency-domain calculations, and a novel time-domain approach with MicroWave Studio. These simulations were used to adjust the coupling coefficient and calculate the powerloss distribution on the coupling slot. The cooling of this slot is a rather challenging thermal management project.