  A high-current emittance-compensated RF photoinjector is a key enabling
technology for a high-power CW FEL. A preliminary design of a
normal-conducting, 2.5-cell pi-mode, 700-MHz CW RF photoinjector that will be
built for demonstration purposes, is completed. This photoinjector will be
capable of accelerating a 100-mA electron beam (3 nC per bunch at 35 MHz bunch
repetition rate) to 2.7 MeV while providing an emittance below 7 mm-mrad at the
wiggler. More than 1 MW of RF power will be fed into the photoinjector cavity
through two ridge-loaded tapered waveguides. The waveguides are coupled to the
cavity by "dog-bone" irises cut in a thick wall. Due to CW operation of the
photoinjector, the cooling of the coupler irises is a rather challenging
thermal management project. This paper presents results of a detailed
electromagnetic modeling of the coupler-cavity system, which has been performed
to select the coupler design that minimizes the iris heating due to RF power
loss in its walls.
