Controlling coupler-kick emittance growth in the cornell erl main linac

One of the main concerns in the design of a high brightness Energy Recovery Linac X-ray source is the preservation of beam emittance. Discussed is one possible source of emittance dilution due to transverse electromagnetic fields in the accelerating cavities of the linac caused by the power coupler geometry. This has already been found to be a significant effect in Cornell´s ERL injector cavities if only one coupler per cavity is chosen. Here we present results of simulations for Cornell´s main ERL linac with six possible coupler configurations and compare them with regards to total emittance growth after one complete pass through the linac. We explain why the sign of the phase between the transverse kick and the accelerating force alternates each cavity when the cavities are arranged in a mirror symmetric way. In this case each cavity with its coupler before the structure is followed by one where the coupler is placed after the structure. This leads to a cancellation of the emittance growth to acceptable values. We find that cavity detuning of individual cavities does not destroy this cancellation. Furthermore we analyze other methods of compensating coupler kicks and find that symmetrization of the cavity geometry in the coupler region with the addition of a stub opposite the coupler is very efficient.