Injection arc design for ERL operation at MESA*

MESA is a recirculating superconducting accelerator under construction at Johannes Gutenberg-Universitaet Mainz. It will be operated in two different modes: the first is the external beam (EB) mode, where the beam is dumped after being used at the experiment. The required beam current in EB mode is 150 μA with polarized electrons at 155 MeV. In the second operation mode MESA will be run as an energy recovery linac (ERL) with an unpolarized beam of 1 mA at 105 MeV. In a later construction stage of MESA the achievable beam current in ERL-mode shall be upgraded to 10 mA. The careful design of the 5 MeV injection arc of MESA and the control of the optical parameters is crucial to allow for the energy recovery mode to reach the desired performance. Optimization of the arc optics with MadX and the implementation of a cavity model in elegant are presented. The required parameters for the final arc optics were determined via backtracking through the first Cryomodule.