Beam breakup simulations for the Mainz Energy recovering Superconducting Accelerator MESA

MESA is a two pass energy recovery linac (ERL) currently under construction at the Johannes Gutenberg-University in Mainz. MESA uses four 1.3 GHz TESLA type cavities with 12.5 MVm-1 of accelerating gradient in two modified ELBE type cryomodule with improved thermal connection of the HOM antennas and cw operation. In the first stage of MESA operation 1mA of beam current is foreseen, which will later be upgraded to 10 mA. One potential limit to maximum beam current in ERLs is the transverse beam breakup (BBU) instability induced by dipole Higher Order Modes (HOMs). These modes can be excited by bunches passing through the cavities off axis. Following bunches are then deflected by the HOMs, which results in even larger offsets for recirculated bunches. This feedback can even lead to beam loss. Simulation results for HOM spectra of a single TESLA cavity are available for example in [1]. It was possible to measure the HOM spectra in the cold, not tuned cavities at DESY and in the cold string tuned to the 1.3 GHz fundamental mode at Mainz. Results for the maximum beam current for MESA, limited by BBU, for the various HOM spectra are presented