Concept of a Beam Diagnostics System for the Multi-Turn ERL Operation at the S-DALINAC

The S-DALINAC is a thrice-recirculating linear electron accelerator operating in cw-mode at a frequency of 3 GHz. Due to the implementation of a path-length adjustment system capable of a 360 phase shift, it is possible to operate the accelerator in ERL mode. For the multi-turn ERL operation the beam will be accelerated twice and subsequently decelerated twice again. For this mode, a non-destructive beam diagnostics system is necessary in order to measure the beam position and phase of both, the accelerated and the decelerated beam simultaneously in the same beam line. A particular challenge will be the operation at low beam currents of 100 nA, which corresponds to bunch charges of about 30 aC. The conceptional study of a 6 GHz resonant cavity beam position monitor will be presented together with alternative solutions.