Dutine, M. Arnold, M. Bahlo, T. Grewe, R. Jürgensen, L.E. Pietralla, N. Schließmann, F. Steinhorst, M. JACoW Publishing Geneva, Switzerland 2673-5350 978-3-95450-204-2 10.18429/JACoW-IBIC2019-WEPP004 English 513-516 WEPP004 cavity linac operation recirculation impedance Contribution to a conference proceedings 2019 2019-11 https://doi.org/10.18429/JACoW-IBIC2019-WEPP004 http://jacow.org/ibic2019/papers/wepp004.pdf The S-DALINAC* is a thrice-recirculating linear electron accelerator operating in cw-mode at a frequency of 3 GHz. A path-length adjustment system in the second recirculation beam line allows to shift the beam phase by 360° and thus to operate in ERL mode. For the multi-turn ERL operation, the beam will be accelerated twice and subsequently decelerated twice again (not demonstrated yet). For this mode, it is necessary to develop a nondestructive beam diagnostics system in order to measure the beam position, phase and beam current of both, the accelerated and the decelerated beam, simultaneously in the same beamline. 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.