• J.K. Sekutowicz, M. Ebert, F. Mittag
  DESY, Hamburg
• P. Kneisel
  JLAB, Newport News, Virginia
• R. Nietubyc
  The Andrzej Soltan Institute for Nuclear Studies, Centre Swierk, Swierk/Otwock

The European XFEL will use superconducting TESLA cavities operating with 650 μs long bunch trains. With 220 ns bunch spacing and 10 Hz RF-pulse repetition rate up to 27000 high quality bunches/s will be delivered to insertion devices generating unprecedented high average brilliance photon beams at very short wavelength. While many experiments can take advantage of full bunch trains, others prefer an increased several μ-seconds intra pulse distance between bunches, or short bursts with kHz repetition rate. With the nominal RF-pulse structure these features will lead to a substantially reduced number of bunches per second and therefore to significantly lower average brilliance. We discuss here an R&D program aiming for a far future upgrade of the European XFEL; operation in the cw and/or near-cw mode. The program profits from the continuous improvement in performance of TESLA cavities, which allows for longer RF-pulses in comparison with the current design. We present test results of a SRF electron injector and a new RF-power source, and some modification of the HOM damping scheme, which will avoid the necessity of re-assembly of the XFEL accelerator for the upgraded operations.