Few Femtosecond level electron bunch diagnostic at quasi-cw electron accelerators

At the SRF based prototype cw accelerator ELBE a new electron beamline, providing for femtosecond electron bunches with nC bunch charges and repetition rates in the 1-200 KHz regime and with pC bunch charge and repetition rates of 13 MHz, is currently being constructed. The 40 MeV electrons will be used in photon-electron interaction experiments with TW and PW class lasers and the generation of broad and narrow bandwidth coherent THz pulses. Discussed here are ideas for novel online diagnostics of the electron bunch properties (e.g. arrival time and bunch form) based on the time and frequency domain analysis of the emitted coherent THz radiation, but also based on direct measurements by e.g. electro-optic sampling. The suitability of ELBE as a testbed for diagnostic of future cw X-ray photon sources (e.g. energy recovery linacs) will be discussed.