Online Longitudinal Bunch Profile and Slice Emittance Diagnostics at the European XFEL

The longitudinal current profile and slice emittance are important bunch parameters for the operation of an X-ray free-electron laser. At the European XFEL, dedicated diagnostic sections equipped with transverse deflecting RF structures (TDS) have been installed for the control and optimisation of these parameters. Travelling-wave TDS in combination with fast kicker magnets and off-axis screens allow for the study of individual bunches without affecting the other bunches in the super-conducting linear accelerator which can generate bunch trains of up to 2700 bunches at 4.5 MHz within 600 microsecond RF pulses at a repetition rateof 10 Hz. The measurement of the slice emittance is realised in a static F0D0 lattice equipped with four individual screen stations.Variations of the longitudinal bunch profile or slice emittance along the bunch train may lead to degraded FEL performance for parts of the train which reduces the effective available number of bunches for FEL operation. By gradually adjusting the timing, individual bunches along the bunch train can be measured in order to optimise the overall beam parameters for all bunches in the train. In this paper, we describe in detail the diagnostic concept and present first measurement results of the projected and slice emittance along the bunch train