• C. Kaya, U. Lehnert
  FZD, Dresden

At the ELBE Free Electron Laser (FEL) at Forschungszentrum Dresden Rossendorf (FZD) electron bunches having lengths between 1 to 4 ps are generated. It is required to compress these electron bunches to lengths below 1 ps which necessitates diagnosis of the electron bunch parameters. We use a Martin-Puplett interferometer (MPI) which is a modification of the Michelson interferometer, where the beams are linearly polarized at specific orientations. It measures the autocorrelation function of the coherent transition radiation (CTR) from a view screen which is an optical replication of the electron bunch. The interferometer setup consists of various optical components like polarizers, beam splitter, mirrors and Golay cell detectors. In our measurement a wire grid was used as a polarizer and also as a beam splitter. A thorough understanding of the response of the optical components, as a function of the CTR wavelength range of our interest, is required for correct analysis of the measured signal. We have therefore simulated the response of the entire interferometer setup including the diffraction losses and the window transmission and compared the results to experimental measurements.

• C. Gerth, F. Ludwig, Ch. Schmidt
  DESY, Hamburg

To exploit the short radiation pulses in pump-probe experiments at single-pass free-electron lasers (FELs), stabilization of the longitudinal profile and arrival time of the electron bunches is an essential prerequisite. Beam energy fluctuations, induced by the cavity field regulation in the accelerating modules, transform into an arrival time jitter in subsequent magnetic chicanes used for bunch compression due to the longitudinal dispersion. The development of beam based monitors is of particular importance for the validation and optimization of the cavity field regulation. In this paper we present bunch-resolved energy jitter measurements that have been recorded with a synchrotron radiation monitor at the Free-electron LASer in Hamburg (FLASH). The cavity field detectors of the accelerating module have been identified as the main source of the stochastical noise which corresponds to a beam energy jitter of 0.012%. The reduction of deterministic cavity field imperfections by applying an adaptive feedforward learning algorithm for the cavity field regulation is demonstrated.