• V. R. Arsov, M.K. Bock, M. Felber, P. Gessler, K.E. Hacker, F. Löhl, F. Ludwig, K.-H. Matthiesen, H. Schlarb, A. Winter
  DESY, Hamburg
• S. Schulz, L.-G. Wißmann, J. Zemella
  Uni HH, Hamburg

The need for timing and bunch-length monitors for free-electron lasers and other accelerators makes the electro-optic spectral decoding (EOSD) a promising diagnostic technique. Being non-destructive, it allows a single shot-measurement within the accelerator environment. In some cases, e.g. low charge or necessity to resolve time-structures below 20 fs, the bunch length is measured indirectly, using the spectrum of the coherent transition radiation (CTR). We present results of EOSD measurements on the CTR beam line at FLASH, using a chirped Ti:Sapphire oscillator pulse focused simultaneously with the THz radiation on a GaP crystal in vacuum. The CTR spectrum is in the range 200 GHz-100 THz and and the pulse energy in the focus is over 10 mJ. The measured narrow CTR temporal profiles in the order of 400 fs FWHM demonstrate, that the ultra short THz-pulses, emitted by the compressed electron bunches are transported through the 19 m long beam line without significant temporal broadening.

• 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.