THOA  —  Synchronization and Stability   (26-Aug-10   08:30—10:00)

Chair: G.J. Hirst, STFC/RAL, Chilton, Didcot, Oxon

Paper Title Page
THOAI1 Femtosecond Synchronization of Laser Systems for the LCLS 534
 
  • J.M. Byrd, L.R. Doolittle, G. Huang, J.W. Staples, R.B. Wilcox
    LBNL, Berkeley, California
  • J. Arthur, J.C. Frisch, W.E. White
    SLAC, Menlo Park, California
 
 

The scientific potential of femtosecond x-ray pulses at linac-driven FELs such as the LCLS is tremendous. Time-resolved pump-probe experiments require a measure of the relative arrival time of each x-ray pulse with respect to the experimental pump laser. To achieve this, precise synchronization is required between the arrival time diagnostic and the laser which are often separated by hundreds of meters. For seeded FELs, synchronization is necessary between the seed and pump laser. We describe an optical timing system based on stabilized fiber links which has been developed for the LCLS. Preliminary results show stability of the timing distribution at the sub-20 fsec level. We present details of the results measured during LCLS operation for the first pump-probe experiment in October 2009 and the present user run starting in April 2010. We conclude with a discussion of potential for development.

 

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THOAI2 Intra-train Longitudinal Feedback for Beam Stabilization at FLASH 537
 
  • W. Koprek, C. Behrens, M.K. Bock, M. Felber, P. Gessler, H. Schlarb, Ch. Schmidt, S. Schulz, B. Steffen, S. Wesch
    DESY, Hamburg
  • J. Szewinski
    The Andrzej Soltan Institute for Nuclear Studies, Centre Swierk, Swierk/Otwock
 
 

The Free electron LASer at Hamburg (FLASH) is a linear accelerator of 330m length. It provides laser pulses with pulse duration between 10 and hundreds fs in the soft X-ray wavelength range below 5nm produced in SASE process from electron bunches with an energy up to 1.2 GeV. FLASH works in pulse mode with repetition rate of 10 Hz where up to 800 bunches at a bunch spacing of 1 us are accelerated in one macro-pulse. The electron beam time structure is well suited for fast intra-train feedbacks using beam based measurements incorporated to the Low Level Radio Frequency (LLRF) control system of the accelerator structures to further improve the bunch compressions, bunch arrival and bunch energy stability directly impacting the quality of the FEL photon beam. In this paper, we present the beam based signal pre-processing, the implementation into LLRF system, the mandatory exception handling for robust operation and the imbedding of the real-time ~ 2us latency fast intra-train feedback with feedbacks for the removal of slow and repetitive errors. First results of the achieved intra-train bunch arrival and peak current stability will be presented together with observed limitations.

 

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THOA3 RF-based Synchronization of the Seed and Pump-Probe Lasers to the Optical Synchronization System at FLASH 544
 
  • M. Felber, M.K. Bock, P. Gessler, K.E. Hacker, T. Lamb, F. Ludwig, H. Schlarb, B. Schmidt
    DESY, Hamburg
  • J. Breunlin, S. Schulz, L.-G. Wißmann
    Uni HH, Hamburg
 
 

At FLASH, UV and soft X-Ray pulses with durations in the order of 10 fs are generated. To fully exploit the opportunities provided by these short laser pulses, an optical synchronization system provides the possibility to synchronize external lasers and stabilize the electron bunch arrival time with 10 fs precision. A seeded free-electron-laser (FEL) section, called sFLASH, is installed upstream of the existing SASE undulators. After higher-harmonic-generation, the femtosecond seed laser pulse needs to be temporarily and spatially overlapped with the electron bunch. Furthermore, for time-resolved pump-probe experiments, using an experimental laser and the FEL pulse, either of sFLASH or of the ordinary SASE process, the synchronization between pump and probe laser pulses is crucial. While the best performance for synchronizing these lasers within 10 fs will be achieved by using an optical cross-correlator, in this paper we present a precursor that relies on an RF-based locking mechanism. The setup includes a coarse and a fine phase measurement between the laser pulses of the reference and the synchronized system after their conversion to an RF signal.

 

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THOA4 On-Line Arrival Time and Jitter Measurements Using Electro-Optical Spectral Decoding 548
 
  • N. Čutić, F. Lindau, S. Werin
    MAX-lab, Lund
  • E. Mansten
    Lund University, Division of Atomic Physics, Lund
 
 

Electro-optical spectral decoding was used to on-line monitor the arrival time of the electron bunches relative to the seed laser pulse at the test FEL facility at MAX-lab. An infrared chirped pulse coming from the seed laser is influenced by an electron bunch induced birefringence in a ZnTe birefringent crystal and the arrival time is determined from its spectrum. The possibility of running simultaneously with the FEL allowed for a feedback scheme to be built to compensate for the long term drifts in the system. Also, the whole system (the accelerator and the lasers) were synchronized to the power grid frequency. This lock increased the stability and was monitored by the EO setup. Measurements of the bunch length were performed and their correlation with arrival time pointed towards main contributors to the jitter in the system.

 

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