• H.-S. Kang, J. Choi, T.-Y. Lee
  PAL, Pohang, Kyungbuk

Funding: The Ministry of Science and Technology, Korea

PAL-XFEL has been designed to generate 0.3-nm SASE radiation with 3.7-GeV electron beam and 4-mm gap in-vacuum undulator. The requirement of energy spread in undulator is tighter than LCLS and EU-FEL. Laser beam heating to reduce the micro-bunching instability inevitably induces an increase of energy spread during the bunching process in bunch compressor. Two factors are contradictory, which should be compromised. Transverse higher modes have comparatively large growth rates which results in poor transverse coherency. Growth rates of transverse modes are calculated with different beam conditions.

• S. Reiche
  UCLA, Los Angeles, California

The Linac Coherent Light Source operates as a Self-Amplified Spontaneous Emission Free-Electron Laser (SASE FEL), where transverse coherence is achieved by the domination of the FEL Eigenmode with the largest growth rate. However complete transverse coherence is not guaranteed because there are multiple eigenmodes with similar growth rates for a low-diffracting FEL, such as the LCLS. In addition the mode purity can be degraded by collective electron beam motion. In this presentation the transverse coherence for the LCLS pulse is investigated with respect to scattering on crystals. The degradation in the contrast and size of the Bragg peaks is analysed for a step wise improved modeling of the experiment (stead-state, time-dependent and start-end simulations). The impact on diffraction experiments, including the proposed experiment to measure the transverse coherence, is discussed.