Nuhn, H.-D.

Paper	Title	Page
MOPPH040	Transverse Coherence Properties of the LCLS X-Ray Beam	126
	S. Reiche UCLA, Los Angeles, California H.-D. Nuhn SLAC, Menlo Park, California
	Self-amplifying spontaneous radiation free-electron lasers, such as the LCLS or the European XFEL, rely on the incoherent, spontaneous radiation as the seed for the amplifying process. Though this method overcomes the need for an external seed source one drawback is the incoherence of the effective seed signal. The FEL process allows for a natural growth of the coherence because the radiation phase information is spread out within the bunch due to slippage and diffraction of the radiation field. However, at short wavelengths this spreading is not sufficient to achieve complete coherence. In this presentation we report on the results of numerical simulations of the LCLS X-ray FEL. From the obtained radiation field distribution the coherence properties are extracted to help to characterize the FEL as a light source.
THBAU02	Electron Beam Alignment Strategy in the LCLS Undulators	529
	H.-D. Nuhn, P. Emma, G. L. Gassner, C. M. LeCocq, F. Peters, R. E. Ruland SLAC, Menlo Park, California
	The x-ray FEL process puts very tight tolerances on the straightness of the electron beam trajectory (2 μm rms) through the LCLS undulator system. Tight but less stringent tolerances of 80 μm rms vertical and 140 μm rms horizontally are to be met for the placement of the individual undulator segments with respect to the beam axis. The tolerances for electron beam straightness can only be met through beam-based alignment (BBA) based on electron energy variations. Conventional alignment will set the start conditions for BBA. Precision-fiducialization of components mounted on remotely adjustable girders and the use of beam-finder wires (BFW) will enable to satisfy placement tolerances. Girder movement due to ground motion and temperature changes will be monitored continuously by an alignment monitoring system (ADS) and remotely corrected. This stabilization of components as well as the monitoring and correction of the electron beam trajectory based on BPMs and correctors will increase the time between BBA applications. Undulator segments will be periodically removed from the undulator Hall and measured to monitor radiation damage and other effects that might degrade undulator tuning.
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	Talk
THBAU05	Precision Measurement of the Undulator K Parameter using Spontaneous Radiation	548
	J. J. Welch, J. Arthur, P. Emma, J. B. Hastings, Z. Huang, H.-D. Nuhn, P. Stefan SLAC, Menlo Park, California R. M. Bionta LLNL, Livermore, California R. J. Dejus, B. X. Yang ANL, Argonne, Illinois
	Obtaining a precise and uniform value of the undulator parameter, K, over the full undulator length is critical for producing high-gain FEL radiation, especially in a hard x-ray source such as the LCLS. At an FEL wavelength of 1.5-Å the relative variation of K over the full undulator must be (dK/K)rms < 0.015%. Transverse misalignments, construction errors, radiation damage, and temperature variations all contribute to a different K value in each few-meter-long undulator segment. It is therefore important to measure relative K precisely, after installation and alignment, using beam-based techniques, if possible. We propose a fairly simple method using the angle-integrated spontaneous radiation spectrum of two interfering undulators, and the natural shot-to-shot energy centroid jitter of the electron beam, to measure the relative K error between two segments using both ideal and measured undulator fields. By 'leap-frogging' to different pairs of undulators with extended separations we hope to confirm or correct the value of K, including proper tapering, over the entire 130-m long FEL undulator.
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	Talk