Yang, B. X.

Paper	Title	Page
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.
	Slides
	Talk
THPPH055	Nonintercepting Beam Size and Position Monitor Using ODR for X-Ray FELs	710
	A. H. Lumpkin, W. Berg, N. Sereno, B. X. Yang, C. Yao ANL, Argonne, Illinois D. W. Rule NSWC, West Bethesda, Maryland
	Interest in nonintercepting (NI) beam size and position diagnostics in the undulators of x-ray free-electron lasers (XFELs) is driven by the requirement of beam-emittance matching, as well as by the need to minimize radiation damage to the undulator's permanent magnets from scattered beam produced by the insertion of converter screens. For these reasons, our investigations on optical diffraction radiation (ODR) as NI relative beam size and position diagnostics are particularly relevant to XFELs. We report the extensions of our studies at 7-GeV beam energy to aspects of the vertical and horizontal polarization components of the ODR near-field and far-field images. The near-field, vertically polarized data are particularly interesting since the vertical field lines at the metal more directly reflected the actual beam sizes. Although our experiments to date are with larger beams and impact parameters of 1-2 mm, our analytical model indicates that this technique scales with beam size and has sensitivity at the 20- to 50-micron regime with an impact parameter, d = 5 times σ-y =100 microns. This is the x-ray FEL intraundulator beam size regime.