Author: Viefhaus, J.
Paper Title Page
WED01 Commissioning of the Delta Polarizing Undulator at LCLS 757
  • H.-D. Nuhn, S.D. Anderson, R.N. Coffee, Y. Ding, Z. Huang, M. Ilchen, Yu.I. Levashov, A.A. Lutman, J.P. MacArthur, A. Marinelli, S.P. Moeller, F. Peters, Z.R. Wolf
    SLAC, Menlo Park, California, USA
  • J. Buck
    XFEL. EU, Hamburg, Germany
  • G. Hartmann, J. Viefhaus
    DESY, Hamburg, Germany
  • A.O. Lindahl
    University of Gothenburg, Gothenburg, Sweden
  • A.B. Temnykh
    Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
  Funding: Work was supported by U.S. DOE, Office of Basic Energy Sciences, under Contract DE-AC02-76SF00515. A.B. Temnykh is supported U.S. National Science Foundation awards DMR-0807731 and DMR-DMR-0936384.
The LCLS generates linearly polarized, intense, high brightness x-ray pulses from planar fixed-gap undulators, which provides only limited taper capability and lacks polarization control. The latter is of great importance for soft x-ray experiments. A new 3.2-m-long compact undulator (based on the Cornell University fixed-gap Delta design) has been developed and installed as the last LCLS undulator segment (U33) in October 2014. The Delta undulator provides full control of the polarization degree and K parameter through array position adjustments. Used on its own, it produces fully polarized spontaneous radiation in the selected state (linear, circular or elliptical). To increase the output power by orders of magnitude, the electron beam is micro-bunched by several (5-15) upstream LCLS undulator segments operated in the linear FEL regime. This micro-bunching process produces horizontally linear polarized (background) radiation. This unwanted radiation component has been greatly reduced by a reversed taper configuration, as suggested by Schneidmiller. Full elimination of the linear polarized component was achieved through spatial separation combined with transverse collimation. The paper will describe the methods tested during commissioning and will also present results of polarization measurements showing high degrees of circular polarization in the soft x-ray wavelength range.
slides icon Slides WED01 [10.165 MB]  
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An Online Diagnostic Unit for FEL Photon Parameter Determination and Optimization  
  • J. Viefhaus
    DESY, Hamburg, Germany
  Originally developed for the Variable Polarization XUV Beamline P04 at PETRA III, an online diagnostic unit for beamline quality control was successfully applied to FEL radiation as well. The relevant photon properties include relative photon flux, photon energy and bandpass, beam position and especially the degree of linear polarization of the photon beam. The device consist of sixteen individual electron time-of-flight electron spectrometers which are oriented in a plane perpendicular to the incoming radiation. A gas-phase target (usually rare gases at pressures of about 10-08 to 10-06 hPa) with very low absorption allows to put the device in front of the user experiment for online diagnostics of the x-ray beam. The efficient time-of-flight method with flight times in the order of tens of ns together with a fast multi-channel digitizer system and multi-threaded data evaluation algorithms allows true online shot-by-shot visualization at high repetition rates with minimal delay. Results from different FEL sources showing the flexibility as well as the real-time capabilities of the system will be presented. This work was only made possible by fruitful collaborations and extensive support from the facilities PETRA III and FLASH (DESY, Hamburg), European XFEL (Hamburg), FERMI (Elettra, Sincrotrone Trieste) and LCLS (SLAC, Menlo Park).  
slides icon Slides WED02 [180.803 MB]  
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