THOA  —  FEL Technology II: Post-Accelerators   (27-Aug-09   09:00—10:45)

Chair: B.C. Kuske, BESSY GmbH, Berlin

Paper Title Page
THOA01 Undulators for the SwissFEL 706
 
  • T. Schmidt, S. Reiche
    PSI, Villigen
 
 

The proposed SwissFEL will provide both hard x-rays down to 1Å and soft x-rays with full polarization control with a rather small electron energy of 5.8GeV. This continues the strategy of the medium energy synchrotron facilities, namely the SLS. The U15 and UE40 undulators are based on the experience with small period, small gap in-vacuum undulators and of APPLE II type respectively but are optimized for FEL operation. The undulator design including room temperature versus cryogenic principle, field optimization, materials and the demands for a series production will be discussed.

 

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THOA02 LCLS Undulator Commissioning, Alignment, and Performance 714
 
  • H.-D. Nuhn
    SLAC, Menlo Park, California
 
 

The LCLS x-ray FEL has recently achieved its 1.5-Angstrom lasing and saturation goals upon first trial. This was achieved with an extensive effort of pre-beam checkout, both traditional and beam-based component alignment techniques, and a high electron beam brightness. The x-ray FEL process demands very tight tolerances on the straightness of the electron beam trajectory (<5 μm) through the LCLS undulator system. Tight, but less stringent tolerances of ~100 μm rms were met for the transverse placement of the individual undulator segments with respect to the beam axis. The tolerances for electron beam straightness can only be met through a beam-based alignment (BBA) method, which is implemented using large electron energy variations and sub-micron cavity beam position monitors (BPM), with precise conventional alignment used to set the starting conditions. Precision-fiducialization of components mounted on remotely adjustable girders, and the use of special beam-finder wires (BFW) at each girder have been used to achieve these challenging alignment tolerances.

 

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THOA03 Selection of the Optimum Undulator Parameters for the NLS: A Holistic Approach 722
 
  • J.A. Clarke, D.J. Dunning, B.D. Fell, K.B. Marinov, N. Thompson
    STFC/DL/ASTeC, Daresbury, Warrington, Cheshire
  • N. Bliss
    STFC/DL, Daresbury, Warrington, Cheshire
 
 

The choice of undulator design and minimum magnet gap is crucial in the definition of every short wavelength FEL and is ultimately a cost driver for that project. The magnet gap selection is a compromise between wanting to minimise harmful wakefield effects whilst at the same time generating high magnetic fields with short periods. The NLS project has tried to take a holistic approach in the definition of the undulators. This has been carried out by first assessing the impact of resistive wall wakefields in general on the FEL performance and then selecting the maximum level of wakefield which has a just tolerable impact on the FEL. This wakefield is then translated into equivalent circular and elliptical vessel geometries. Suitable vessel thickness and mechanical tolerances are then added to define the undulator magnet gap for the case of a circular vessel (Delta undulator) and an elliptical vessel (APPLE-2 undulator). Finally, the two types of undulator have been modelled, their parameters compared, and a selection made. This paper summarises this global self-consistent approach to undulator definition and reports on the result for the NLS.

 

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THOA04 Undulator Options for Soft X-ray Free Electron Lasers 726
 
  • S. Prestemon, D. Schlueter
    LBNL, Berkeley, California
 
 

Soft X-ray free electron laser sources require significant photon energy tuning and ideally provide variable polarization to users. The LBNL proposed facility, BLASER, will provide multiple FEL lines with varying spectral characteristics to satisfy a broad array of soft X-ray physics. A variety of undulator technologies are being investigated to satisfy these requirements. We evaluate the performance characteristics of the key competing technologies, including superconducting options, and outline the impact of technology choice on overall facility design and cost. We review the key R&D issues that must be addressed to validate the different technologies for soft X-ray FEL application.

 

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THOA05 Undulator K-Parameter Measurements at LCLS 730
 
  • J.J. Welch, A. Brachmann, F.-J. Decker, Y.T. Ding, P. Emma, A.S. Fisher, J.C. Frisch, Z. Huang, R.H. Iverson, H. Loos, H.-D. Nuhn, P. Stefan, J.L. Turner, J. Wu, D. Xiang
    SLAC, Menlo Park, California
  • R.M. Bionta
    LLNL, Livermore, California
  • D.F. Ratner
    Stanford University, Stanford, California
  • H. Sinn
    European X-ray Free Electron Laser Project Team, c/o DESY, Hamburg
 
 

Precision in-situ measurements of relative undulator segment K parameters were made at the LCLS and are reported here. We describe the methods used, systematics errors, and signal levels. A method for determining the central ray from each undulator segment was developed to control the effect of angle-energy correlation of the spontaneous radiation on the photon energy spectrum. A variety of photon-energy sensitive detectors were employed, including: Ni foil, the yttrium component in a YAG screen, and a narrow band monochromator followed by either a photodiode or a YAG screen. Different harmonics of the spontaneous radiation were also used.

 

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