TUC —  FEL Oscillators   (22-Aug-17   13:30—15:00)
Chair: Y.K. Wu, FEL/Duke University, Durham, North Carolina, USA
Paper Title Page
TUC01 Polarization Control of Storage Ring FELs Using Cross Polarized Helical Undulators 1
  • J. Yan, H. Hao, S.F. Mikhailov, V. Popov, Y.K. Wu
    FEL/Duke University, Durham, North Carolina, USA
  • S. Huang
    PKU, Beijing, People's Republic of China
  • J.L. Li
    IHEP, Beijing, People's Republic of China
  • V. Litvinenko
    Stony Brook University, Stony Brook, USA
  • N.A. Vinokurov
    BINP SB RAS, Novosibirsk, Russia
  For more than two decades, accelerator researchers have been working to gain control of polarization of synchrotron radiation and FELs using non-optical means. In 2005, the first experimental demonstration of polarization control of an FEL beam was realized with the Duke storage-ring FEL. With the recent upgrade of the undulator system, the Duke FEL can be operated with up to four helical undulators simultaneously. Using two sets of helical undulators with opposite helicities, for the first time, we have demonstrated full polarization control of a storage ring FEL. First, the helicity switch of the FEL beam has been realized with good lasing up to a few Hz. Second, the linearly polarized FEL beam has been generated with a high degree of polarization (Plin>0.95). The FEL polarization direction can be fully controlled using a buncher magnet. Furthermore, the use of non-optical means to control the FEL polarization allows us to extend polarization control to gamma-ray beams generated using Compton scattering. This has been experimentally demonstrated with the production of linearly polarized Compton gamma-ray beams with rotatable polarization direction based upon helical undulators.  
slides icon Slides TUC01 [5.916 MB]  
TUC02 Thermal and Mechanical Stability of Bragg Reflectors under Pulsed XFEL Radiation 1
  • I. Bahns, C.P. Maag, J. Roßbach, P. Thiessen
    University of Hamburg, Institut für Experimentalphysik, Hamburg, Germany
  • H. Sinn, V. Sleziona
    XFEL. EU, Hamburg, Germany
  • J. Zemella
    DESY, Hamburg, Germany
  Funding: BMBF FKZ 05K16GU4
Free-electron laser(FEL) x-ray radiation can deliver pulses with a huge amount of energy in short time duration. X-ray optics like Bragg reflectors therefore must be chosen in a way that they can withstand radiation-material interaction without getting damaged so that they can maintain their technical functionality. Therefore thermal and mechanical reactions of Bragg reflectors to the radiation induced thermal strain and force (radiation pressure) have been considered in this study. The theory of thermoelasticity has been used to simulate the strain conditions at saturation of the amplifying process in an X-ray free-electron laser oscillator(XFELO). One aim of this study was to investigate, if the radiation pressure could be an effect that gives a considerable contribution to the strain propagation. The results of the simulations have shown that, if Bragg backscattering of the X-ray pulse by a diamond crystal with 99% reflectivity and 1% absorptivity is assumed, the value of the thermally induced strain is about two magnitudes higher than the radiation pressure induced strain.
High-Flux, Fully Coherent X-Ray FEL Oscillator  
  • K.-J. Kim, S.P. Kearney, T. Kolodziej, R.R. Lindberg, X. Shi, D. Shu, Yu. Shvyd'ko
    ANL, Argonne, Illinois, USA
  • K.L.F. Bane, Y. Ding, P. Emma, W.M. Fawley, J.B. Hastings, Z. Huang, J. Krzywinski, G. Marcus, T.J. Maxwell
    SLAC, Menlo Park, California, USA
  • V.D. Blank, S. Terentiev
    TISNCM, Troitsk, Russia
  • W.M. Fawley
    Elettra-Sincrotrone Trieste S.C.p.A., Basovizza, Italy
  • C. Grizolli
    LNLS, Campinas, Brazil
  • W. Qin
    PKU, Beijing, People's Republic of China
  • S. Stoupin
    Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
  • J. Zemella
    DESY, Hamburg, Germany
  Funding: The ANL part of this work is supported by the U.S. DOE Office of Science under Contract No. DE-AC02-06CH11357 and the SLAC part under contract No. DE-AC02-76SF00515.
By optimizing the parameters of the accelerator, undulator, and the optical cavity, an XFELO driven by an 8-GeV superconducting linac is predicted to produce 10zEhNZeHn photons per pulse at the important photon energies around 14.4 keV.* This is an order of magnitude larger than that in previous designs.** With a BW of 3 meV (FWHM), rep rate of 1 MHz, and taking into account the full coherence, the spectral brightness is then 2×1026 photons per (mm2mr2 0.1\% BW), which is higher than any other source currently operating or anticipated in the future. Experiments at APS beam lines have shown that a high-quality diamond crystal can survive the power density (~15 kW/mm2) expected at the XFELO intra-cavity crystals preserving the high reflectivity.*** The compound refractive lenses can serve as the focusing element. Adding an XFELO to the suite of other FEL sources will, at a minor incremental cost but with a major scientific payoff, significantly expand the scientific capabilities at superconducting linac-based XFEL facilities, such as the European XFEL, the proposed LCLS-II High Energy upgrade and the XFEL project in Shanghai.
* W. Qin et al., this conference.
** R.R. Lindberg et al., Phys. Rev. ST Accel. Beams, vol 14, 403 (2011).
*** T. Kolodziej et al., this conference.
TUC04 Enhancement of Radiative Energy Extraction in an FEL Oscillator by Post-Saturation Beam Energy Ramping 1
  • H. S. Marks, A. Gover
    University of Tel-Aviv, Faculty of Engineering, Tel-Aviv, Israel
  • E. Dyunin, Yu. Lurie
    Ariel University, Ariel, Israel
  We present experimental results showing a greater than 50% increase in post-saturation radiation power extraction from a Free Electron Laser oscillator based on an electrostatic accelerator. Electrostatic accelerator free electron laser oscillators have the potential for CW operation. Present day operating oscillators rely on long pulses of electrons, tens of microseconds in duration; they generate correspondingly long radiation pulses, at a single longitudinal mode after a mode competition process. The presented post-saturation power extraction enhancement process is based on temporal tapering (up-ramping) of the beam energy, enabling a large synchrotron oscillation swing of the trapped electron bunches in passage along the interaction length. We further discuss the theoretical limits of the temporal tapering efficiency enhancement process.  
Start-to-End Simulations for an X-Ray FEL Oscillator at the LCLS-II and LCLS-II-HE  
  • W. Qin, K.L.F. Bane, Y. Ding, Z. Huang, G. Marcus, T.J. Maxwell
    SLAC, Menlo Park, California, USA
  • S. Huang, K.X. Liu
    PKU, Beijing, People's Republic of China
  • K.-J. Kim, R.R. Lindberg
    ANL, Argonne, Illinois, USA
  The proposed high repetition-rate electron beam from the LCLS-II and LCLS-II High Energy (LCLS-II-HE) upgrade are promising sources as drivers for an X-ray FEL Oscillator (XFELO) operating at both the harmonic and fundamental frequencies. In this contribution we present start-to-end simulations for an XFELO operating at the fifth harmonic with 4 GeV LCLS-II beam and at the fundamental with 8 GeV LCLS-II-HE beam. The electron beam longitudinal phase space is optimized by shaping the photoinjector laser and adjusting various machine parameters. The XFELO simulations show that high-flux output radiation pulses with 1010 photons and 3 meV (FWHM) spectral bandwidth can be obtained with the 8 GeV configuration.