TUB —  Seeded FELs   (22-Aug-17   10:30—12:00)
Chair: K.X. Liu, PKU, Beijing, People's Republic of China
Paper Title Page
TUB01 Seeding Experiments and Seeding Options for LCLS II 1
  • E. Hemsing, R.N. Coffee, W.M. Fawley, Y. Feng, B.W. Garcia, J.B. Hastings, Z. Huang, G. Marcus, D.F. Ratner, T.O. Raubenheimer
    SLAC, Menlo Park, California, USA
  • G. Penn, R.W. Schoenlein
    LBNL, Berkeley, California, USA
  We discuss the present status of FEL seeding experiments toward the soft x-ray regime and on-going studies on possible seeding options for the high repetition soft x-ray line at LCLS-II. The seeding schemes include self-seeding, cascaded HGHG, and EEHG to reach the 1-2 nm regime with the highest possible brightness and minimal spectral pedestal. We describe relevant figures of merit, performance expectations, and potential issues.  
Fresh Slice Self-Seeding and Fresh Slice Harmonic Lasing at LCLS  
  • C. Emma, C. Pellegrini
    UCLA, Los Angeles, USA
  • J.W. Amann, M.W. Guetg, J. Krzywinski, A.A. Lutman, C. Pellegrini, D.F. Ratner
    SLAC, Menlo Park, California, USA
  • D.C. Nguyen
    LANL, Los Alamos, New Mexico, USA
  We present results from the successful demonstration of fresh slice self-seeding at the Linac Coherent Light Source (LCLS).* The performance is compared with SASE and regular self-seeding at photon energy of 5.5 keV, resulting in a relative average brightness increase of a factor of 12 and a factor of 2 respectively. Following this proof-of-principle we discuss the forthcoming plans to use the same technique** for fresh slice harmonic lasing in an upcoming experiment. The demonstration of fresh slice harmonic lasing provides an attractive solution for future XFELs aiming to achieve high efficiency, high brightness X-ray pulses at high photon energies (>12 keV).***
* C. Emma et al., Applied Physics Letters, 110:154101, 2017.
** A. A. Lutman et al., Nature Photonics, 10(11):745-750, 2016.
*** C. Emma et al., Phys. Rev. Accel. Beams 20:030701, 2017.
slides icon Slides TUB02 [13.759 MB]  
TUB03 ASU Compact XFEL 1
  • W.S. Graves, J.P.J. Chen, P. Fromme, M.R. Holl, R. Kirian, L.E. Malin, K.E. Schmidt, J. Spence, M. Underhill, U. Weierstall, N.A. Zatsepin, C. Zhang
    Arizona State University, Tempe, USA
  • K.-H. Hong, D.E. Moncton
    MIT, Cambridge, Massachusetts, USA
  • C. Limborg-Deprey, E.A. Nanni
    SLAC, Menlo Park, California, USA
  Funding: This work was supported by NSF Accelerator Science award 1632780, NSF BioXFEL STC award 1231306 and DOE contract DE-AC02-76SF00515.
ASU is pursuing a concept for a compact x-ray FEL (CXFEL) that uses nanopatterning of the electron beam via electron diffraction and emittance exchange to enable fully coherent x-ray output from electron beams with an energy of a few tens of MeV. This low energy is enabled by nanobunching and use of a short-pulse laser field as an undulator, resulting in an XFEL with 10 m total length and modest cost. The method of electron bunching is deterministic and flexible, rather than dependent on SASE amplification, so that the x-ray output is coherent in time and frequency. The phase of the x-ray pulse can be controlled and manipulated with this method so that new opportunities for ultrafast x-ray science are enabled using e.g. attosecond pulses, very narrow linewidths, or extremely precise timing among multiple pulses with different colors. These properties may be transferred to large XFELs through seeding with the CXFEL beam. Construction of the CXFEL accelerator and laboratory are underway, along with initial experiments to demonstrate nanopatterning via electron diffraction. An overview of the methods, project, and new science enabled are presented.
TUB04 Recent On-Line Taper Optimization on LCLS 1
  • J. Wu, X. Huang, T.O. Raubenheimer
    SLAC, Menlo Park, California, USA
  • A. Scheinker
    LANL, Los Alamos, New Mexico, USA
  Funding: The work was supported by the US Department of Energy (DOE) under contract DE-AC02-76SF00515 and the US DOE Office of Science Early Career Research Program grant FWP-2013-SLAC-100164.
High-brightness XFELs are demanding for many users, in particular for certain types of imaging applications. Self-seeding XFELs can respond to a heavily tapered undulator more effectively, therefore seeded tapered FELs are considered as a path to high-power FELs in the terawatts level. Due to many effects, including the synchrotron motion, the optimization of the taper profile is intrinsically multi-dimensional and computationally expensive. With an operating XFEL, such as LCLS, the on-line optimization becomes more economical than numerical simulation. Here we report recent on-line taper optimization on LCLS taking full advantages of nonlinear optimizers as well as up-to-date development of artificial intelligence: deep machine learning and neural networks.
First Demonstration of Fully Coherent Super-Radiant Pulses From a Short-Pulse Seeded FEL  
  • X. Yang
    BNL, Upton, Long Island, New York, USA
  • L. Giannessi
    Elettra-Sincrotrone Trieste S.C.p.A., Basovizza, Italy
  The generation of a single X-ray isolated spike of radiation with peak power at the GW level and femtosecond temporal duration represents an almost unique opportunity for time-resolved non-linear spectroscopy. Such a condition is met by an FEL operating in superradiance. The resulting pulse has a self-similar shape deriving from the combined dynamics of saturation and slippage of the radiation over fresh electrons. The pulse is followed by a long pedestal, resulting from the complex dynamics occurring in the tail after saturation. This tail consists of a train of pulses with both transverse and longitudinal coherence and decaying amplitudes. We analyze the dynamical conditions on slippage and pulse length leading to the formation of the main pulse and the following tail. We study the correlation of the tail structure with the longitudinal phase space of the e-beam and provide recipes to partially suppress it near the background level leading to the fully coherent super-radiant pulse. Our analytical prediction of the intensity peak of the leading pulse evolving along the undulator before, during, and after becoming a super-radiant pulse agrees well with the simulations.