MOD —  Monday - Late Afternoon   (26-Aug-19   16:15—17:45)
Chair: A. Gover, University of Tel-Aviv, Faculty of Engineering, Tel-Aviv, Israel
Paper Title Page
Physics of Post-Saturation Tapered FEL Towards Single-Frequency Terawatt Output Power  
  • C.-Y. Tsai
    HUST, Wuhan, People’s Republic of China
  Funding: This work is supported by the Fundamental Research Funds for the Central Universities under Project No. 5003131049 and the U.S. DOE under Contract No. DE-AC02-76SF00515 and No. FWP-2013-SLAC-100164.
TW-level output power of a single-pass high-gain x-ray FEL has recently attracted much attention. GW-level power can be achieved out of beneficial FEL instability. The remaining factor of 50 before reaching TW poses great challenges, including sustaining power growth and retaining spectral purity in the post-saturation regime. Post-saturation FEL physics involves the sideband instability [1], radiation diffraction [2], undulator tapering [3] etc. In this talk I will introduce FEL sideband and diffraction effects, and give an estimate of individual effects to the overall performance. Undulator tapering is known an effective route to enhance power extraction efficiency. We recently proposed an efficiency-enhancement scheme based on preserving the longitudinal beam phase space, which ensures trapping of resonant particles in the ponderomotive bucket and meanwhile takes advantage of the increasing radiation amplitude to precipitate the particle deceleration process. Analysis shows that the optimal power efficiency based on the proposed scheme, together with a prebunched beam, can be greatly improved within a relative short taper length and the sideband effects effectively suppressed.
[1] C.-Y. Tsai et al., Phys. Rev. Accel. Beams 20, 120702 (2017)
[2] C.-Y. Tsai et al., Phys. Rev. Accel. Beams 21, 060702 (2018)
[3] C.-Y. Tsai et al., NIM A 913 (2019) 107-119
slides icon Slides MOD01 [8.530 MB]  
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Microbunch Rotation and Coherent Undulator Radiation from a Kicked Electron Beam  
  • J.P. MacArthur
    SLAC, Menlo Park, California, USA
  Microbunches form perpendicular to the electron travel direction, and the conventional understanding is that they shear rather than rotate in response to a transverse kick, locking FEL facilities into a single-user operating mode. We show that microbunches rotate toward the new direction of travel if the electron beam is kicked and defocused. We provide evidence that microbunch rotation explains the unexpectedly large amount of off-axis radiation observed during experiments at the Linac Coherent Light Source. We demonstrate that LCLS can be multiplexed into at least three soft X-ray beams using this principle. Finally, we report on a more sophisticated scheme of offset quadrupoles that was used to produce two distinct hard X-ray spots at LCLS.  
slides icon Slides MOD02 [25.347 MB]  
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Hanbury Brown and Twiss Interferometry at XFEL Sources  
  • I. Vartaniants
    DESY, Hamburg, Germany
  The invention of optical lasers led to a revolution in the field of optics as well as to the birth of quantum optics. The reasons were the unique statistical and coherence properties of lasers. Short-wavelength free-electron lasers (FELs) are sources of bright, coherent extreme-ultraviolet and X-ray radiation with pulse duration on the order of tens of femtoseconds and are presently considered to be laser sources at these energies. Hanbury Brown and Twiss (HBT) interferometry that is based on intensity correlations allows fast and comprehensive analysis of the FEL statistical properties. We demonstrate that self-amplified spontaneous emission (SASE) FELs are highly spatially coherent to the first-order, but despite their name, statistically behave as chaotic sources. HBT measurements performed at an externally seeded FEL FERMI showed that it behaves as a real laser-like source according to a Glauber definition.  
slides icon Slides MOD03 [8.298 MB]  
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Post-Saturation Dynamics and Superluminal Propagation of a Super-Radiant Spike in a Free-Electrons Laser Amplifier  
  • X. Yang
    BNL, Upton, New York, USA
  • L. Giannessi
    ENEA C.R. Frascati, Frascati (Roma), Italy
  • L. Giannessi, N.S. Mirian
    Elettra-Sincrotrone Trieste S.C.p.A., Basovizza, Italy
  The generation of a single X-ray isolated spike of radiation with peak power at multi-GW level and femtosecond temporal duration represents a unique opportunity in time resolved diffraction imaging of isolated molecules or non-periodic structures for acquiring single shot images before the Coulomb explosion of the sample takes place. Such a condition is met by an FEL operating in superradiant regime for the possibility of simultaneously reducing the pulse duration while increasing the peak power and the pulse energy. We study the dynamics of an isolated spike of radiation in superradiant regime. We show that conditions exist where the pulse moves with a group velocity larger than one and is followed by a pedestal resulting from a complex post-saturation dynamic. The tail is constituted by a train of sub-pulses with both transverse and longitudinal coherence and decaying amplitudes. We analyze the dynamical conditions leading to the formation of the main pulse and the tail. We study the correlation of the tail structure with the longitudinal phase space of the electrons and provide a recipe to partially suppress this tail.  
slides icon Slides MOD04 [7.203 MB]  
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