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MOC01 Regenerative Amplifier FEL - from IR to X-Rays 20
  • D.C. Nguyen, P.M. Anisimov, C.E. Buechler, Q.R. Marksteiner, R.L. Sheffield
    LANL, Los Alamos, New Mexico, USA
  The Regenerative Amplifier FEL (RAFEL) feeds back a small fraction of the radiation exiting a high-gain undulator as the seed for the next pass, and achieves narrow linewidth and saturation in a few passes. For the IR RAFEL, we used an optical cavity with annular mirrors to reinject ~10% of the IR radiation back into a two-meter undulator [1]. Since then, a number of researchers have proposed RAFEL and XFELO to achieve full temporal coherence in VUV and X-ray FELs [2-5]. For the XFELO, symmetric Bragg backscattering off high-quality diamond crystals can provide very high reflectivity for the XFELO cavity [6]. The required reflectivity for a RAFEL feedback cavity is much lower than the XFELO. We show that 6% feedback is sufficient for the X-ray RAFEL at 9.8 keV to saturate and achieve 0.5-eV bandwidth. We discuss options to out-couple more than ~50% of the RAFEL intra-cavity power and discuss challenges associated with X-ray absorption in the out-coupler.
[1] D. Nguyen et al. NIMA 429 125 (1999)
[2] B. Faatz et al. NIMA A429 424 (1999)
[3] Z. Huang et al. PRL 96 144801 (2006)
[4] B.W.J. McNeil et al. NJP 9 239 (2007)
[5] G. Marcus et al. FEL17 MOP061 (2017)
[6] K. Kim et al. PRL 100 244802 (2008)
slides icon Slides MOC01 [1.260 MB]  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-MOC01  
About • paper received ※ 21 August 2019       paper accepted ※ 28 August 2019       issue date ※ 05 November 2019  
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Microbunching Instability and Laser Heater Usage in Seeded Free-Electron Lasers  
  • E. Roussel
    PhLAM/CERLA, Villeneuve d’Ascq, France
  • E. Ferrari
    PSI, Villigen PSI, Switzerland
  Seeded FELs are tremendous coherent sources in the EUV and soft X-rays domain. They are driven by high-quality electron beams produced in linear accelerators. However, spatio-temporal instabilities can develop in the magnetic compressor chicanes degrading the beam quality. The so-called microbunching instability is a major limitation for the generation of coherent pulses in the X-ray domain. Laser-heater systems are nowadays routinely used to control the electron beam energy spread in order to dump this instability. In the last years, there have proven to be also a powerful device to influence the FEL amplification process. We present here several LH usages in FELs with specific examples achieved on seeded FELs such as improving the harmonic up-conversion process in HGHG seeded FEL, generating short FEL pulses or also producing multi-color FEL pulses. We also report on microbunching instability issues and its effect on FEL spectral quality with strategies to benefit from that. Direct observation of the microbunching level in the electron beam remains also an open challenge. Innovative strategies to measure the electron beam and potentially its microbunching will be presented.  
slides icon Slides MOC02 [8.713 MB]  
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Laser Heater Impact on the Performances of Seeded High Gain FELs  
  • E. Ferrari
    PSI, Villigen PSI, Switzerland
  • E. Roussel
    PhLAM/CERCLA, Villeneuve d’Ascq Cedex, France
  Laser-heater systems have been demonstrated to be an important component for the accelerators that drive high gain free electron laser (FEL) facilities, dramatically improving the performance of such lightsources. They are nowadays routinely used in most of the operating FEL facilities around the world. The improvement is achieved by suppressing the longitudinal microbunching instability via a controllable increase in the slice energy spread. The system has also been extensively used to manipulate the properties of the electron beam e.g., to produce short pulses of radiation, or for multicolor FEL generation. In this contribution we focus on the usage and impact of such system on a seeded FEL facility, with particular focus on the possibility of efficient generation of short wavelength radiation with unexpected power levels.  
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Accelerator Challenges for XFELs with Very High X-Ray Energies  
  • B.E. Carlsten
    LANL, Los Alamos, New Mexico, USA
  Future X-ray Free-Electron Lasers (XFELs) will produce coherent X-rays with energies much greater than 20 keV, requiring electron beams with lower laboratory emittances and relative energy spreads than those in current XFELs. To satisfy this requirement, electron beam energies would need to be significantly higher than in current XFEL designs if conventional accelerator architectures are used, leading to increased construction and operation costs. To provide design margin for these future XFELs at the lowest possible electron beam energies, novel schemes may be employed to suppress or eliminate limitations introduced by coherent synchrotron radiation, undulator resistive wall wakes, the microbunch instability, and intrabeam scattering.  
slides icon Slides MOC04 [1.689 MB]  
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