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Outline and Acknowledgment

A 50-keV X-ray Free-Electron Laser with Periodically Bunched Electron Beam Dinh Nguyen, Steve Russell, Bruce Carlsten and Cris Barnes Los Alamos National Laboratory XFEL Workshop Lawrence Berkeley Laboratory October 23-25, 2008. Outline and Acknowledgment. MaRIE Signature Science Facility

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Outline and Acknowledgment

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  1. A 50-keV X-ray Free-Electron Laser withPeriodically Bunched Electron BeamDinh Nguyen, Steve Russell, Bruce Carlsten and Cris BarnesLos Alamos National LaboratoryXFEL WorkshopLawrence Berkeley LaboratoryOctober 23-25, 2008

  2. Outline and Acknowledgment • MaRIE Signature Science Facility • 50-keV X-ray FEL • Issues with Energy Diffusion • Pre-bunched X-ray FEL • Electrons Channeling in Si Crystals • MEDUSA Simulation Results We thank Henry Freund (SAIC) for the use of MEDUSA FEL simulation code, and Paul Channell (LANL) for helpful discussion.

  3. MaRIE(Matter-RadiationInteractionsinExtremes) • The Multi-probe Diagnostic Hall will provide unprecedented probes of matter. • X-ray scattering capability at high energy and high repetition frequency with simultaneous proton imaging. • The Fission and Fusion Materials Facility will create extreme radiation fluxes. • Unique in-situ diagnostics and irradiation environments comparable to best planned facilities. • The M4 Facility dedicated to making, measuring, and modeling materials will translate discovery to solution. • Comprehensive, integrated resource for controlling matter, with national security science infrastructure. LANSCE Accelerator

  4. Why 50 keV XFEL? MaRIE seeks to probe insidemultigranular samples of condensed matter that represent bulk performance properties with sub-granular resolution. With grain sizes of tens of microns, "multigranular" means 10 or more grains, and hence samples of few hundred microns to a millimeter in thickness. For medium-Z elements, this requires photon energy of 50 keV or above. This high energy also serves to reduce the absorbed energy per atom per photon in the probing, and allows multiple measurements on the same sample. Interest in studying transient phenomena implies very bright sources, such as an XFEL.

  5. MaRIE XFEL Temporal Pulse Format Two RF pulses separated by an adjustable delay ~1 ms 1 - 100 ms Each macropulse consists of 10-100 micropulses 30 ns Each micropulse has ~1011 x-ray photons <1 ps 0.3 ns

  6. Electron Injector Linear Accelerator Beam Conditioning Undulator X-rays Beam Electron Beam Dump MaRIE 50-keV XFEL Baseline Concept

  7. -19 2 (1.264 x10cm) Energy diffusion limits the maximum beam energy and undulator length Energy diffusion due to quantum fluctuations* 2 gK0N 3 2 dg u g 2 2 l (1 + 1.33K0 + 0.4K0 ) u * Z. Huang and K.J. Kim, “Review of x-ray free-electron laser theory” Phys. Rev. Special Topics – Accelerators and Beams, 10, 034801 (2007)

  8. Periodically bunch electron beams with density modulations on the order of a radiation wavelength • Start up with coherent bunched beam radiation. P(z) = Pc e • With higher start-up radiation intensity and optical guiding, FEL will saturate in shorter undulator length, thus reducing energy diffusion. Energy diffusion ~ Lu z/LG Pre-bunched beam FEL minimizes energy diffusion by reducing undulator length

  9. Start-up in SASE and Pre-bunched FEL • SASE starts up with spontaneous radiation from randomly distributed N electrons in a bunch Radiation intensity ~ N • Pre-bunched beam FEL starts up with coherent radiation from periodically bunched beam with N electrons Radiation intensity ~ N2 Coherent start-up with pre-bunched beams • shortens the saturation length • reduces energy spread due to quantum fluctuations • extends XFEL to higher photon energy (shorter l)

  10. g g g f f f How do we pre-bunch electron beam with a period of 0.25 Angstrom? Si crystal as a spatial mask with 1-2 Å spacing Chicane transforms energy modulations in x into density modulations in z with a compression factor D.C. Nguyen and B.E. Carlsten, “Amplified coherent emission with electron beams prebunched in a masked chicane” Nucl. Instr. Meth. Phys. Res. A 375 (1996) 597-601.

  11. Axial Channeling of Relativistic Electrons in Silicon Crystals Energy loss rate vs. electron incident angle Interplanar spacing 1.36 Å 1.92 Å 2.35 Å Lindhard angle 30 mrad V.M. Biryukov, Y.A. Shesnokov and V.I. Kotov, “Crystal channeling and its applications at high energy accelerators” Springer Press, Berlin 1997. J.F. Bak et al., “Channeling radiation from 2 to 20 GeV/c electrons and positrons. Pt. 2. Axial case” Nucl. Phys. B (1988-06-13) Vol.302, iss.4, p.525-558.

  12. Experimental Results of GeV Electron Channeling in Thin Silicon Crystals S.P. Fomin et al., “Features of Angular Distributions of 1 GeV Electrons Scattered by Thin Silicon Monocrystals” Problems of Atomic Science and Technology. 2001, pp. 138-143

  13. MaRIE XFEL Baseline Parameters Beam energy       35.32 GeV Bunch charge 0.25 nCBunch length (FWHM) 74 fs (at undulator) Peak current  3.4 kA (at undulator) Transverse emittance 0.2-0.6 mm (at undulator) rms Energy spread 0.01% Undulator period (lu) 3 cm Undulator parameter (Krms) 2.624 Undulator total length 20 m Wavelength 0.0248 nm Pierce parameter 1.77 x 10-3 1D Gain length 77.7 cm Saturation power (prebunched) 1.7 x 1010 watts Micropulse energy 1.2 mJ # photons per micropulse 1.5 x 1011

  14. Time-independent Simulations for50-keV SASE FEL with MEDUSA FEL peak power (blue) shows saturation at z > 115 m FEL mode radius (red) shows no optical guiding at z < 70 m

  15. Time-independent Simulations forPre-bunched Beam FEL with MEDUSA FEL peak power (blue) shows saturation at z = 20 m FEL mode radius (red) shows optical guiding at z < 2 m Undulator Length

  16. Power (W) Undulator length (m) Pre-bunched beam start-up power is 300,000 times SASE start-up noise Pre-bunched beam XFEL saturates while SASE is still in lethargy region.

  17. Time-dependent MEDUSA Calculations for Pre-bunched Beam XFEL at 50 keV Power vs z for slice # 146 Peak power vs slice number Power (GW) Time-dependent calculations show pre-bunched beam FEL to have smooth temporal profile (spectral profile is also expected to be smooth) instead of the usual SASE spiky profile.

  18. Effect of Normalized rms Emittance on SASE and Pre-bunched XFEL Very low emittance is not necessary for pre-bunched XFEL as it is for SASE. Pre-bunched FEL can accept emittance up to 1 mm-mrad.

  19. Conclusions • Los Alamos is studying the feasibility of a 50-keV X-ray FEL for materials science research. • We propose to pre-bunch the electron beam periodically at the radiation wavelength before injecting the electron beam into the undulator. • The periodically bunched beam 50-keV XFEL saturates in 20 meters of undulator length, while SASE takes 115 meters to saturate. • Unlike SASE, the periodically bunched beam FEL does not require exceptionally small rms emittance.

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