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Possible Single-bunch Hard x-ray Self-seeding for LCLS-II

Possible Single-bunch Hard x-ray Self-seeding for LCLS-II. Juhao Wu May 12, 2010. Two-stage self-seeding to reduce the FEL bandwidth Previous schemes: four-crystal fixed-exit monochromator in Bragg geometry

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Possible Single-bunch Hard x-ray Self-seeding for LCLS-II

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  1. Possible Single-bunch Hard x-ray Self-seeding for LCLS-II Juhao Wu May 12, 2010

  2. Two-stage self-seeding to reduce the FEL bandwidth Previous schemes: four-crystal fixed-exit monochromator in Bragg geometry Single-bunch scheme will require long electron beam bypass (Feldhaus et al., Optics Comm. 140, (1997 ) 341.) Two-bunch scheme (Geloni et al., DESY 10-033; Ding et al., SLAC-PUB-???) New scheme: single crystal in the transmission direction Use Bragg geometry (Geloni et al., DESY 10-053) Use Laue geometry (P.M. Stefan, J.B. Hastings, J. Arthur, et al., SLAC/LCLS) Laue geometry do not need to invoke the fresh bunch technique for electron bunch Fresh bunch is required for the Bragg geometry (Geloni et al., DESY 10-053) Outline

  3. Single crystal and Short magnetic chicane Offset for single crystal installation Chicane removes the microbunching in the electron bunch generated in the first undulator Chicane provides a delay line for temporal window Simple scheme with monochromator G. Geloni, V. Kocharyan, and E. Saldin, DESY 10-053, April 2010 .

  4. Three section of undulator with fresh bunch technique and monochromator Brief review of DESY scheme G. Geloni, V. Kocharyan, and E. Saldin, DESY 10-053, April 2010 .

  5. Details Brief review of DESY scheme G. Geloni, V. Kocharyan, and E. Saldin, DESY 10-053, April 2010 .

  6. Details, think it as superposition of two pulses (spectrum): one wider and one narrower (with p-phase shift) Brief review of DESY scheme Notch in spectrum Wake in time used as the seed G. Geloni, V. Kocharyan, and E. Saldin, DESY 10-053, April 2010 .

  7. Bragg Geometry, but look at the transmission Brief review of DESY scheme G. Geloni, V. Kocharyan, and E. Saldin, DESY 10-053, April 2010 .

  8. Consider a 3D array of atoms arranged on planes Get constructive interference between x-rays scattered from atoms P and K in same plane when there is no path difference for the scattered rays Bragg Diffraction • Need to have symmetrical diffraction so that QK –PR = PK cosq– PK cosq= 0 • Get constructive interference between x-rays scattered from atoms in different planes when the path length is a multiple of l. Consider atoms K and L: ML + LN = d’sinq + d’ sinq = 2 d’ sinq = nl. • Bragg “reflection” is really “diffraction” 2 d sinq = nl

  9. Laue Diffraction • Consider a row of atoms scattering x-rays • S0 is a vector describing the incoming x-ray beam and S describes the scattered beam • To get constructive interference between the x-rays scattered from each atom • a(cosa – cosa0) = h l where h is an integer • If we have a 2D periodic array of atoms we also have to satisfy • b(cosb – cosb0) = k l where k is an integer • If we have a 3D periodic array of atoms we also have to satisfy • c(cosg – cosg0) = l l where l is an integer

  10. Laue Diffraction • Consider Si(220) for 1.5 Å FEL • Lattice constant: a = 5.43095 Å • Atom plane distance: • Bragg condition: 2d sin q = l;  q = 23o. Forward-diffraction beam Diffraction beam q Incident beam

  11. Rocking curve • Laue forward-diffraction beam as the seed • Thick crystal to make only s polarization on the a branch of the dispersive curve transmit most • The forward-diffraction beam intensity • m0 = 150 cm-1 • h’ proportional to Dl/l • m0t0 increase, peak height little change  diffraction; but width shrinks quickly

  12. Electron Chicane • Take t0 = 0.5 mm • Path Length Difference (PLD): 40 mm • Chicane R56 = 80 mm • Excursion: 195 mm Laue forward-diffraction beam as seed

  13. Single-bunch hard x-ray 13 Schematics of Self-Seeded FEL chicane 1st undulator 2nd undulator FEL SASE FEL Seeded FEL electron electron dump

  14. Electron current profile entering the undulator LCLS SASE FEL Parameters tail head

  15. Slice emittance entering the undulator LCLS SASE FEL Parameters Slice Emittance small  Gain Length Short

  16. FEL power along the undulator LCLS SASE FEL Parameters Saturation early with power on order of GW

  17. FEL bandwidth along the undulator LCLS SASE FEL Parameters Bandwidth on order of 1E-3

  18. FEL temporal profile at 60 m LCLS SASE FEL Parameters

  19. FEL spectrum at 60 m LCLS SASE FEL Parameters

  20. For a Gaussian photon beam LCLS electron bunch flat top, sz ~ 10 mm Transform limited sw / w0 ~ 2E-06 Room toimprove the coherence  bandwidth reduces by 2 order of magnitude (?) Transform limited

  21. Seeding the second undulator vs. single undulator followed by x-ray optics Power loss in the monochromator is recovered in the second undulator (FEL amplifier) The shot-to-shot FEL intensity fluctuation reduced due to the nonlinear regime of the FEL amplifier The peak power after the first undulator is less than the saturation power, the damage to the optical elements is reduced Two-stage FEL with monochromator

  22. FEL power along the second undulator LCLS SASE FEL Parameters Saturation early with power on order of GW

  23. FEL temporal profile at 40 m LCLS SASE FEL Parameters

  24. FEL spectrum at 40 m LCLS SASE FEL Parameters FWHM 10-5

  25. LCLS excellent electron beam quality leads to short gain length, early saturation. This makes possible to add more functions Two-stage FEL with monochromator reduce the bandwidth from 1E-3 to a few 1E-5 with similar peak power  increase the brightness by factor of 100 Laue forward diffraction beam makes chicane small (J. Galayda suggested to test in LCLS baseline) Summary

  26. Thanks for your attention! Special thanks to: P.M. Stefan, J.B. Hastings, J. Arthur, U. Bergmann, P. Emma, J. Galayda, J.B. Murphy (NSLS/BNL), T.O. Raubenheimer ……

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