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A Grating Spectrograph for the LCLS

Observe the spontaneous radiation spectrum of individual undulators and FEL radiation with a spectrograph optimized for 800-8000 eV photon energy range. Achieve a spectral resolution of <1x10-3 and a spectral window of >1x10-2. Consider damage from FEL radiation.

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A Grating Spectrograph for the LCLS

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  1. A Grating Spectrograph for the LCLS Philip Heimann Advanced Light Source • Observe the spontaneous radiation spectrum of the individual undulators • Observe the spectrum of FEL radiation

  2. Design criteria • Goals • Photon energy range: 800 – 8000 eV. • Spectral resolution: Dl/l < 1 x 10-3 set by the FEL radiation bandwidth • Spectral window Dl/l > 1 x 10-2 set by the single undulator harmonic energy width • Single shot sensitivity for single undulator spectra. • Consider damage for FEL radiation

  3. Individual undulator spectrum • From Steve Hulbert • We would like to display ~800 eV energy window. • The energy resolution requirement is modest < 60 eV.

  4. Can we use gratings from 800 – 8000 eV? • Heimann, Koike and Padmore Rev. Sci. Instrum. 76, 63102 (2005) • Shimadzu grating: 600 l/mm, laminar, h = 6.5 nm, Au coating, 2.4 A rms • Peak efficiency 14 % • Measured with Surface Normal Rotation (SNR) of 0o and 60o, equivalent to changing d

  5. LCLS grating spectrometer layout • One VLS grating in -1 order • Length of spectrometer 1.3 m

  6. Raytracing of the grating spectrometer: 800 eV 799.2 eV 800 eV 800.8 eV • Source 130 mm diameter (fwhm) 799.2, 800, 800.8 eV or 760, 800, 840 eV • At the detector 3.5 mm (h) x 4 mm (v) (fwhm) DE = 0.4 eV (2 x103 RP, limited by detector pixel size 13 mm) 760 eV 800 eV 840 eV 3 mm

  7. Raytracing of the grating spectrometer: 8000 eV 7992 eV 8000eV 8008 eV • At the detector 1.1 mm (h) x 2 mm (v) (fwhm) DE = 14 eV (6x102 RP , limited by detector pixel size 13 mm, in FEL case could use inclined detector) • Source 90 mm diameter (fwhm) 7992, 8000, 8008 eV or 7600, 8000, 8400 eV 7600 eV 8000 eV 8400 eV 800 mm

  8. Is there single shot sensitivity for spontaneous radiation? • Undulator (1) • Flux F = 1.4 x 1014 N Qn I = 3 x 106 1/(pulse 0.1% bw) • Bandwidth DE/E = 1/N = 8.8 x10-3 • Divergence sr‘ = l/2L = 15 mrad (800 eV) and 4.8 mrad (8 keV) • Spectrometer • Vertical angular acceptance 60 mrad (800 eV) and 20 mrad (8 keV) • Efficiency e = RM1.eG = 0.13 (800 eV) and 0.08 (8000 eV) • Flux at detector 2 - 4 x 105, N noise ~ 0.2 % Yes

  9. Beam damage with LCLS FEL radiation • M1 mirror preliminary estimate: mirror would be OK at 800 eV (0.2 eV/atom) but not at 8 keV (10 eV/atom). • Detector: focusing in one dimension, silicon at normal incidence, would damage at 800 eV (12 eV/atom) and at 8 keV (1.5 eV/atom). • It would be possible to attenuate the FEL beam, e.g. with the gas filter.

  10. Grating beam splitter for spectrometer as a passive diagnostic • Optimize grating efficiency for 0 order, “bad” grating • Parameters: 800 eV, 50 l/mm, q = 89.5o, h = 4 nm, Pt coating (damage?) • E0 = 0.88, E1 = 0.005

  11. Summary: the Grating Spectrograph for the LCLS • Photon energy range: 800 – 8000 eV. • Resolving power: E/DE = 2000 at 800 eV and 600 at 8 keV. • For FEL radiation the resolution could be improved with an inclined detector. • Spectral window: DE/E = 10%. • Single shot sensitivity for single undulator spectra.

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