A Grating Spectrograph for the LCLS

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 lt 1 x 10-3 set by the
  FEL radiation bandwidth
• Spectral window Dl/l gt 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 lt 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.