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:  < 1 x set by the FEL radiation bandwidth » Spectral window  > 1 x 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? 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 0 o and 60 o, equivalent to changing d Heimann, Koike and Padmore Rev. Sci. Instrum. 76, (2005)

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 Source 130  m diameter (fwhm) 799.2, 800, eV or 760, 800, 840 eV At the detector 3.5 mm (h) x 4  m (v) (fwhm)  E = 0.4 eV (2 x10 3 RP, limited by detector pixel size 13  m) eV 800 eV eV 760 eV 800 eV 840 eV 3 mm

7 Raytracing of the grating spectrometer: 8000 eV Source 90  m diameter (fwhm) 7992, 8000, 8008 eV or 7600, 8000, 8400 eV At the detector 1.1 mm (h) x 2  m (v) (fwhm)  E = 14 eV (6x10 2 RP, limited by detector pixel size 13  m, in FEL case could use inclined detector) 7992 eV 8000eV 8008 eV 7600 eV 8000 eV 8400 eV 800  m

8 Is there single shot sensitivity for spontaneous radiation? Undulator (1) » Flux F = 1.4 x N Q n I = 3 x /(pulse 0.1% bw) » Bandwidth  E/E = 1/N = 8.8 x10 -3 » Divergence  r ‘ = /2L = 15  rad (800 eV) and 4.8  rad (8 keV) Spectrometer » Vertical angular acceptance 60  rad (800 eV) and 20  rad (8 keV) » Efficiency  = R M1.  G = 0.13 (800 eV) and 0.08 (8000 eV) » Flux at detector x 10 5, 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,  = 89.5 o, h = 4 nm, Pt coating (damage?) » E 0 = 0.88, E 1 = 0.005

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