Bragg Spectrographs for LCLS Diagnostics and Science D. Peter Siddons Zhong NSLS Brookhaven National Laboratory Upton, NY 11973

Outline ● Basic idea ● Problems ● The SPPS prototype ● Resolution limits ● Reflection-transmission design

Basic idea ● Find a geometry which presents a range of Bragg angles to an x-ray beam in a correlated manner. ● Detect spatially-dispersed spectrum using a rapid- readout integrating 1-D position-sensing detector. Xtal Detector Source

Problems ● Not very efficient: – Each element of crystal surface only reflects one energy. – Each element of crystal surface only sees one 'ray' from incident beam. ● but LCLS is very bright! ● LCLS beam is collimated and small: – Usual schemes need big beams – No beam-divergence-driven Bragg angle variations Xtal Detector Source

Problems ● Not very efficient: – Each element of crystal surface only reflects one energy. – Each element of crystal surface only sees one 'ray' from incident beam. ● but LCLS is very bright! ● LCLS beam is collimated and small: – Usual schemes need big beams – No beam-divergence-driven Bragg angle variations Xtal Detector Source

The SPPS prototype spectrograph ● Described in SRI2003 paper ● Uses convex bend to generate Bragg angle range ● Uses highly asymmetric cut to spatially expand small beam ● Detector is close to spectrograph – compact arrangement, can fit anywhere. ● Some possibilities for chirped pulse manipulation

Details ● Si (4 2 2) reflection at Ni or Cu K-edges – Different asymmetries needed for different energies ● cut for 4 degrees incidence angle at edge energy – ~ 0.3eV resolution

Results ● Spectra measured at NSLS for Ni K- edge ● One spectrum from SPPS at Cu edge ● Would work well in the far experimental hall at LCLS (beam size ~1mm).

Resolution limits ● Resolution limit comes from intrinsic Bragg width of reflection order chosen – highest order from silicon at 8keV is (4 4 4) – intrinsic resolution in symmetric case is 39 meV – asymmetry makes it 100 meV ● Can use asymmetry to help – but it is in the wrong sense for the designs shown earlier. – In the right sense, the bending radius would need to be very small, strains could remove any gain, and the spatial dispersion would be absent.

Transmission-Reflection design ● Ideal spectrograph would use all of the beam at all energies to generate the spectrum. – This is what Zhong was describing in his talk on Laue designs – Laue case offers no opportunity for spatial dispersion within the optic. – The T-R design does so in a piecewise manner. – Can seperate spatial dispersion and asymmetry. ● Many consecutive thin Bragg elements at progressively different angles ● Each diffracting element operates in the thin-Bragg regime.

Details ● Many thin lamellae (~10um) microfabricated on curved substrate – Si (4 2 2) or diamond (3 3 1) both provide < 40meV ● Each lamella picks an energy from same part (in fact, all) of incident beam ● What about absorption? – 8keV has mu = 14/mm, so max elements is ~10 – Diamond is 10 x better, mu=1.4/mm, so 100 elements could work. – They would both work better at higher energies! e.g. 3 rd 24keV -> 1meV for diamond (9 3 3), mu=0.4/mm

How to make it? ● Reactive Ion Etching can deliver high aspect-ratio structures in silicon, so this structure should be straightforward. ● People have deposited single-crystal diamond on silicon using CVD techniques. – Can orientation be controlled? – Can it be RIE'd? – how good crystals are they? ● How serious are strain issues for resolution?

Detector ● All spectrographs need a 1D position-sensitive detector. ● Should have readout time << inter-bunch time – 120Hz for LCLS ● Should have noise level << signal – 10^12/pulse -> 10^8 per 1% pixel ● Poisson noise 10^-4 ● 50nC of charge produced in a Si diode: electronic noise should not be an issue.

Sumary ● A simple flash spectrograph has been built and (minimally) tested at SPPS. – < 0.4eV resolution – ~ 150eV coverage ● Suitable for XANES experiments and diagnostics. ● A suggestion for a microstructured device which could answer some of the 'problems' with the simple device. ● At 24keV, a Laue version of this could provide optimised resolution/coverage.