FEL Offset Mirrors

1. FEL Offset Mirrors LCLS Week * FAC Meeting 26–27 October 2005

2. Overview • FEL offset mirrors will serve as a low-pass filter, eliminating the high-energy spontaneous spectrum • Basic concept is to rely on the relatively sharp, step-function nature of grazing-incidence reflectivity curves • Original plan calls for two sets of mirrors • Be: operates below 2 keV @13 mrad • SiC: operates above 2 keV @1.5 mrad

3. Approach • Actual implementation must account for: • Safety • Cost • Complexity • Guided by ISM Core Functions • Define work scope • Analyze work for hazards (e.g., use of beryllium) • Must start with physics requirements

4. Original concept • Mirrors made from monolithic blanks • Graze angles of 1.5 and 13 mrad cut off reflectance at 2 and 24 keV • Mirror pairs spaced to give FEL 24 mm of offset • Mirrors must not increase divergence (i.e., decrease brilliance)  Very stringent specs

5. Length of the mirror Projected Footprint on Mirror • Mirror length L must be larger than the projection of FEL beam of width w : • Driven by lowest energy to be focused • Be mirror must be ~200 mm long • SiC mirror must be ~600 mm long

6. General approach to mirror specifications • Need to worry about three regimes: • Low-spatial frequency (i.e., figure): • Length scales > 1 mm • High-spatial frequency (i.e., finish) • Length scales < 1 micron • Mid-spatial frequency (typically called “mids”) • 1 micron < length scales < 1 mm • Overall goal is to increase divergence less than 10%

7. Impact of errors • High- and low-spatial frequency errors mainly impact throughput • Mids will broaden PSF Figure 7: JE Harvey, Applied Optics, 34, 3715, 1996

8. Figure specifications • Mirrors should not increase divergence more than 10% • At 8 keV, divergence Ds= 1 mrad • Two independent reflections, Dsmirror≤ 71 nrad (0.015″) • 7.1 nm PV over 100 mm • Very challenging! • 600 mm long CVD-SiC mirror made for ESRF had slope error of 3 μrad (0.3″)

9. Mids specifications • Past experience with grazing incidence optics indicates mids are often the limiting factor in performance. • Specs will be determined through analytic methods and Monte Carlo simulations.

10. Finish specifications Normalized SiC throughput versus s • To first order, finish or micro-roughness will only impact reflectivity (throughput). • Must consider how R2 (two mirrors) degrades as micro-roughness s increases. • Essentially little impact as long as s≤ 6 Å.

11. Concerns with current baseline plan • Material choices: • Use of beryllium requires additional safety measures • Be is incredibly difficult to polish • Current work indicates best finish achievable is the range s = 15–25 Å • Fabrication method: • Monolithic mirrors are very expensive • State-of-the-art mirrors may not even meet spec Is there another approach that can reduce cost and risk?

12. Thin coatings on silicon substrates • Deposit SiC and B4C (instead of Be) on super-polished, figured Si substrates • Advantages • Lower cost • Eliminates beryllium-related safety issues • Leverages expertise and infrastructure developed at LLNL for the EUVL project • SiC and B4C properties currently being optimized for their use in multilayer applications for LCLS

13. Sputtered SiC 41 nm of a-SiC has performance similar to a thick mirror

14. Sputtered B4C 47 nm of B4C has performance similar to a thick Be mirror

15. Conceptual approach • Fabricate Si substrates with appropriate figure and finish • Deposit thin films on substrates • Substrate length limited to < 150–200 mm with current deposition chambers • Tile pieces into long mirrors • Mount coated-segments into mirror fixtures coated Si substrate fixture

16. Outstanding issues • Need to perform FEA to determine optimal substrate shapes • Requires mirror specifications and detailed design of fixtures (e.g., gravitational sag) • Determine if gaps will impact performance • Verify that silicon substrates will not be affected by high-energy spontaneous beam