1. Michael Pivovaroff pivovaroff1@llnl.gov 26-27 OCT 2005 UCRL-PRES-216326 1 FEL Offset Mirrors LCLS Week * FAC Meeting 26–27 October 2005

2. Michael Pivovaroff pivovaroff1@llnl.gov 26-27 OCT 2005 UCRL-PRES-216326 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. Michael Pivovaroff pivovaroff1@llnl.gov 26-27 OCT 2005 UCRL-PRES-216326 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. Michael Pivovaroff pivovaroff1@llnl.gov 26-27 OCT 2005 UCRL-PRES-216326 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. Michael Pivovaroff pivovaroff1@llnl.gov 26-27 OCT 2005 UCRL-PRES-216326 5 Length of the 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 E (keV) Beam Size (mm) FWHMprojected Be mirror,  = 13 mrad 0.831.11886 1.000.99076 2.000.55142 SiC mirror,  = 1.5 mrad 2.00.551367 3.20.367245 4.80.272181 6.70.216144 8.30.190127 Projected Footprint on Mirror

6. Michael Pivovaroff pivovaroff1@llnl.gov 26-27 OCT 2005 UCRL-PRES-216326 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. Michael Pivovaroff pivovaroff1@llnl.gov 26-27 OCT 2005 UCRL-PRES-216326 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. Michael Pivovaroff pivovaroff1@llnl.gov 26-27 OCT 2005 UCRL-PRES-216326 8 Figure specifications Mirrors should not increase divergence more than 10% At 8 keV, divergence  = 1  rad Two independent reflections,  mirror ≤ 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 μ rad (0.3″)

9. Michael Pivovaroff pivovaroff1@llnl.gov 26-27 OCT 2005 UCRL-PRES-216326 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. Michael Pivovaroff pivovaroff1@llnl.gov 26-27 OCT 2005 UCRL-PRES-216326 10 Finish specifications To first order, finish or micro-roughness will only impact reflectivity (throughput). Must consider how R 2 (two mirrors) degrades as micro-roughness  increases. Essentially little impact as long as  ≤ 6 Å. Normalized SiC throughput versus 

11. Michael Pivovaroff pivovaroff1@llnl.gov 26-27 OCT 2005 UCRL-PRES-216326 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  = 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. Michael Pivovaroff pivovaroff1@llnl.gov 26-27 OCT 2005 UCRL-PRES-216326 12 Thin coatings on silicon substrates Deposit SiC and B 4 C (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 B 4 C properties currently being optimized for their use in multilayer applications for LCLS

13. Michael Pivovaroff pivovaroff1@llnl.gov 26-27 OCT 2005 UCRL-PRES-216326 13 Sputtered SiC 41 nm of a-SiC has performance similar to a thick mirror

14. Michael Pivovaroff pivovaroff1@llnl.gov 26-27 OCT 2005 UCRL-PRES-216326 14 Sputtered B 4 C 47 nm of B 4 C has performance similar to a thick Be mirror

15. Michael Pivovaroff pivovaroff1@llnl.gov 26-27 OCT 2005 UCRL-PRES-216326 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 fixture coated Si substrate

16. Michael Pivovaroff pivovaroff1@llnl.gov 26-27 OCT 2005 UCRL-PRES-216326 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