FEL Offset Mirrors

1
FEL Offset Mirrors

• LCLS Week FAC Meeting
• 2627 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 _at_13 mrad
• SiC operates above 2 keV _at_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
E (keV) Beam Size (mm) Beam Size (mm)
E (keV) FWHM projected
Be mirror, q 13 mrad Be mirror, q 13 mrad Be mirror, q 13 mrad
0.83 1.118 86
1.00 0.990 76
2.00 0.551 42
SiC mirror, q 1.5 mrad SiC mirror, q 1.5 mrad SiC mirror, q 1.5 mrad
2.0 0.551 367
3.2 0.367 245
4.8 0.272 181
6.7 0.216 144
8.3 0.190 127

• 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 gt 1 mm
• High-spatial frequency (i.e., finish)
• Length scales lt 1 micron
• Mid-spatial frequency (typically called mids)
• 1 micron lt length scales lt 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 1525 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 lt 150200 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