Optics for next generation light sources Anton Barty Centre for Free Electron Laser Science Hamburg, Germany
Key issues Optical specifications Metrology (mirror surfaces) Metrology (wavefront, focal spot) Coatings and damage Novel optical structures
Overview of the LCLS front-end optics Source - ~30 µm Gaussian electron beam near undulator exit - Can easily incorporate a variety of source models Beamline - Apertures - Slits - etc. Offset mirrors - Measured figure - Measured PSD - Known geometry Endstation - Intensity at endstation - Intensity at focus - Total wavefront error
LCLS front end mirrors direct the beam into different end stations Front end enclosure Near experiment hall 0.8-2 kev LCLS X-ray beam M2S M3S2 M3S1 0.8-2 kev 2-24 kev Tunnel M1S M1H M2H Hutch 1 (AMO) Hutch 2 (SXR) Hutch 3 (XPP) Soft X-ray offset mirrors (SOMS, 0.8 to 2 kev) Hard X-ray offset mirrors (HOMS, 2 to 24 kev)
A wave optics formalism is must be used for coherent X-ray beams M1S M2S M3S/M4S Source Beamline Offset mirrors Endstation???? + + + = (Gaussian profile) (Apertures) (Measured 1D figure) (Statistical roughness) (Intensity at focus)
Complete mirror PSD is obtained from interferometry, surface microscopy and AFM data Interferometer ~ 0.16 µrad rms! ~ 0.3 µrad rms! Si test substrate by Vendor 1 Si test substrate by Vendor 2! = 0.19 nm rms!! = 0.19 nm rms!! = 0.16 nm rms!! = 0.36 nm rms! Zygo 2x Zygo 20x AFM AFM 2.95 mm 0.37 mm 10 µm 2 µm Regina Soufli, LLNL regina.soufli@llnl.gov
Interferometry measurements are used to create composite mirror surfaces Interferometer data Zygo Figure Model surface (Measured 1D figure) PSD Model surface AFM (Statistical roughness)
Grazing incidence optics lead to a dilation of the mirror surface power spectrum Incident wavefront Reflected wavefront (x, y )! Mirror surface h(x,y) W=2h sin(!) y = y x =x sin(!) z y mirror f y y beam f y x mirror f x x beam f x Mirror surface height (viewed normal to surface) Mirror surface PSD (isotropic distribution) Wavefront error (mirror reflecting at angle!) Wavefront PSD (elliptical distribution)
Coherent X-rays demand wavefront propagation to model the optical system At M1S At M2S At M3S Intensity Intensity Intensity Source (0.0139 radian from grazing) (0.0417 radian from grazing) (0.0417 radian from grazing) SOMS #4 Figure (phase) SOMS #4 Figure (phase) SOMS #4 Figure (phase) (Not to scale) (colour = phase) ~1cm Note: For testing purposes the same figure and PSD was used for all SOMS mirrors. This is trivial to change when we have new metrology data - the use of beamline scripting simplifies changes (although some format checking is still required).
Mid spatial frequency range gives rise to additional beam structure At AMO endstation no MSFR At AMO endstation with measured MSFR ~5 mm at enstation
Beam structure is affected by the mid spatial frequency components Figure only (no MSFR) With MSFR included
We expect measurable intensity variation at the entrance to the AMO endstation 1.0 0.8 0.8 kev 0.6 0.4 0.2 0.0 0 100 200 300 400 500 600 1.0 0.8 2.0 kev 0.6 0.4 0.2 Without MSFR With measured MSFR ~5 mm at endstation 0.0 0 100 200 300 400 500 600
Specifications for SOMS mirrors Spatial frequency Spatial wavelength Specification range range CA to 10-3 µm -1 CA to 1 mm <2 nm RMS and <0.25 µrad RMS MSFR 10-3 µm -1! 0.5 µm -1 2 µm! 1 mm < 0.25 nm RMS HSFR 0.5 µm -1! 50 µm -1 20 nm! 2 µm < 0.4 nm RMS Mirror # Figure (nm RMS) Slope error (µrad RMS) SN1 1.8 0.19 SN2 1.3 0.2 SN3 1.2 0.37 SN4 0.64 0.14 SN5 1.4 0.37
Intensity at K-B focus is less severely degraded
Specifications for HOMS mirrors Mirror # Figure (nm RMS) Slope error (µrad RMS) SN1 2.4 0.27 SN2 1.0 0.27 SN3 2.0 0.22 SN4 1.5 0.23
HOMS performance at CXI endstation
Predicted focal spot of the CXI instrument
The LCLS mirrors are coated with B4C J.Bozek, R.Soufli
A compromise must be drawn between surface roughness and stress J.Bozek, R.Soufli
Optical mounts must be precision engineered for ultra high stability ~300m to FEH 1 µrad => 0.3 mm 1 nrad => 0.3 µm Offset mirrors - Measured figure - Measured PSD - Known geometry
J. Krzywinski
Focal spot size is inferred from crater size and damage models J. Krzywinski
and now for something completely different...
Sasa Bajt, PXRMS 2010
Pump-probe geometry with small windows allows us to measure only the delayed diffraction signal i) Drive the sample ii) Probe the sample some time later iii) Collect diffraction from probe
We demonstrated that a tamper is effective in containing nanoparticle expansion Low fluence High fluence Tampered Untampered Aluminium dot Substrate (silicon nitride) Silicon tamper Aluminium dot Substrate (silicon nitride)
Multilayer optics for two-colour pump-probe experiments Andy Aquila, PXRMS 2010
Multilayers for X-ray two-colour time delay experiments Andy Aquila, PXRMS 2010
Disposable zone plates for extreme focussing Anne Sakdinawat, LBNL
Chirped multilayers for X-ray pulse compression Andy Aquila, PXRMS 2010
Coherent beams demand wavefront propagation for optics analysis and optic specification Precision metrology is essential Anything in the beam will diffract (apertures, solid attenuators - everything!) Some optics are designed to be blown up! Credits: LLNL X-ray optics group Sasa Bajt, Andy Aquila (CFEL) Jacek Kryzwinski (SLAC)
Measured HOMS figure error is used to determine the optical surface profiles 10.00 8.00 6.00 Figure (nm) 4.00 2.00 0.00-2.00 HOMS #1 HOMS #2 HOMS #3 HOMS #4-4.00-6.00-8.00-250.00-200.00-150.00-100.00-50.00 0.00 50.00 100.00 150.00 200.00 250.00 Position (mm)
We expect overfilling of HOMS mirrors to degrade performance in the far experimental hall 1.5 Intensity at 2.0 kev and z=383 m 1.5 Intensity at 8.0 kev and z=383 m 1.0 1.0 Position (mm) 0.5 0.0-0.5 Position (mm) 0.5 0.0-0.5-1.0-1.0-1.5-1.5-1.0-0.5 0.0 0.5 1.0 1.5 Position (mm) -1.5-1.5-1.0-0.5 0.0 0.5 1.0 1.5 Position (mm)