1. Soft x-ray optics and beamlines for next generation light sources Mark D Roper Accelerator Science & Technology Centre STFC Daresbury Laboratory

2. mark.roper@stfc.ac.uk 5 March 2012 FLS2012, Thomas Jefferson National Laboratory Talk Outline Photon Properties Transportation Diagnostics Conclusion Questions If I could summarise everything that was of concern in soft x-ray optics for future light sources in 30 minutes, this lecture would probably not be worth giving.

3. mark.roper@stfc.ac.uk 5 March 2012 FLS2012, Thomas Jefferson National Laboratory Properties of a FLS Light Beam Coherent wavefront –Diffraction limited Pulsed –Shot to shot variation Short pulse length –Transform limited High pulse energy –Damage Wavelength dependence –In ways not familiar from conventional sources

4. mark.roper@stfc.ac.uk 5 March 2012 FLS2012, Thomas Jefferson National Laboratory Transportation

5. mark.roper@stfc.ac.uk 5 March 2012 FLS2012, Thomas Jefferson National Laboratory Where is the source? As optic gets closer than Z R, source will look like it is at infinity. –Not very likely for an x-ray source Still need to ask –Where is the source? –How big is the source? –What is the M 2 propagation factor? –Is it the same horizontally and vertically? –How do these factor vary with wavelength?

6. mark.roper@stfc.ac.uk 5 March 2012 FLS2012, Thomas Jefferson National Laboratory Source characteristics for NLS FEL Deduced from Genesis simulations using wavefront propagation (FOCUS code) and second moment analysis Roper, Thompson, Dunning. J.Mod.Opt. (2011)

7. mark.roper@stfc.ac.uk 5 March 2012 FLS2012, Thomas Jefferson National Laboratory Source shape for NLS FEL Deduced from Genesis simulations using wavefront propagation Four undulator modules The transport system has to cope with a source of changing size, position & quality

8. mark.roper@stfc.ac.uk 5 March 2012 FLS2012, Thomas Jefferson National Laboratory Preserving the wavefront Reflection imprints defects in the mirror surface onto the wavefront Small defects also give “speckle” diffraction patterns M. Zangrando FERMI@Elettra

9. mark.roper@stfc.ac.uk 5 March 2012 FLS2012, Thomas Jefferson National Laboratory Preserving the wavefront The demands on optical manufacturing and metrology are unprecedented WavelengthAoIP-V shape error (nm) (nm) (°)  =0.25°  =0.10° 4064718 4039538 401.519176 103239 1023514 1017128 53125 52187.2 513614 1.67342 M. Zangrando FERMI@Elettra Typical SR mirror

10. mark.roper@stfc.ac.uk 5 March 2012 FLS2012, Thomas Jefferson National Laboratory Preserving the wavefront 100 eV. Simulation with FOCUS code Implications for: Diffraction limited focusing Wavefront dividing beam-splitters Knife-edge position monitors Don’t forget that a coherent wave will diffract from the edges of mirrors!!

11. mark.roper@stfc.ac.uk 5 March 2012 FLS2012, Thomas Jefferson National Laboratory Diffraction Limited Focusing The fringes will not be (so) visible at a focus –Size of focus limited by the aperture through diffraction f = 0.2 m 8  : +2.5% 6  : +11% 4  : +38% Relative to infinite aperture

12. mark.roper@stfc.ac.uk 5 March 2012 FLS2012, Thomas Jefferson National Laboratory Focus limit from surface errors Focus source with imperfect ellipsoid at 37.5:1 demagnification Field @ source Field @ focus Temporal profile @ source Temporal profile @ focus PSD of mirrors FLASH BL3, 98 eV M.A.Bowler B.Faatz F.Siewert

13. mark.roper@stfc.ac.uk 5 March 2012 FLS2012, Thomas Jefferson National Laboratory Beam splitters Significant demand for multi-photon experiments Wavefront division –Technologically easier - knife-edged mirror –Diffraction effects –Auto-correlator (beam splitter & delay line) at FLASH Amplitude division –Reflection-reflection or reflection-transmission –Gratings, multi-layers, (crystals) –Pulse length effects –Flatness of thin membranes

14. mark.roper@stfc.ac.uk 5 March 2012 FLS2012, Thomas Jefferson National Laboratory Metrology & Manufacturing Achieving the highest possible figure accuracy requires collaboration between the manufacturer and the metrology laboratory Reduction in form error of elliptical focusing mirrors by factor of 3 1.6 µrad to 0.5 µrad RMS 61 nm to 22 nm PV H. Thiess, H. Lasser, F. Siewert, NIM A (2009) F. Siewert, J. Buchheim, T. Zeschke, NIM A (2010) HZB: NOM Metrology Data + Zeiss: Ion Beam Finishing After Before Height (nm) 6.04.02.00.0 Slope (arcsec) 3020100-10-20 F. Siewert HZB

15. mark.roper@stfc.ac.uk 5 March 2012 FLS2012, Thomas Jefferson National Laboratory Preserving the Pulse Length To preserve the pulse length to 1 fs, the optical path length must be the same to 0.3 µm for all positions across the wavefront from source to final image (distance 10’s to 100’s metres). –Tight control of all aberrations –Control of penetration depth into multi-layers –Special attention to dispersing elements like gratings The pulse bandwidth must be preserved –Transform limit

16. mark.roper@stfc.ac.uk 5 March 2012 FLS2012, Thomas Jefferson National Laboratory Gratings and short pulses The path length difference with a diffraction grating will stretch the pulse Low line density gratings and controlled illumination* Conical diffraction geometry Double gratings * Roper, NIM A (2010)

17. mark.roper@stfc.ac.uk 5 March 2012 FLS2012, Thomas Jefferson National Laboratory Photon Induced Damage Damage from the high fluence pulses to the optical surfaces is a major concern The main approach to protection is –Use the most robust coating (lighter elements) –Spatially dilute the beam (distance & grazing angle) –Calculate absorbed dose per atom (geometry, reflectivity, penetration) and make sure it is below the “damage threshold” Amorphous carbon (a-C) most popular XUV coating (FLASH) but no good >280 eV Cr, Ni, even Pt may be needed. Damage mechanisms are complicated and not fully understood –What is the “damage threshold” (e.g. function of wavelength) Effect on structured surfaces

18. mark.roper@stfc.ac.uk 5 March 2012 FLS2012, Thomas Jefferson National Laboratory a-C Single-shot Damage Chalupský et al., Appl Phys Lett 95 031111 (2009) Threshold fluence for damage as a function of grazing angle. Nomarski microscope images of damage by 13.5 nm radiation. Beam at normal incidence (left) and 18.5° grazing angle (right). Two regions of damage  Central ablation  Peripheral expansion (graphitization) FLASH Measurements Electron transport in the a-C is key in determining the absorbed dose per atom below the critical angle Damage occurs well below the melt threshold

19. mark.roper@stfc.ac.uk 5 March 2012 FLS2012, Thomas Jefferson National Laboratory Multi-shot damage in a-C Multi-shot damage observed in a-C Each shot is below threshold for single shot damage –0.5 J/cm 2, 46.9 nm, 1.7 ns, CDL –5 shots  no observable damage –10 - 40 shots  progressive erosion a-C complex behaviour  Low fluence multi-shot => photo-induced erosion without chemical change  High fluence => expansion due to graphitization 10 shots 40 shots Juha et al., J. Appl. Phys 105, 093117 (2009) AFM Image University of L’Aquila

20. mark.roper@stfc.ac.uk 5 March 2012 FLS2012, Thomas Jefferson National Laboratory Active Optics Significant usage of active optics is certain Achieving better quality foci –Use plane surfaces (easier to make) and benders –Correct residual errors in the manufactured surface –Correct wavefront distortion caused by errors in the surface of other optics Tailored focusing –Different spot sizes (without sitting off-focus) –Tailored spot shapes (e.g. top hat, Lorentzian) Compensating for the moving source position

21. mark.roper@stfc.ac.uk 5 March 2012 FLS2012, Thomas Jefferson National Laboratory FERMI@Elettra K-B System Both DiProI and Low Density Matter will use a KB active optics system to give a small spot taking into account the source variation between FEL1 and FEL2 and the necessary optical quality of the surfaces, achievable only on plane surfaces. M. Zangrando FERMI@Elettra

22. mark.roper@stfc.ac.uk 5 March 2012 FLS2012, Thomas Jefferson National Laboratory Modelling Geometric vs physical optics –Ray-tracing will still play a big part in designing a beamline Checking aberrations, alignment tolerances etc Because it’s fast!! –Previous slides show physical optics simulations are essential Modelling with Genesis source simulations Determining the actual source properties Coherence effects from apertures and mirror imperfections

23. mark.roper@stfc.ac.uk 5 March 2012 FLS2012, Thomas Jefferson National Laboratory Diagnostics

24. mark.roper@stfc.ac.uk 5 March 2012 FLS2012, Thomas Jefferson National Laboratory Diagnostics The challenge of the ideal diagnostic –Measure every pulse in real time (@ Hz to MHz) –Non-invasive Transparent to the beam Require no special optics –In situ Always “on-line” To measure –Pulse energy –Pulse length –Longitudinal and transverse intensity profiles –Timing jitter (relative to something useful) at fs or as level –Spectral content (and phases) –Polarisation

25. mark.roper@stfc.ac.uk 5 March 2012 FLS2012, Thomas Jefferson National Laboratory Pulse Length Measurement Cross-correlation with IR laser –Side-band generation in the presence of an intense IR field during the photo-ionisation of a noble gas by the FEL beam Meyer, M., et al., Two-colour photoionization in xuv free- electron and visible laser fields. Phys Rev, 2006. 74, 011401 FEL beam needs to be focused - impacts on beamline layout Multi-shot (scan laser delay) Photo-electrons must be spectrally analysed

26. mark.roper@stfc.ac.uk 5 March 2012 FLS2012, Thomas Jefferson National Laboratory Pulse Length Measurement Single shot cross-correlation by looking at the intensity and number of the sidebands Also gives jitter information (relative to IR laser) Radcliffe, P., et al., Single-shot characterization of independent femtosecond extreme ultraviolet free electron and infrared laser pulses. Appl Phys Lett, 2007. 90, 131108 FEL at 89.9 eV Other approaches include “time to space” mapping Single shot cross-correlation by looking at the intensity and number of the sidebands Also gives jitter information (relative to IR laser) Cunovic, S., et al., Time-to-space mapping in a gas medium for the temporal characterization of vacuum-ultraviolet pulses. Appl Phys Lett, 2007. 90, 121112

27. mark.roper@stfc.ac.uk 5 March 2012 FLS2012, Thomas Jefferson National Laboratory Pulse Length “Holy Grail” Intensity autocorrelation gives only limited pulse profile information A soft x-ray analog of FROG or SPIDER is needed for complete pulse characterisation –Requires a non-linear process to give a signal that is proportional to the autocorrelation function Beam mixing (FROG) or spectral shear (SPIDER) –(Almost) certainly will involve measuring photo-electrons Two-photon ionisation (one or two colours) Single-photon multiple-ionisation Optical phase and spectral information encoded onto photo- electrons, requires electron spectrometers Challenging experiments, limited by spectrometer performance and wavelength coverage may be limited by gases available –Autocorrelation, so no timing jitter info Remetter, T., et al., Attosecond electron wave packet interferometry. Nat Phys, 2006. 2, 323

28. mark.roper@stfc.ac.uk 5 March 2012 FLS2012, Thomas Jefferson National Laboratory Other Diagnostics Wavefront –Hartmann sensor Pulse energy –Gas cell –Can be expanded to measure wavelength & harmonics Spectrum –VLS grating spectrometer (zeroth order to experiment) Position & angle –Blade monitors (diffraction & damage) –Ionisation chambers (sensitivity and accuracy) Polarisation –Wideband ML Polarimetry (F. Schäfers) –Full Stokes vector in a single shot??

29. mark.roper@stfc.ac.uk 5 March 2012 FLS2012, Thomas Jefferson National Laboratory Conclusions Ultra-short and transversely coherent SXR pulses present a new challenge to the beamline designer –Spectral dependence of even the most basic source properties –Diffractive disruption to the wavefront –Stretching the pulse –The risk of damaging the optical surfaces –Requirement for physical optics modelling We also have to account for the shot to shot variation in the source –Diagnostics need to be an integrated part of the beamline Many areas are at least partly addressed –There is more that needs to be done –Progress will follow as sources come on stream

30. mark.roper@stfc.ac.uk 5 March 2012 FLS2012, Thomas Jefferson National Laboratory Thank you for your attention