Soft x-ray optics and beamlines for next generation light sources

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

2. 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. 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. Transportation

5. Where is the source? As optic gets closer than ZR, 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 M2 propagation factor? Is it the same horizontally and vertically? How do these factor vary with wavelength?

6. 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. 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. Preserving the wavefront Reflection imprints defects in the mirror surface onto the wavefront M. Zangrando FERMI@Elettra Small defects also give “speckle” diffraction patterns

9. Preserving the wavefront Typical SR mirror M. Zangrando FERMI@Elettra The demands on optical manufacturing and metrology are unprecedented

10. Preserving the wavefront Don’t forget that a coherent wave will diffract from the edges of mirrors!! 100 eV. Simulation with FOCUS code • Implications for: • Diffraction limited focusing • Wavefront dividing beam-splitters • Knife-edge position monitors

11. Diffraction Limited Focusing The fringes will not be (so) visible at a focus Size of focus limited by the aperture through diffraction 6: +11% f = 0.2 m 4: +38% 8: +2.5% Relative to infinite aperture

12. Focus limit from surface errors Field @ source Field @ focus FLASH BL3, 98 eV Focus source with imperfect ellipsoid at 37.5:1 demagnification PSD of mirrors Temporal profile @ source Temporal profile @ focus M.A.Bowler B.Faatz F.Siewert

13. 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. Metrology & Manufacturing Before After 0.0 2.0 4.0 6.0 -20 -10 0 10 20 30 Achieving the highest possible figure accuracy requires collaboration between the manufacturer and the metrology laboratory HZB: NOM Metrology Data + Zeiss: Ion Beam Finishing Reduction in form error of elliptical focusing mirrors by factor of 3 H. Thiess, H. Lasser, F. Siewert, NIM A (2009) F. Siewert, J. Buchheim, T. Zeschke, NIM A (2010) Slope (arcsec) Height (nm) F. Siewert HZB 61 nm to 22 nm PV 1.6 µrad to 0.5 µrad RMS

15. 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. 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. 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. a-C Single-shot Damage Threshold fluence for damage as a function of grazing angle. Chalupský et al., Appl Phys Lett 95 031111 (2009) • Two regions of damage • Central ablation • Peripheral expansion (graphitization) 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 Nomarski microscope images of damage by 13.5 nm radiation. Beam at normal incidence (left) and 18.5° grazing angle (right). FLASH Measurements

19. Multi-shot damage in a-C 10 shots 40 shots • Multi-shot damage observed in a-C • Each shot is below threshold for single shot damage • 0.5 J/cm2, 46.9 nm, 1.7 ns, CDL • 5 shots  no observable damage • 10 - 40 shots  progressive erosion Juha et al., J. Appl. Phys 105, 093117 (2009) University of L’Aquila • a-C complex behaviour • Low fluence multi-shot => photo-induced erosion without chemical change • High fluence => expansion due to graphitization AFM Image

20. 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. FERMI@Elettra K-B System M. Zangrando FERMI@Elettra 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.

22. 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. Diagnostics

24. 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. 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 FEL beam needs to be focused - impacts on beamline layout Multi-shot (scan laser delay) Meyer, M., et al., Two-colour photoionization in xuv free-electron and visible laser fields. Phys Rev, 2006. 74, 011401 Photo-electrons must be spectrally analysed

26. 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) • 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 Other approaches include “time to space” mapping 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 FEL at 89.9 eV

27. 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. 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. 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. Thank you for your attention