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Advancements in Photon Beamlines and Optics: A Summary of the 2010 Meeting

Explore key topics discussed, including preserving wavefronts, diffraction-limiting focii, manufacturing processes, metrology, and innovative optics applications.

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Advancements in Photon Beamlines and Optics: A Summary of the 2010 Meeting

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  1. Photon Beamlines and Diagnostics 2010 Meeting Summary

  2. Optics • To preserve the coherent wavefront of the FEL pulse • To achieve diffraction limited focii • Limit or control the phase error • Need nm level height errors • Metrology «» Manufacturing • Manufacturer and end-user collaboration • Classical polishing • Spheres with <0.2 µrad slope errors • Deterministic figuring • IBF, EEM • Large substrates • Fast figuring - ELID • Fast metrology - stitching interferometry for 2-D maps

  3. Optics “beyond the mirror” • Diamond optics for use in an X-Ray cavity FEL • Need “perfect” crystals • 4 needed for tunable FEL • Demonstrated the required 99% reflectivity is achievable • Feedback control to stabilise angle also proven • Cryo cooling to reduce heat deformation • Proven technologies, no reason XFELO should not work

  4. Optics “beyond the mirror” • Multilayers • Large NA in the hard X-ray • Improved reflectivity • More convenient geometry • Delay lines • Smaller optics • Spectral filtering (isolate harmonic) • Beam splitters • BW match to FEL pulse • Damage (Mo:Si) • Thermal in nature - diffusion • Use experience with EUV lithography

  5. Optics / Metrology • There is a limit to what can be made by direct figuring • Change the shape with a bender • Allow more extreme shapes • Allow change in focal length • Be clever when setting up the bender • Measure response function of each actuator - derive response matrix • Can work out which actuator to use to correct residual errors, shift FL etc. Saves a lot of time! • Accuracy depends on quality of original measurements • More accurate metrology, lower systematic errors

  6. Optics / Metrology • Why stop at just defining the basic mirror shape with a bender? • Add actuators along mirror surface to give control to smaller spatial frequencies • Deterministic control • Find response matrix with metrology • Dial in required figure • More degrees of freedom = better figure accuracy • Adaptive control • Phase compensator • Deform mirror to correct for errors in a downstream mirror • In situ interferometry

  7. Metrology • Profilers - the end-users favourite • Accuracy 0.05 µrad for flats (10x improvement) • Known through comparative measurements to ESAD (Deutsch national standard) • 0.1 to 0.2 µrad for spheres • Systematic errors more problematic • Characterize with angle comparator • Not the best choice for the manufacturer (too slow)

  8. Metrology / Diagnostics • Hartmann wavefront sensors • Developed technology for XUV and SXR • Commercially available • Simple Hartmann plate (no micro lenses) • Calibrate with spherical wavefront from pin hole • HXR more difficult - convert to optical  • Calibration more critical (imaging optics) and harder to achieve (can’t make spherical wavefront) • Measuring beam wavefront allows determination of source properties

  9. Metrology / Diagnostics • Grating interferometry • At-wavelength metrology of optics with 10 nrad sensitivity • Source characterisation • Longitudinal position to 1 m • Shot-by-shot potential • Facilitated by advanced nanofabrication techniques • But an instrumentally simple and robust diagnostic

  10. Photon Beam Transport • Time preserving monochromators • A diffraction grating stretches a pulse • Double grating systems • Complete reversal of pulse stretch • Complicated, reduced efficiency • Conical diffraction • Pulse stretch down to few fs • The only practical single grating solution in VUV to XUV • Working example on HHG source (RAL) • Classical diffraction mount (match to FEL source in SXR) • Also few fs pulse stretch • Conceptual only • Needs model of FEL source • >~250 eV only

  11. Photon Beam Transport • Multi-beam experiments (TIMER) • Beam splitters • Wavefront (knife-edged mirror) • Harmonic (multilayers) • Delay lines • Multilayer • Multiple mirrors to vary angles • Three beams coincident in time and space at the experiment, with defined angles and energies

  12. Photon Beam Transport • HHG Seeding • Essential for single mode SXR FELs (?) • Successful demonstration at SCSS Test FEL and SPARC • As demanding as a user beamline • Focus and spatial overlap with electron beam • Steering and focusing mirrors • Temporal overlap • Diagnostics • Spectrometer, position & angle

  13. SXR Beamlines • Use with a FEL source • Reduce the bandwidth of a SASE pulse with many longitudinal modes • Remove the background of spontaneous emission • Remove SASE emission outside the spectral bandwidth of a seeded pulse • Define the photon energy reaching the experiment (eliminate energy jitter) • Reduce the intensity of the higher harmonics • Remove the fundamental when working on the harmonics • Experiments requiring HSRP

  14. SXR Beamlines • Design challenges • 1st mirror distance / mirror lengths • Coatings (conflicting edges) • Carbon K-edge • Mirror damage (what is safe?) • Grating damage (effect of structure?) • Transport efficiency • Pulse stretch • Diffraction limited focusing (quality, aperture)

  15. Photon Diagnostics • Experiments on FELs demand a wide range of diagnostics of the photon beam • Pulse energy • Pulse length • Pulse spectrum • Timing jitter • Polarisation • Beam position and angle • Etc • Non-invasive and every pulse

  16. Photon Diagnostics • Timing and pulse length • Streak camera • Over come limits of camera with single photon counting and in-camera processing • Achieved 280 fs FWHM resolution • Single shot, 4.25 kHz

  17. Photon Diagnostics • Pulse spectrum • Grating spectrometer • On-line (minimally invasive) • Pulse by pulse analysis (detector ltd) • Photon energy • eTOF and iTOF • Central wavelength, modes, harmonics • On-line, non-invasive • Robust and high accuracy

  18. Photon Diagnostics • Generation of CP in a FEL with mixed planar and helical ID’s means measuring the DOP is essential • Multilayer-based Polarimetry • Broad-band in SXR • Full Stokes vector • High accuracy, proven • Slow (inherently time averaged) • Needs very high quality multilayers

  19. Photon Diagnostics • Gas-based polarimetry • Angle resolved photo electron spectrometer using TOF • Broad-band XUV and SXR • Full Stokes vector (need coincidence for CP) • Requires data base of gas yields • Can be shot by shot (if good cross-section) • Complicated instrument (16 TOFs) • Also measures photon energy, photon flux and beam position • A universal, in situ, pulse-by-pulse diagnostic

  20. Global Perspective • Japan • Success of SCSS Test FEL &rapid progress towards SCSS operation • Common themes • Shot by shot diagnostics • Diffraction limited focusing • Polarization control • Pulse duration, delay and synchronisation • X-ray autocorrelator using thin Si Bragg beam splitters and channel-cut Si for delay line • Spectrum analysis • Crystal spectrometer • Compton back-scattering from a diamond crystal

  21. Global Perspective • USA • Stunningly rapid commissioning of LCLS • Excellent performance already achieved • Huge user demand • Proof of the importance of FEL sources • Extensive use of B4C coatings on mirrors • Have seen some carbon deposits • Absorber uses N2 gas and Be foils • Up to 7 Torr • Measure fluorescence of nitrogen • See speckle from Be • Gas detector compared to DESY GMD • LUSI diagnostic suite for HXR experiments

  22. Global Perspective • Europe • FLASH II • Huge over-subscription of FLASH has to be addressed • Complementary to FLASH • Seeding to go beyond the limits of FLASH SASE • Timing stability • Frequency stability • Single mode • Polarisation control • Details still being worked out

  23. Global Perspective • EuroFEL • Preparatory phase now been running for 2 years • Created a vibrant community of experts with • A diverse range of expertise • But a common interest in delivering photons from a FEL source to a users experiment • Outcomes • Exchange ideas, experiences and knowledge • R & D collaborations • Faster solutions to common problems • Is it useful? I invite you to draw your own conclusion: • This meeting has demonstrated Global interest in FEL sources and common problems that face all the sources • Does anyone think they can solve every problem on their own?

  24. Open Forum “gardener’s question time”

  25. Closing Remarks • Thanks to the organisers • Thanks to the speakers • Thanks to all the participants for making the meeting a success • Reminder to submit your papers

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