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Soft x-ray optics and beamlines for next generation light sources Mark D Roper Accelerator Science & Technology Centre STFC Daresbury Laboratory
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Soft x-ray optics and beamlines for next generation light sources
Mark D RoperAccelerator Science & Technology Centre
STFC Daresbury Laboratory
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.
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
5 March 2012
FLS2012, Thomas Jefferson National Laboratory
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?
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)
5 March 2012
FLS2012, Thomas Jefferson National Laboratory
Source shape for NLS FEL
Deduced from Genesis simulations using wavefront propagation
Four undulator modulesThe transport system has to cope with a source of changing size, position & quality
5 March 2012
FLS2012, Thomas Jefferson National Laboratory
Preserving the wavefront
• Reflection imprints defects in the mirror surface onto the wavefront
QuickTime™ and a decompressor
are needed to see this picture.
Small defects also give “speckle” diffraction patterns
M. Zangrando
FERMI@Elettra
5 March 2012
FLS2012, Thomas Jefferson National Laboratory
Preserving the wavefront
QuickTime™ and a decompressor
are needed to see this picture.
The demands on optical manufacturing and metrology are unprecedented
Wavelength AoI P-V shape error (nm)
(nm) (°) =0.25° =0.10°
40 6 47 18
40 3 95 38
40 1.5 191 76
10 3 23 9
10 2 35 14
10 1 71 28
5 3 12 5
5 2 18 7.2
5 1 36 14
1.67 3 4 2
M. Zangrando
FERMI@Elettra
Typical SR mirror
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!!
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
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
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
5 March 2012
FLS2012, Thomas Jefferson National Laboratory
Metrology & ManufacturingAchieving 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
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
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)
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
5 March 2012
FLS2012, Thomas Jefferson National Laboratory
a-C Single-shot DamageChalupský 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
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/cm2, 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 graphitization10 shots
40 shots
Juha et al., J. Appl. Phys 105, 093117 (2009)
AFM Image
University of L’Aquila
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
5 March 2012
FLS2012, Thomas Jefferson National Laboratory
FERMI@Elettra K-B System
QuickTime™ and a decompressor
are needed to see this picture.
QuickTime™ and a decompressor
are needed to see this picture.
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
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
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
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
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
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
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??
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
5 March 2012
FLS2012, Thomas Jefferson National Laboratory
Thank you for your attention
