Transcript Photon Beamlines and Diagnostics 2010

Photon Beamlines and
Diagnostics 2010
Meeting Summary
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
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
Optics “beyond the mirror”
• Multilayers
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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
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
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
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)
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
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
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)
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Also few fs pulse stretch
Conceptual only
Needs model of FEL source
>~250 eV only
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
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
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
SXR Beamlines
• Design challenges
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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)
Photon Diagnostics
• Experiments on FELs demand a wide range
of diagnostics of the photon beam
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Pulse energy
Pulse length
Pulse spectrum
Timing jitter
Polarisation
Beam position and angle
Etc
• Non-invasive and every pulse
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
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
Photon Diagnostics
• Generation of CP in a FEL with mixed planar
and helical ID’s means measuring the DOP
is essential
• Multilayer-based Polarimetry
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Broad-band in SXR
Full Stokes vector
High accuracy, proven
Slow (inherently time averaged)
Needs very high quality multilayers
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
Global Perspective
• Japan
– Success of SCSS Test FEL &rapid progress towards SCSS
operation
– Common themes
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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
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
Global Perspective
• Europe
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FLASH II
Huge over-subscription of FLASH has to be addressed
Complementary to FLASH
Seeding to go beyond the limits of FLASH SASE
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Timing stability
Frequency stability
Single mode
Polarisation control
– Details still being worked out
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?
Open Forum
“gardener’s question time”
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