Transcript X-ray Free Electron Laser (FEL) Beamline Challenges

X-ray Free Electron Laser (FEL)
Beamline Challenges
Philip Heimann (SLAC)
Armin Busse, Yiping Feng, Joe Frisch, Nicholas Kelez,
Jacek Krzywinski, Stefan Moeller, Michael Rowen,
Peter Stefan and Jim Welch (SLAC)
Ken Chow and Howard Padmore (LBNL)
X-ray optics from LCLS and LCLS-II
X-ray diagnostics from LCLS and LCLS-II
High repetition rate from NGLS
X-ray FEL radiation has novel properties
Photons/pulse 1012 - 1013
Pulse length ~10 - 500 fs (fwhm)
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Atoms may absorb more than one photon
Must consider damage
Diagnostics may respond non-linearly
Bandwidth 0.2 - 0.5 % (fwhm)
High transverse coherence
Repetition rate 120 Hz
l jitter similar to bandwidth
Intensity fluctuations ~ 10 %
›
Each x-ray pulse is different from the last one.
These properties require novel x-ray optics and diagnostics .
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X-ray mirrors
X-ray mirrors
› Separate FEL radiation from Bremsstrahlung.
› Switch x-ray beam to different instruments.
› Focusing.
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LCLS HOMS mirror distortion
Current HOMS substrate – 450mm length.
› 1nm RMS polished substrate
› 2-3nm RMS as coated and mounted
Current state of the art leads to distortion of FEL x-rays.
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Intensity variations away from focus
Fringes result from edge diffraction and wavefront distortion
caused by figure error.
However, focused beam has good quality.
8 keV x-ray beam downstream of hard x-ray offset mirrors.
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Characterization of x-ray focus
• Use the FEL to make ablative imprint in a solid with variable
attenuation.
• Measure damage area with AFM or Nomarski microscope.
• This technique has been successfully used to characterize
~1 mm focus at LCLS instruments.
• Not an “in situ” measurement.
In PbWO4
From Chalupsky et al., NIMA 631, 130
(2011).
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Optical coatings
Reflectivity of coating for mirror system
› B4C, SiC, C are well understood
› Un-coated silicon may be used around carbon edge
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Optical Damage
Principle: stay significantly below melt dose.
› At LCLS, the guideline is 0.1 eV/atom for B4C < melt dose of 0.62
eV/atom.
Damage has not been observed on LCLS optics.
In damage studies surface roughening, ablation and cracking
has been observed.
Multishot damage is observed at a lower threshold than single
shot damage. It is an area of current development.
2.6 J/cm2
5.4 J/cm2
At 830 eV.
From Hau-Riege et al., Optics Express
18, 23933 (2010).
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Mirror contamination
• Carbon contamination is observed on LCLS mirror surfaces.
• It is possible to clean with UV-ozone.
– However, B4C optical coating is partially or completely removed.
– Requires recoating.
Optical profilometry
From Soufli et al., SPIE 8077, 807702 (2011).
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X-ray Diagnostics
X-ray diagnostics
›Characterize pulse energy, beam profile, spectrum and timing.
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Energy Monitors
Performance requirements
- S. Moeller
Operating energy range
› 250 eV to 13 keV
Capable of sustaining full un-focused FEL power
› Maximum fluence: 12 mJ @ 250 eV
Single-shot measurement, non-destructive
Relative pulse energy accuracy
› 1%
Sensitivity
› 10 mJ
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Energy Monitors
FLASH – Gas Monitor Detector
LCLS Gas Monitor
› N2 photoluminescence (UV)
proportional to FEL intensity
› Relative intensity determination
› X-ray ionization of rare gases
(Xe and Kr)
› Ion-current proportional to FEL
intensity
› Capable of absolute intensity
determination
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Energy Monitors
LCLS gas monitor performance
› Energy range 480 eV to 9.5 keV
› 1% relative accuracy
Single-shot measurement
950 eV
Correlation of two identical devices
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Calorimeters
- S. Moeller
Performance requirements
Operating energy range
› 250 eV to 13 keV
Capable of sustaining full un-focused FEL power
› Maximum fluence: 12 mJ @ 250 eV
Average measurement, destructive
Absolute average pulse energy accuracy
› 10%
Sensitivity
› 10 mJ
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Calorimeters
Design based on Electrical Substitution Radiometer (ESR)*
› Equivalence of electrical and radiant heating
› Average, absolute pulse energy measurement
› Previously used at synchrotrons as primary standard, e.g. NIST,
PTB, NMIJ, and also UV-FEL at SPring-8 SCCS
Being developed at the LCLS
› Absorber material
› Testing
*Rabus et al. Applied Optics 36, 22, (1997)
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X-ray Spectrometer
Performance requirements
Grating spectrometer covers energy range from 400 eV to 2 keV
Crystal spectrometer covers energy range from 2 keV to 10 keV
Spectral range > 2%
› Capable of capturing full FEL spectrum
Spectral resolution (FWHM) better than 1x10-3
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Soft X-ray Spectrometer
LCLS SXR spectrometer
› Variable-line-spacing (VLS) plane grating w/ pre-mirror focusing
› X-ray scintillation of 1st order light & optical imaging
VLS
G ratings G 1, G 2
Soft X -Ray
D etector
D ispersed
Soft X -Rays
Beam
YAG:Ce
screen
M 1 M irror
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Soft X-ray Spectrometer
Energy resolution
100 l/mm
200 l/mm
Photon énergies
(eV)
Measured
resolution (eV)
Measured
resolving power
Calculated
resolution (eV)
674
0.21
3200
0.26
867
0.51
1700
0.42
867
0.30
2900
0.25
1678
0.75
2200
0.79
High resolution E/DE > 104 needed to characterize seeding
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X-ray optical timing
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The natural jitter of the LCLS x-ray FEL relative to an optical laser is ~180 fs
(rms).
The x-ray pulses create free carriers via photoionization of core electrons,
which altered the optical properties of the Si3N4.
X-ray arrival time located to within 25 fs (rms).
Techniques need to be converted into an on-line diagnostic.
From Bionta et al., Optics Express 19, 21855 (2011).
Spectrometer
Chirped optical
pulse
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X-ray pulse duration
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Electron and x-ray pulse durations need not be the same. The x-ray pulse
duration is a critical parameter for many experiments.
Laser-assisted Auger decay: Auger electrons in an intense NIR field exchange
photons with the field causing sidebands in the electron kinetic energy spectrum.
Analysis of single-shot Auger spectra suggests pulse durations of Dt(x-ray) = (
40 ± 20 fs) for Dt(electron) = 75 fs.
This measurement is an experiment. An on-line diagnostic is needed.
From Duesterer et al., New Journal
of Physics 13, 93024 (2011).
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High repetition rate at the NGLS
E (eV)
Incident
power (W)
Dx= Dy (mm,
fwhm) at 50 m
R (14 mrad)
Absorbed
power (W)
276
376
620
827
1240
410
373
301
249
104
1.46
1.01
0.70
0.68
0.36
0.869
0.883
0.911
0.924
0.937
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44
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6.5
Absorbed
power
density
(W/mm2)
0.31
0.53
0.67
0.50
0.61
At NGLS, SASE beamline has106 repetition rate and seeded beamline
have105 repetition rate.
For SASE beamline, the absorbed power is not high compared with a
synchrotron undulator beamline, e.g. ALS BL6.0 M201 absorbs a factor of
~10 higher.
The absorbed power density is high (similar to ALS BL6.0 M201).
› The absorbed power density is for B4C mirror at 50 m and qi = 14 mrad.
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Mirror FEA Analysis
- K. Chow
Water cooled silicon mirror (frit-bonded)
› Internal water cooling channels
› 1x5 mm cooling channels, 2 mm pitch
› 5000 W/m2-K convection coefficient (0.75 gpm)
50 x 50 x 400 or 800 mm3
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Tangential slope error, 14 mrad grazing angle
Moving mirror further from source helps.
Bending mirror reduces slope error about a factor of 2.
Both are not enough to preserve brightness.
7.6 km
528 km bend radius
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Tangential slope error, 7 mrad grazing angle
A viable NGLS first mirror design: bent, internally-cooled silicon
mirror with grazing angle of 7 mrad at ~50 meters from end of
undulators.
8.0 km
1065 km
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Diffraction gratings
Grating parameters: 100 l/mm, grating order m=1, and laminar
profile.
Efficiency calculations performed with GSolver.
The sum of the diffracted orders < mirror reflectivity by a large factor.
X-rays are preferentially absorbed at land leading edges.
› For optical distortion and damage, gratings are a significantly
more difficult case than mirrors.
6X
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h
w
Laminar profile
Summary of Challenges
X-ray Optics
Challenges
Mirrors
Preservation of x-ray wavefront
Surface contamination
X-ray Diagnostics
Challenges
Energy monitors / Calorimeters
Absolute intensity
Timing
X-ray / optical delay <
Pulse durations
X-ray pulse duration
On-line diagnostic
High repetition rate
Challenges
Mirrors
Damage
Gratings
Optical distortion
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Attenuator
- J. Frisch
Performance requirements gas attenuator
Operating energy range from 250 eV to 2 keV
Energy above 2 keV covered by solid attenuator
Attenuator factor from 1 to10-6
› Only 10-3 for LCLS, users requested higher attenuation
Avoid operating near an absorption edges of attenuating medium
Minimize small angle scattering to the extent possible
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Attenuator
Design concept similar to LCLS-I
› Use multiple gases, i.e. Kr and Xe
› Differential pumping w/ 1st variable (impedance) apertures to reduce
conductance
› Harmonics are not well attenuated
2-7 mm
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Imagers
- Y. Feng
Performance requirements
Operating energy range
› 250 eV to 13 keV
Single-shot measurement, destructive
Spatial resolution
› 10% of beam size or better
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X-ray Imagers
X-ray scintillation using single-crystal YAG:Ce @ normal incidence
Optical imaging using 45° mirror, zoom lens, and camera
pixelated
camera
vertical
stage
zoom
lens
neutral
density
filter
YAG:Ce
screen
500 mm
FEL
45°
mirror
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XPP Profile Monitor
2 mm resolution