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-IIX-ray diagnostics from LCLS and LCLS-IIHigh repetition rate from NGLS

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X-ray FEL radiation has novel properties

Photons/pulse 1012 - 1013

Pulse length ~10 - 500 fs (fwhm)› Atoms may absorb more than one photon› Must consider damage› Diagnostics may respond non-linearly

Bandwidth 0.2 - 0.5 % (fwhm)High transverse coherenceRepetition rate 120 Hz 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 focusFringes 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

In PbWO4

From Chalupsky et al., NIMA 631, 130 (2011).

• 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 m focus at LCLS instruments.• Not an “in situ” measurement.

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Reflectivity of coating for mirror system› B4C, SiC, C are well understood› Un-coated silicon may be used around carbon edge

Optical coatings

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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

Optical profilometryFrom Soufli et al., SPIE 8077, 807702 (2011).

• 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.

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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 requirementsOperating energy range

› 250 eV to 13 keVCapable of sustaining full un-focused FEL power

› Maximum fluence: 12 mJ @ 250 eVSingle-shot measurement, non-destructiveRelative pulse energy accuracy

› 1%Sensitivity

› 10 J

- S. Moeller

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Energy MonitorsLCLS Gas Monitor› N2 photoluminescence (UV) proportional to

FEL intensity› Relative intensity determination

FLASH – Gas Monitor Detector› 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

950 eV

Correlation of two identical devices

Single-shot measurement

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Calorimeters

Performance requirementsOperating energy range

› 250 eV to 13 keVCapable of sustaining full un-focused FEL power

› Maximum fluence: 12 mJ @ 250 eVAverage measurement, destructiveAbsolute average pulse energy accuracy

› 10%Sensitivity

› 10 J

- S. Moeller

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Calorimeters

*Rabus et al. Applied Optics 36, 22, (1997)

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

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X-ray Spectrometer

Performance requirementsGrating spectrometer covers energy range from 400 eV to 2 keVCrystal spectrometer covers energy range from 2 keV to 10 keVSpectral range > 2%

› Capable of capturing full FEL spectrumSpectral 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

Soft X-RayDetector

DispersedSoft X-Rays

Gratings G1, G2

M1 Mirror

Beam

VLS

YAG:Cescreen

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Soft X-ray SpectrometerEnergy resolution

Photon énergies (eV)

Measured resolution (eV)

Measured resolving power

Calculated resolution (eV)

100 l/mm 674 0.21 3200 0.26867 0.51 1700 0.42

200 l/mm 867 0.30 2900 0.251678 0.75 2200 0.79

High resolution E/E > 104 needed to characterize seeding

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X-ray optical timing• 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.

Chirped opticalpulse

Spectrometer

From Bionta et al., Optics Express 19, 21855 (2011).

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X-ray pulse duration

From Duesterer et al., New Journal of Physics 13, 93024 (2011).

• 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 t(x-ray) = ( 40 ± 20 fs) for t(electron) = 75 fs.

• This measurement is an experiment. An on-line diagnostic is needed.

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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 i = 14 mrad.

High repetition rate at the NGLS

E (eV) Incident power (W)

xymm, fwhmat 50 m

R (14 mrad) Absorbed power (W)

Absorbed power density

(W/mm2)276 410 1.46 0.869 54 0.31376 373 1.01 0.883 44 0.53620 301 0.70 0.911 27 0.67827 249 0.68 0.924 19 0.50

1240 104 0.36 0.937 6.5 0.61

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Mirror FEA Analysis

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

- K. Chow

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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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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.

Diffraction gratings

6XLaminar profile

wh

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Summary of ChallengesX-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

Performance requirements gas attenuatorOperating 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 attenuationAvoid operating near an absorption edges of attenuating mediumMinimize small angle scattering to the extent possible

- J. Frisch

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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

Performance requirementsOperating energy range

› 250 eV to 13 keV

Single-shot measurement, destructiveSpatial resolution

› 10% of beam size or better

- Y. Feng

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X-ray ImagersX-ray scintillation using single-crystal YAG:Ce @ normal incidenceOptical imaging using 45° mirror, zoom lens, and camera

pixelatedcamera

zoomlens

neutraldensityfilter

verticalstage

45°mirror

YAG:Cescreen

FEL

XPP Profile Monitor2 m resolution

500 m