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

2 X-ray FEL radiation has novel properties Photons/pulse Pulse length ~ fs (fwhm) ›Atoms may absorb more than one photon › Must consider damage › Diagnostics may respond non-linearly Bandwidth % (fwhm) High transverse coherence Repetition 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.

3 X-ray mirrors › Separate FEL radiation from Bremsstrahlung. › Switch x-ray beam to different instruments. › Focusing.

4 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.

5 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.

6 Characterization of x-ray focus In PbWO 4 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.

7 Reflectivity of coating for mirror system ›B 4 C, SiC, C are well understood ›Un-coated silicon may be used around carbon edge Optical coatings

8 Optical Damage Principle: stay significantly below melt dose. ›At LCLS, the guideline is 0.1 eV/atom for B 4 C < 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/cm J/cm 2 At 830 eV. From Hau-Riege et al., Optics Express 18, (2010).

9 Mirror contamination Optical profilometry From Soufli et al., SPIE 8077, (2011). Carbon contamination is observed on LCLS mirror surfaces. It is possible to clean with UV-ozone. –However, B 4 C optical coating is partially or completely removed. –Requires recoating.

10 X-ray Diagnostics X-ray diagnostics ›Characterize pulse energy, beam profile, spectrum and timing.

11 Energy Monitors Performance requirements Operating energy range ›250 eV to 13 keV Capable of sustaining full un-focused FEL power ›Maximum fluence: eV Single-shot measurement, non-destructive Relative pulse energy accuracy ›1% Sensitivity ›10  J - S. Moeller

12 Energy Monitors LCLS Gas Monitor ›N 2 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

13 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

14 Calorimeters Performance requirements Operating energy range ›250 eV to 13 keV Capable of sustaining full un-focused FEL power ›Maximum fluence: eV Average measurement, destructive Absolute average pulse energy accuracy ›10% Sensitivity ›10  J - S. Moeller

15 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

16 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

17 Soft X-ray Spectrometer LCLS SXR spectrometer ›Variable-line-spacing (VLS) plane grating w/ pre-mirror focusing ›X-ray scintillation of 1 st order light & optical imaging VLS YAG:Ce screen

18 Soft X-ray Spectrometer Energy resolution Photon énergies (eV) Measured resolution (eV) Measured resolving power Calculated resolution (eV) 100 l/mm l/mm High resolution E/  E > 10 4 needed to characterize seeding

19 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 Si 3 N 4. X-ray arrival time located to within 25 fs (rms). Techniques need to be converted into an on-line diagnostic. Chirped optical pulse Spectrometer From Bionta et al., Optics Express 19, (2011).

20 X-ray pulse duration From Duesterer et al., New Journal of Physics 13, (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.

21 At NGLS, SASE beamline has10 6 repetition rate and seeded beamline have10 5 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 B 4 C mirror at 50 m and  i = 14 mrad. High repetition rate at the NGLS E (eV) Incident power (W)  x  y  mm, fwhm  at 50 m R (14 mrad) Absorbed power (W) Absorbed power density (W/mm 2 )

22 Mirror FEA Analysis Water cooled silicon mirror (frit-bonded) ›Internal water cooling channels ›1x5 mm cooling channels, 2 mm pitch ›5000 W/m 2 -K convection coefficient (0.75 gpm) 50 x 50 x 400 or 800 mm 3 - K. Chow

23 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

24 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

25 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 6X Laminar profile w h

26 Summary of Challenges X-ray OpticsChallenges MirrorsPreservation of x-ray wavefront Surface contamination X-ray DiagnosticsChallenges Energy monitors / CalorimetersAbsolute intensity Timing X-ray / optical delay < Pulse durations X-ray pulse durationOn-line diagnostic High repetition rateChallenges MirrorsDamage GratingsOptical distortion

27 Attenuator 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 for LCLS, users requested higher attenuation Avoid operating near an absorption edges of attenuating medium Minimize small angle scattering to the extent possible - J. Frisch

28 Attenuator Design concept similar to LCLS-I ›Use multiple gases, i.e. Kr and Xe ›Differential pumping w/ 1 st variable (impedance) apertures to reduce conductance ›Harmonics are not well attenuated 2-7 mm

29 Imagers Performance requirements Operating energy range ›250 eV to 13 keV Single-shot measurement, destructive Spatial resolution ›10% of beam size or better - Y. Feng

30 X-ray Imagers X-ray scintillation using single-crystal normal incidence Optical imaging using 45° mirror, zoom lens, and camera pixelated camera zoom lens neutral density filter vertical stage 45° mirror YAG:Ce screen FEL XPP Profile Monitor 2  m resolution 500  m