Xray Free Electron Laser FEL Beamline Challenges Philip

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) › › › • • › 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. 2

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

LCLS HOMS mirror distortion • Current HOMS substrate – 450 mm length. › 1 nm RMS polished substrate › 2 -3 nm RMS as coated and mounted • Current state of the art leads to distortion of FEL x-rays. 4

Intensity variations away from focus • Fringes result from edge diffraction and wavefront distortion caused by figure error. • However, focused beam has good quality. 8 ke. V x-ray beam downstream of hard x-ray offset mirrors. 5

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 Pb. WO 4 From Chalupsky et al. , NIMA 631, 130 (2011). 6

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

Optical Damage • Principle: stay significantly below melt dose. › At LCLS, the guideline is 0. 1 e. V/atom for B 4 C < melt dose of 0. 62 e. V/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 2 5. 4 J/cm 2 At 830 e. V. From Hau-Riege et al. , Optics Express 18, 23933 (2010). 8

Mirror contamination • 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. Optical profilometry From Soufli et al. , SPIE 8077, 807702 (2011). 9

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

Energy Monitors • Performance requirements - S. Moeller – Operating energy range › 250 e. V to 13 ke. V – Capable of sustaining full un-focused FEL power › Maximum fluence: 12 m. J @ 250 e. V – Single-shot measurement, non-destructive – Relative pulse energy accuracy › 1% – Sensitivity › 10 m. J 11

Energy Monitors • LCLS Gas Monitor • FLASH – Gas Monitor Detector › N 2 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 12

Energy Monitors • LCLS gas monitor performance › Energy range 480 e. V to 9. 5 ke. V › 1% relative accuracy Single-shot measurement 950 e. V Correlation of two identical devices 13

Calorimeters - S. Moeller • Performance requirements – Operating energy range › 250 e. V to 13 ke. V – Capable of sustaining full un-focused FEL power › Maximum fluence: 12 m. J @ 250 e. V – Average measurement, destructive – Absolute average pulse energy accuracy › 10% – Sensitivity › 10 m. J 14

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) 15

X-ray Spectrometer • Performance requirements – Grating spectrometer covers energy range from 400 e. V to 2 ke. V – Crystal spectrometer covers energy range from 2 ke. V to 10 ke. V – Spectral range > 2% › Capable of capturing full FEL spectrum – Spectral resolution (FWHM) better than 1 x 10 -3 16

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 17

Soft X-ray Spectrometer • Energy resolution 100 l/mm 200 l/mm Photon énergies (e. V) Measured resolution (e. V) Measured resolving power Calculated resolution (e. V) 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 18

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. From Bionta et al. , Optics Express 19, 21855 (2011). Spectrometer Chirped optical pulse 19

X-ray pulse duration • • 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). 20

High repetition rate at the NGLS E (e. V) 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 54 44 27 19 6. 5 Absorbed power density (W/mm 2) 0. 31 0. 53 0. 67 0. 50 0. 61 At NGLS, SASE beamline has 106 repetition rate and seeded beamline have 105 repetition rate. For SASE beamline, the absorbed power is not high compared with a synchrotron undulator beamline, e. g. ALS BL 6. 0 M 201 absorbs a factor of ~10 higher. The absorbed power density is high (similar to ALS BL 6. 0 M 201). › The absorbed power density is for B 4 C mirror at 50 m and qi = 14 mrad. 21

Mirror FEA Analysis - K. Chow • Water cooled silicon mirror (frit-bonded) › Internal water cooling channels › 1 x 5 mm cooling channels, 2 mm pitch › 5000 W/m 2 -K convection coefficient (0. 75 gpm) • 50 x 400 or 800 mm 3 22

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 23

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 24

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. 6 X 25 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 26

Attenuator - J. Frisch • Performance requirements gas attenuator • Operating energy range from 250 e. V to 2 ke. V – Energy above 2 ke. V covered by solid attenuator • Attenuator factor from 1 to 10 -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 27

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 28

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

X-ray Imagers • X-ray scintillation usingle-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 30 XPP Profile Monitor 2 mm resolution