Ultrafast x-ray-matter interaction at LCLS Optics design, photon diagnostics, and imaging

Transcription

1 Ultrafast x-ray-matter interaction at LCLS Optics design, photon diagnostics, and imaging Stefan P. Hau-Riege Lawrence Livermore National Laboratory Advanced Instrumentation Seminar (AIS) Stanford Linear Accelerator (SLAC) This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA /2/08 sphr_slac_ais 1

2 Thanks to all collaborators: LLNL: LBNL: R. Bionta, R. London, D. Ryutov, A. Barty, M. Bogan, M. Frank, S. Friedrich, M. Pivovaroff, N. Rohringer, R. Soufli, A. Szoke, B. Woods, and R. Lee S. Marchesini DESY/FLASH: H. Chapman, S. Bajt, and K. Tiedtke SLAC: Uppsala U.: CAS Prague: PAS Warsaw: S. Boutet and J. Krzywinski J. Hajdu L. Juha and J. Chalupsky R. Sobieraski 4/2/08 sphr_slac_ais 2

3 Outline 1. Introduction to XFELs 2. Fundamentals of XFEL x-ray-matter interaction 3. Applications: 1. Optics design and damage 2. Photon diagnostics (e.g. gas detector) 3. Coherent x-ray imaging 4/2/08 sphr_slac_ais 3

4 X-ray free electron lasers will produce extremely bright, ultrashort, coherent x-ray pulses Peak Brightness operational 2012 (photons/s/mm2/mrad2/0.1% bandwidth) nm, < 100 fs, >1012 photons Euro XFEL LCLS 1032 Euro XFEL DESY, Hamburg 1030 FLASH LCLS operational nm, < 100 fs, >1012 photons Linac Coherent Light Source (LCLS), SLAC, Stanford 1024 APS undulator 1022 FLASH ALS undulator operational now 48-6 nm, < 25 fs, > 1012 photons DESY, Hamburg /2/ Photon Energy (kev) sphr_slac_ais 4

5 The principle of SASE x-ray free electron lasers electron injector 1 2 3,4 linac undulator experiments electrons x rays 1. 1 An electron bunch is accelerated and compressed 2. 2 The short electron bunch is injected into an undulator 3. 3 The undulator radiation interacts with the electrons: Undulator radiation overtakes electrons by one wavelength per undulator period, leading to the formation of electron bunches ( microbunching ) 4. 4 Microbunched electron beam radiate coherently Huang&Kim, Phys. Rev. ST Accel. Beams 10, (2007) 4/2/08 sphr_slac_ais 5

6 Overview of LCLS electron injector ~ linac -132 undulator optics and diagnostics near experimental hall far experimental hall z (m) electrons x rays The raw LCLS beam contains FEL and a spontaneous halo 3 mj High energy core Eγ > 400 kev 2-3 mj FEL 20 mj Spontaneous 4/2/08 sphr_slac_ais 6

7 Coherence properties of the LCLS beam Temporal (longitudinal) coherence - beam s ability to interfere with a delayed (but not spatially shifted) version of itself LCLS spectral power profile (~10% of the pulse) Each SASE spike is temporally coherent, t c ~ 300as at 8keV Phase relation of SASE spikes is random Huang&Kim, Phys. Rev. ST Accel. Beams 10, (2007) Spatial (transversal) coherence - beam s ability to interfere with a spatially shifted (but not delayed) version of itself LCLS is transversally fully coherent 4/2/08 sphr_slac_ais 7

8 Outline 1. Introduction to XFELs 2. Fundamentals of XFEL x-ray-matter interaction 3. Applications: 1. Optics design and damage 2. Photon diagnostics (e.g. gas detector) 3. Coherent x-ray imaging 4/2/08 sphr_slac_ais 8

9 X-ray interaction with matter 1. Absorption Bound-bound, bound-free, free-free Bound-free (photoionization) tends to dominate for x-rays 2. Scattering Elastic ( coherent, Rayleigh scattering) Inelastic ( incoherent, Bound-electron Compton scattering) 3. Emission Inverse process Fluorescence occurs for hot plasmas on a longer timescale Auger electron emission tends to dominate for low-z atoms 4/2/08 sphr_slac_ais 9

10 Schematic energy diagram of an atom For our applications, chemical bonding is secondary, so that an atomistic description of matter is often sufficient. Hydrogenic energy levels energy continuum shell N M principal quantum number 4 3 max. number electrons L 2 8 K 1 2 4/2/08 sphr_slac_ais 10

11 Total interaction cross sections Nitrogen (Z=7) Tantalum (Z=73) Cross section (barn) K photoabsorption elastic scattering M L K inelastic scattering X-ray energy (ev) Veigele, Atomic Data 5, 51 (1973) Low-Z materials absorb less photons than high-z materials For large x-ray energies, inelastic scattering dominates over elastic scattering 4/2/08 sphr_slac_ais 11

12 Photoionization by atomic shell Subshell photoionization cross sections Nitrogen (Z=7) 10 6 Cross section (barn) p (L) 1s (K) Verner et al., Atomic Data 55, 233 (1993) s (L) X-ray energy (ev) Inner-shell photoionization dominates 4/2/08 sphr_slac_ais 12

13 Emission direction of photoelectrons y x θ r E r B preferred direction of emitted electrons is in the direction of the electric field z XFEL beam θ (degrees) H.K. Tseng et al., Phys. Rev. A 17, 1061 (1978) Polarization-dependent energy deposition r E r E r E d straggle d range max(d xray,d straggle ) max(d xray,d range ) max(d xray,d straggle ) normal incidence grazing incidence 4/2/08 sphr_slac_ais 13

14 Elastic vs. inelastic scattering Photon scattering by a free electron Classical treatment (Thomson formula) Relativistic QM treatment (Klein-Nishima formula) Photon scattering by an atom Elastic scattering (without atomic excitation): Rayleigh ( coherent ) scattering d" elastic d# = d" T homson F2 d# (F=atomic form factor) Inelastic scattering (with atomic excitation): Bound-electron Compton ( incoherent ) scattering d" inelastic d# = S d" Klein$Nishima d# (S= incoherent scattering function) 4/2/08 sphr_slac_ais 14

15 Scattering direction Differential scattering cross sections at 1, 10, and 100 kev (in units of Å 2 /sterad) Nitrogen (Z=7) elastic inelastic dσ elastic dω 10 1 kev dσ inelastic dω kev At larger x-ray energies, elastic scattering occurs primarily in the forward direction Inelastic scattering is more homogeneous Low- and high-z materials behave similarly 4/2/08 sphr_slac_ais 15

16 Effect of subshell ionization on atomic form factor Atomic form factor is the Fourier transform of the electron density Ff 2 1 Example: Carbon 2s neutral C C w/ core hole 1s 2p q (a -1 0 photoionization core relaxation Details of ionization states have strong effect on diffraction pattern Phys. Rev. A 76, (2007) 4/2/08 sphr_slac_ais 16

17 Atomic processes in low-z materials after x-ray absorption photoionization Auger relaxation electron impact ionziation L L L L L L K K K K K K electron-ion coupling three-body recombination electron equilibration L L L L K K K K recombination L K L K Most of these processes take place during the pulse Continuum processes (e.g. melting, spallation, or fracture) take place after the pulse Non-thermal ion motion can take place during and after the pulse 4/2/08 sphr_slac_ais 17

18 Outline 1. Introduction to XFELs 2. Fundamentals of XFEL x-ray-matter interaction 3. Applications: 1. Optics design and damage 2. Photon diagnostics (e.g. gas detector) 3. Coherent x-ray imaging 4/2/08 sphr_slac_ais 18

19 LCLS photon beam diagnostics and offset mirrors in the FEE Overview of LCLS electron injector linac undulator optics and diagnostics near experimental hall far experimental hall (FEE) Slit Solid Attenuators K Spectrometer Soft X-Ray Imager Thermal Sensor Fixed Mask Gas Detector Gas Attenuator Gas Detector Direct Imager (Scintillator) beam direction FEL Offset Mirror Systems 4/2/08 sphr_slac_ais 19

20 Damage modes and material selections Damage = optics degradation or failure Possible damage mechanisms: Melting Phase change High-pressure effects (e.g. spallation) Thermal stress effects and fatigue Photo-chemical processes Both single- and multiple-pulse effects are of concern Low-Z materials with high melting points are expected to exhibit a higher damage resistance since they absorb less light so that the energy density is smaller Since XFEL s are not available yet, we have performed damage experiments on existing light sources 4/2/08 sphr_slac_ais 20

21 Single-shot damage experiments at FLASH We have performed single-shot damage experiments at the FEL FLASH (32-6 nm wavelength, 25 fs pulse length, 20 µm beam diameter) Threshold Fluences in mj/cm 2 calculated melt measured damage Si ± 65 SiC ± 100 B 4 C ± 145 a-c (45nm on Si) 95 ± 50 CVD diamond ± 115 The damage threshold is somewhat higher than the expected melt threshold (except Si) This supports main tenet for designing the x-ray optics Possible error sources: beam diameter, small number of exposures, and pulse energy measurements Appl. Phys. Lett. 90, (2007) 4/2/08 sphr_slac_ais 21

22 Discovery at FLASH: Damage-resistant single-shot optics During 25 fs pulse (10 14 W cm -2 ) 32 nm wavelength After the pulse Si/C multilayer 40µ mm R (%) Reflectivity unchanged After 4 16% pulse Low-fluence % increasing fluence A B A C B C D D E E F F Angle of incidence (degrees) 200nm 50nm Phys. Rev. Lett. 98, (2007) 4/2/08 sphr_slac_ais 22

23 Multiple-shot damage experiments Experimental results will be posted after publication 4/2/08 sphr_slac_ais 23

24 Summary of optics design and damage Low-Z, high-melting-point materials are expected to be most resistant to damage Experiments at FLASH suggest that 1.) The single-pulse damage threshold for bulk materials is comparable to the melting damage threshold. 2.) Thin films have a somewhat lower damage threshold Multiple-pulse experiments using a UV laser to emulate the XFEL-induced temperature profile suggest that multiplepulse damage occurs below the melting threshold 1.) Grazing-incidence optics should be ok (but: compare 10 5 pulses with 10 7 pulses/day on LCLS) 2.) There may be concerns for higher-z normal-incidence optics exposed to the full FEL beam This learning will be directly applicable to optics in the experimental halls 4/2/08 sphr_slac_ais 24

25 Outline 1. Introduction to XFELs 2. Fundamentals of XFEL x-ray-matter interaction 3. Applications: 1. Optics design and damage 2. Photon diagnostics (e.g. gas detector) 3. Coherent x-ray imaging 4/2/08 sphr_slac_ais 25

26 Summary of Pulse-Energy Diagnostics in the FEE Direct (scintillator) imager using a Ce:YAG ~100 nj sensitivity 10 to 25 % absolute calibration destructive Thermal sensor <100 µj pulsed, ~1 µj average sensitivity < 7% absolute calibration destructive Gas Detector ~100 nj sensitivity x 2 absolute calibration non -intrusive 4/2/08 sphr_slac_ais 26

27 Thermal Sensor ( Total Energy Monitor ) Use a thin low-z high-κ substrate for FEL absorption Thermistor deposited on back side to measure temperature rise Temperature rise is proportional to FEL energy Cool down through substrate FEL pulse Cu heat sink 0.5 mm Si substrate Nd 0.67 Sr 0.33 Mn O 3 thermistor Resistance [k!] Nd 0.67 Sr 0.33 MnO 3 sensor on STObufffered Si /R dr/dt [%/K] Temperature [K] -5 Stephan Friedrich et al., LLNL 4/2/08 sphr_slac_ais 27

28 Overview of the LCLS Gas Detector Infer FEL pulse energy from the near-uv fluorescence radiation of a volume where the LCLS beam intersects a N 2 gas The amount of near-uv radiation correlates to the intensity of the LCLS beam PMT vacuum window bandpass filter x rays differential pumping N 2 gas r B differential pumping x rays The gas detectors provide a non-intrusive measure of the FEL pulse energy in real-time, pulse-by-pulse window-less (differentially pumped) 4/2/08 sphr_slac_ais 28

29 Why did we choose this design? Alternative design: Ionization chambers, use electrodes to measure electron and/or ion current At low pressures, ionization ~ pulse energy Successfully used at FLASH Design too large for hard x-rays At high pressures Secondary ionization and space charges Voltage required to quickly remove ions would be large => possible gas breakdown Reasons to use N 2 : Low cost and safe N 2 luminescence is very well understood since it is used in air fluorescence techniques: To determine yield of nuclear explosions through charged particles To detect cosmic ray air showers 4/2/08 sphr_slac_ais 29

30 Overview of the physical processes Auger e- photo e- x-rays r B N 2 gas N 2 molecules absorb a fraction of the x-rays by K-shell photoionization, emitting photoelectrons of energy (E x-ray 0.4 kev) Ionized nitrogen relaxes by Auger decay, emitting Auger electrons of energy ~ 0.4 kev High-energy electrons deposit their energy into the N 2 gas until they are thermalized or reach the detector walls Excited gas relaxes under the emission of near-uv photons The Simple Model 4/2/08 sphr_slac_ais 30

31 Other physical effects (neglected in The Simple Model ) X-rays are scattered into the walls Slow secondary electrons may reach the detector walls Ions may reach the detector walls Space charge effects Spontaneous radiation Long afterglow To test applicability of The Simple Model, we build a prototype and performed experiment at SSRL in quasi-steady-state 4/2/08 sphr_slac_ais 31

32 Gas detector SSRL prototype Port for pumping Photo Multiplier Tube Magnet Coils Be window Gas Feed and Pressure Control Avalanche Photodiode Port for florescence samples Chamber liners: SS, colloidal-graphite, Au 4/2/08 sphr_slac_ais 32

33 Gas Detector Signal (measured at SSRL) 10 8 PMT Signal (# UV photons/ keV photons) Pressure (Torr) B off B=276 Gauss J. Appl. Phys. 103, (2008) 4/2/08 sphr_slac_ais 33

34 Comparison of calculated and measured Gas Detector Signal 10 8 measured calculated PMT Signal (# UV photons/ keV photons) Pressure (Torr) B off B=276 Gauss B= Pressure (Torr) B=0 B=276 Gauss Measured signal (colloidal-graphite coating) agrees with calculations within a factor of < 2!! 4/2/08 sphr_slac_ais 34

35 Gas Detector Signal (measured at SSRL) 10 8 SS colloidal PMT Signal (# UV photons/ keV photons) graphite B off B=276 Gauss Pressure (Torr) Stainless-steel coating results in a 2X larger signal than colloidal-graphite coating 4/2/08 sphr_slac_ais 35

36 Luminescence Signal of Solid Materials 10 8 Al 2 O 3 SS PMT Signal (arb. units) graphite PMMA Al Cu Si Be SiO 2 Au 10 5 Luminescence of SS >> Luminescence of graphite 4/2/08 sphr_slac_ais 36

37 How does LCLS differ from SSRL? Spontaneous radiation Has larger divergence than fundamental and can be shuttered off ~ 360X larger average intensity Pulse energy: SSRL: 8.3 kev LCLS: kev => Low-energy effects, including space charge confinement Pulse length SSRL is quasi-steady-state with ~ x-ray photons/ sec LCLS is pulsed with ~ 3x10 12 xray photons / 100 fs pulse => Measurement of time-dependent signal will provide new insight 4/2/08 sphr_slac_ais 37

38 energy deposition rate (kev/ns) Time dependence of gas detector signal from the 8keV fundamental 6x10 7 4x10 7 2x10 7 energy deposited into N 2 walls end caps time (ns) X rays scattered into walls Photoelectrons hitting walls Photoelectrons hitting end caps UV photons (arb. units) UV photons (arb. units) 6x x x X rays scattered into detector window Secondaries hitting walls and end caps Energy of photoelectrons deposited into N 2 signal from N 2 (!~25ns) walls (!~1ns) end caps (!~1ns) signal to be measured relative amplitude of curves is not known time (ns) UV signal within ~ 1 ns 0 18 ns 0 15 ns ns (?) 0 45 ns 4/2/08 sphr_slac_ais 38?

39 LCLS Gas Detector Summary and Conclusions We have developed a non-intrusive window-less detector to measure the FEL pulse energy in real-time and pulse-by-pulse Calibration will be provided by a calorimeter We have tested the detector in a quasi-steady-state mode of operation at SSRL: Our models capture the relevant physics Most discrepancies can be attributed to different luminescence behaviors of the chamber walls Using the Gas Detector at LCLS is more challenging: Higher intensity Larger wavelength range Shorter pulses Time-dependent measurements hold the promise to provide further insights into the workings of the detector 4/2/08 sphr_slac_ais 39

40 Outline 1. Introduction to XFELs 2. Fundamentals of XFEL x-ray-matter interaction 3. Applications: 1. Optics design and damage 2. Photon diagnostics (e.g. gas detector) 3. Coherent x-ray imaging 4/2/08 sphr_slac_ais 40

41 Diffraction image single biological molecules with ultrashort pulses before the absorbed energy has time to alter the structure Particle injection Neutze et al, Nature 406, 752 (2000) XFEL output: 8 kev, < 70 fs, 2x10 12 photons in 100 nm spot CCD collecting diffraction pattern Our goal is to understand the XFEL pulse and sample requirements. 4/2/08 sphr_slac_ais 41

42 Effect of the limited temporal coherence of the LCLS beam Coherence time ~ maximum time delay of two beams scattered into the highest resolution part of the diffraction pattern Scattering factor for a single SASE spike (Gaussian-shaped): Diffraction pattern is the incoherent sum of multiple SASE spikes. At LCLS at 8 kev, the particles have to be smaller than 500 nm to achieve atomic resolution Optics Express, Vol. 16 Issue 4, pp (2008) 4/2/08 sphr_slac_ais 42

43 Damage dynamics in biological molecules Requires an understanding of the x-ray molecule interactions: Trade-off between the pulse length and pulse intensity versus image resolution Can we, by design, reduce the effect of x-ray damage and thereby enable longer pulses? We have developed several theoretical models to address the x-ray damage question: Hydrodynamics two-fluid model computationally fast: can treat large as well as small molecules Molecular dynamics calculate classical motion of each atom in molecule limited by computer resources to small molecules 4/2/08 sphr_slac_ais 43

44 Charging and trapping of Free Electrons Photo-, Auger-, and secondary electrons are free initially Positive molecule charge increases with time Eventually, free electrons are trapped e- Electron escapes if E E electron electron > > eq r eq 2r (center) (surface) Molecule with positive net charge Q e- E E Electron trapped if electron electron < < eq r eq 2r (center) (surface) 4/2/08 sphr_slac_ais 44

45 One-Dimensional Continuum Model for Radiation Damage Assumptions: Sample is initially a homogeneous continuum Sample has spherical symmetry Treat free electrons and ions as separate fluids that interact by the Coulomb force Rate equations are used to model ionization of each atomic species (H,C,N,O, ) real molecule Continuum model PRE 69, (2004) PRE 77 (2008) 4/2/08 sphr_slac_ais 45

46 Number of Atoms 2x10 4 Damage Dynamics for H 48.6 C 32.9 N 8.9 O 8.9 S 0.7 (2,4) R=50A, τ=40fs, fluence=6x10 12 photons/100nm diameter Carbon 2.0 (K,L) 1.5 1x10 4 (2,3) (2,2) (1,0) R/R0 1.0 (1,2) (2,1) (1,1) (2,0) (0,0) Time (fs) Time (fs) Collisional ionization is dominant initially Heavier atoms get ionized faster Captured photoelectrons accelerate ionization Higher-charged outer layers explode faster than inner layers Inner part of molecule is more strongly ionized 4/2/08 sphr_slac_ais 46

47 Preliminary estimate of pulse parameters The maximum pulse length is determined by a competition between signal and damage using the hydro model Fluence Determined by signal required to classify each image by angular orientation. (*) Pulse length Determined by image degradation due to damage at required fluence. resolution (Å) (*) Ignores effect of damage on classification x x LCLS number of photon/8 kev pulse: (Huldt, Szoke and Hajdu, 2004) Molecule Radius (A) 6fs 9fs 5fs 4fs 3fs Initial LCLS 2fs pulse duration: ~70 fs 1fs molecule radius (Å) Molecule Radius (A) PRE 71, (2005) 4/2/08 sphr_slac_ais 47

48 Effect of a tamper on explosion dynamics Encapsulating the molecule with a sacrificial layer (e.g. H 2 O) reduces damage without tamper charged layer with tamper tamper (sacrificial charged layer) neutralized hot core R (A) R (A) 20 Tamper (H 2 O) molecule time (fs) PRL 98, (2007) time (fs) 4/2/08 sphr_slac_ais 48

49 Repair of diffraction patterns The effect of ionization damage can be significantly reduced if we know the type of atoms in the molecule and (roughly) their ionization physics: 1.) We developed a simplified strategy to repair diffraction pattern for the case of stationary atoms randomly-ionized atoms (on average spatially homogeneous) Given average statistical information about the ionization process, one can show that (i) For the case of mono-atomic ionization-damaged particles, a perfect correction ( repair ) of pulse- and shot-averaged diffraction pattern is possible! (ii) For the case of a more generic particle, partial repair of the diffraction pattern is possible 2.) Apply this limited repair strategy to the actual case in which atoms move and ionization is not spatially homogeneous 4/2/08 sphr_slac_ais 49

50 Effect of tamper on x-ray diffraction imaging Except close to the molecule (ordered solvent), the atoms in the tamper are positioned randomly (unordered solvent) and cannot be seen in the averaged diffraction pattern A tamper reduces the radial atomic motion - H 2 O is a likely choice (natural solvent) - He is less effective since it does not ionize strongly A tamper leads to a radially more homogeneous ionization of the molecule (advantageous for repair of patterns) R factor (%) A tamper does not necessarily reduce the amount of ionization damage - It possibly reduces ionization since it fosters recombination - It possibly accelerates ionization by capturing photoelectrons earlier in the pulse A tamper will make image classification more difficult - As a disordered solvent a tamper will not affect the shot-averaged diffraction pattern number of diffraction patterns 4/2/08 sphr_slac_ais Crystallographic R Factor repair tamper+repair

51 Atomistic Model: Massively-Parallel Molecular Dynamic (in progress) Study damage dynamics of large molecules (>> 10,000 atoms): Check validity of continuum models Check effectiveness of tampers and repair strategies We have developed an MD code for small molecules beyond state-of-the-art: Particle-particle interaction based on Hansen-McDonald potential Relevant atomic physics is included, including three-body recombination 4/2/08 sphr_slac_ais 51

52 We have verified the ionization dynamics in the MD code by comparison to continuum model 500 (K,L) electrons for C in C 540 (2,4) thick = MD thin = continuum model Number of Ions 100 (1,4) (2,2) (1,2) Time (fs) (w/out secondary ionization) C 540 Bucky Ball 4/2/08 sphr_slac_ais 52

53 Movie: Molecular Dynamics Simulations of Lysozyme Irradiated by an LCLS Pulse Pulse energy = 2.3 mj ( kev photons) 1960 atoms, number of particles in the simulation is <N 2 > time = Simulation includes photoionization, Auger decay, and electron impact ionization Light H atoms escape first H C N O S Outer part of molecule expands first due to shielding of center by trapped electrons; this effect will be more pronounced for large molecules Pulse length in simulation is 30 fs; LCLS pulse will be ~70 fs initally, and various schemes to reduce pulse length are planned 4/2/08 sphr_slac_ais 53

54 Results of x-ray imaging of biological molecules Pulse length of fs are necessary to avoid damage Using a molecular tamper helps extending the allowable pulse length Incorporation of an ionization repair technique in the data analysis also helps 4/2/08 sphr_slac_ais 54

55 Conclusions XFEL s will open an exciting new field of ultra-short high-intensity x-ray material interaction A large range of energy densities and physical phenomena can be accessed X-ray optics must be designed to withstand damage Low-Z materials Grazing incidence Placement at a large distance from the undulator Novel photon diagnostics are being developed to characterize the beam properties Studies of x-ray interaction with biomolecules have help defining the necessary pulse requirements for future imaging experiments 4/2/08 sphr_slac_ais 55