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Advanced Instrumentation Seminar (AIS)Stanford Linear Accelerator (SLAC)
Stefan P. Hau-RiegeLawrence Livermore National Laboratory
This work was performed under the auspices of the U.S. Department of Energyby Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344.
Ultrafast x-ray-matter interaction at LCLSOptics design, photon diagnostics, and imaging
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Thanks to all collaborators:
LLNL: 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
LBNL: S. Marchesini
DESY/FLASH: H. Chapman, S. Bajt, and K. Tiedtke
SLAC: S. Boutet and J. Krzywinski
Uppsala U.: J. Hajdu
CAS Prague: L. Juha and J. Chalupsky
PAS Warsaw: R. Sobieraski
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Outline
1. Introduction to XFELs
2. Fundamentals of XFEL x-ray-matter interaction
3. Applications:1. Optics design and damage2. Photon diagnostics (e.g. gas detector)3. Coherent x-ray imaging
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10-2 100 102
1018
1020
1022
1024
1026
1028
1030
1032
1034
Photon Energy (keV)
ALS undulator
APS undulator
FLASH
Euro XFEL
LCLS
Peak Brightness(photons/s/mm2/mrad2/0.1% bandwidth)
X-ray free electron lasers will produce extremely bright,ultrashort, coherent x-ray pulses
FLASH operational now48 - 6 nm,
< 25 fs,> 1012 photons
DESY, Hamburg
operational 20091.5 - 0.15 nm,
< 100 fs,>1012 photons
Linac Coherent Light Source(LCLS), SLAC, Stanford
LCLS
operational 20126 - 0.1 nm,< 100 fs,
>1012 photons
DESY, HamburgEuro XFEL
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The principle of SASE x-ray free electron lasers
1. An electron bunch is acceleratedand compressed
2. The short electron bunch is injectedinto an undulator
3. The undulator radiation interactswith the electrons:Undulator radiation overtakes electronsby one wavelength per undulator period,leading to the formation of electronbunches (“microbunching”)
4. Microbunched electron beam radiatecoherently
electroninjector linac undulator experiments
x rayselectrons
1 2 3,4
1
2
3
4
Huang&Kim, Phys. Rev. ST Accel. Beams 10, 034801 (2007)
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Overview of LCLS
x rayselectrons
electroninjector linac undulator
optics anddiagnostics
nearexperimental
hall
farexperimental
hall
0 111 356 426-132~ -1300z
(m)
2-3 mJFEL
20 mJSpontaneous
3 mJ Highenergy coreEγ > 400 keV
The raw LCLS beam contains FEL and a spontaneous halo
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Coherence properties of the LCLS beam• Temporal (longitudinal) coherence - beam’s ability to
interfere with a delayed (but not spatially shifted) version ofitself
LCLS spectral power profile(~10% of the pulse)
• Each SASE spike is temporally coherent, tc ~ 300as at 8keV• Phase relation of SASE spikes is random
Huang&Kim, Phys. Rev. ST Accel. Beams 10, 034801 (2007)
• Spatial (transversal) coherence - beam’s ability to interferewith a spatially shifted (but not delayed) version of itself• LCLS is transversally fully coherent
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Outline
1. Introduction to XFELs
2. Fundamentals of XFEL x-ray-matter interaction
3. Applications:1. Optics design and damage2. Photon diagnostics (e.g. gas detector)3. Coherent x-ray imaging
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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
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Schematic energy diagram of an atom
For our applications, chemical bonding is secondary, so that anatomistic description of matter is often sufficient.
……
shell…K
L
MN
principalquantumnumber
1
2
34
max.number
electrons
2
8
1832
continuum
energy
Hydrogenic energy levels
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Total interaction cross sections
102 103 104 105 106 102 103 104 105 106
X-ray energy (eV)
106
104
102
1
10-2
10-4
Crosssection(barn)
Nitrogen (Z=7) Tantalum (Z=73)
photoabsorption
elastic scattering
inelastic scattering
• Low-Z materials absorb less photons than high-Z materials• For large x-ray energies, inelastic scattering dominates over elastic scattering
K KLM
Veigele, Atomic Data 5, 51 (1973)
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Photoionization by atomic shellSubshell photoionization cross sections
Nitrogen (Z=7)106
104
102
1
10-2
10-4
Crosssection(barn)
X-ray energy (eV)102 103 104 105 106
1s (K)
2s (L)
2p (L)
Inner-shell photoionization dominates
Verner et al., Atomic Data 55, 233 (1993)
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Emission direction of photoelectrons
XFEL beam
!
r B
!
r E
preferred direction of emitted electronsis in the direction of the electric field θ (degrees)
H.K. Tseng et al.,Phys. Rev. A 17,1061 (1978)
θ
Polarization-dependent energy deposition
!
r E
max(dxray,dstraggle)
drange
dstraggle
!
r E
max(dxray,drange)
!
r E
max(dxray,dstraggle)
normal incidence grazing incidence
x
yz
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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
• Inelastic scattering (with atomic excitation): Bound-electron Compton (“incoherent”) scattering
!
d"elastic
d#= F
2 d"T homson
d#
!
d"inelastic
d#= S
d"Klein$Nishima
d#
(F=atomic form factor)
(S=“incoherent” scattering function)
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Scattering directionDifferential scattering cross sections at 1, 10, and 100 keV
(in units of Å2/sterad)
Nitrogen (Z=7)
• 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
dσelasticdΩ
dσinelasticdΩ
1 keV
10
100 1 keV
10
100
elastic inelastic
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Effect of subshell ionization on atomic form factor
photoionization core relaxation
Atomic form factor is the Fourier transform of the electron density
Details of ionization states have strong effect on diffraction pattern
Phys. Rev. A 76, 042511 (2007)
0 1 20
1
2 neutral C C w/ core hole
2s
2p
1s
f
q (a-1
0)
Example: Carbon
F
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Atomic processes in low-Z materials after x-ray absorption
K
L
photoionization
K
L
K
L
Auger relaxation
K
L
K
L
electron impact ionziation
K
L
electron equilibration
K
L
three-body recombination
K
L
K
L
recombination
K
L
K
L
electron-ion coupling
K
L
• 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
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Outline
1. Introduction to XFELs
2. Fundamentals of XFEL x-ray-matter interaction
3. Applications:1. Optics design and damage2. Photon diagnostics (e.g. gas detector)3. Coherent x-ray imaging
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LCLS photon beam diagnostics and offset mirrors in the FEE
GasDetector
Gas Attenuator
FixedMask
beam direction
SolidAttenuators
K SpectrometerSoft X-Ray Imager
Thermal Sensor
Slit
electroninjector
GasDetector Direct Imager
(Scintillator) FEL Offset Mirror Systems
linac undulator optics anddiagnostics
nearexperimental
hall
farexperimental
hall
Overview of LCLS
(FEE)
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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 toexhibit a higher damage resistance since they absorb lesslight so that the energy density is smaller
• Since XFEL’s are not available yet, we have performeddamage experiments on existing light sources…
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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)
calculated
melt
measured
damage
Si 181 129 ± 65
SiC 82 207 ± 100
B4C 65 289 ± 145
a-C (45nm on Si) 95 ± 50
CVD diamond 107 230 ± 115
Threshold Fluences in mJ/cm2
• 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, 173128 (2007)
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Discovery at FLASH: Damage-resistant single-shot optics
FEDCBA
40µmm
50nm200nm
A FEDCB
During 25 fs pulse (1014 W cm-2)32 nm wavelength
Reflectivity unchanged
Si/C multilayer
Phys. Rev. Lett. 98, 145502 (2007)
After the pulse
After pulse4
50Angle of incidence (degrees)
increasingfluence3
2R
(%)
1
035 40 45
100%
16% Low-fluence
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Multiple-shot damage experiments
Experimental results will be postedafter publication
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Summary of optics design and damage
• Low-Z, high-melting-point materials are expected to be most resistant to damage
• Experiments at FLASH suggest that1.) 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 multiple- pulse damage occurs below the melting threshold
1.) Grazing-incidence optics should be ok (but: compare 105 pulses with 107 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 theexperimental halls
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Outline
1. Introduction to XFELs
2. Fundamentals of XFEL x-ray-matter interaction
3. Applications:1. Optics design and damage2. Photon diagnostics (e.g. gas detector)3. Coherent x-ray imaging
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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
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Thermal Sensor (“Total Energy Monitor”)
Stephan Friedrich et al., LLNL
• 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
Nd0.67Sr0.33MnO3 thermistor
FELpulse
Cu heatsink
0.5 mm Sisubstrate
0
10
20
30
40
50
60
-5
0
5
10
15
90 120 150 180 210 240
Nd0.67
Sr0.33
MnO3
sensor on STO-
bufffered Si
Res
ista
nce
[k!
]
1/R
dR
/dT
[%/K
]
Temperature [K]
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Overview of the LCLS Gas Detector
x rays x rays
PMT
bandpass filtervacuum window
differentialpumping
differentialpumping
N2 gas
• The gas detectors provide a non-intrusive measure of the FEL pulse energy• in real-time,• pulse-by-pulse• window-less (differentially pumped)
• Infer FEL pulse energy from the near-UV fluorescence radiation of a volume where the LCLS beam intersects a N2 gas• The amount of near-UV radiation correlates to the intensity of the LCLS beam
Br
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• 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 N2:• Low cost and safe• N2 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
Why did we choose this design?
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• N2 molecules absorb a fraction of the x-rays by K-shell photoionization, emitting photoelectrons of energy (Ex-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 N2 gas until they are thermalized or reach the detector walls• Excited gas relaxes under the emission of near-UV photons
x-rays
photo e-Auger e-
N2 gas
Br
Overview of the physical processes
“TheSimpleModel”
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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 performedexperiment at SSRL in “quasi-steady-state”
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Gas detector SSRL prototype
Photo Multiplier Tube
Magnet Coils
Gas Feedand
Pressure Control AvalanchePhotodiode
Port for pumping
Be window
Port for florescence samples
Chamber liners: SS, colloidal-graphite, Au
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10 100105
106
107
108
B off B=276 Gauss
Pressure (Torr)
Gas Detector Signal (measured at SSRL)
J. Appl. Phys. 103, 053306 (2008)
PMT Signal(# UV photons/
1012 8keV photons)
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10 100
B=0
B=0 B=276 Gauss
Pressure (Torr)
measured calculated
Measured signal (colloidal-graphite coating) agrees with calculationswithin a factor of < 2 !!
Comparison of calculated and measured Gas Detector Signal
10 100105
106
107
108
B off B=276 Gauss
Pressure (Torr)
PMT Signal(# UV photons/
1012 8keV photons)
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10 100105
106
107
108
colloidalgraphite
SS
B off B=276 Gauss
Pressure (Torr)
Gas Detector Signal (measured at SSRL)
Stainless-steel coating results in a 2X larger signal than colloidal-graphite coating
PMT Signal(# UV photons/
1012 8keV photons)
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Luminescence Signal of Solid Materials
105
106
107
108Al2O3
SSAu
BeSi
O2
Cu
SiAl
grap
hite
PMT
Sign
al(a
rb. u
nits
)
PMM
A
Luminescence of SS >> Luminescence of graphite
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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: 0.83 – 8.3 keV=> Low-energy effects, including space charge confinement
• Pulse length• SSRL is “quasi-steady-state” with ~ 1012 x-ray photons/ sec• LCLS is pulsed with ~ 3x1012 xray photons / 100 fs pulse=> Measurement of time-dependent signal will provide new insight
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0 5 10 15 200
2x107
4x107
6x107
e
nerg
y de
posi
tion
rate
(keV
/ns)
time (ns)
energy deposited into N2 walls end caps
0 – 15 nsPhotoelectrons hitting end caps
~ 1 nsX rays scattered into walls
0 – 200 ns (?)Secondaries hitting walls and end caps0 – 45 nsEnergy of photoelectrons deposited into N2
0 – 18 nsPhotoelectrons hitting walls?X rays scattered into detector window
UV signal within
Time dependence of gas detector signal from the 8keV fundamental
0 20 40 60 80 1000
2x107
4x107
6x107
UV
photo
ns (
arb
. units)
time (ns)
signal from N
2 (!~25ns)
walls (!~1ns) end caps (!~1ns)
relative amplitude of curvesis not known
signalto bemeasured
UV
pho
tons
(arb
. uni
ts)
2
4
6
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LCLS Gas DetectorSummary 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
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Outline
1. Introduction to XFELs
2. Fundamentals of XFEL x-ray-matter interaction
3. Applications:1. Optics design and damage2. Photon diagnostics (e.g. gas detector)3. Coherent x-ray imaging
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Diffraction image single biological molecules with ultrashortpulses before the absorbed energy has time to alter the structure
CCD collectingdiffraction pattern
Particle injection
XFEL output:8 keV,< 70 fs,
2x1012 photonsin 100 nm spot
Our goal is to understand the XFEL pulse and sample requirements.
Neutze et al,Nature 406, 752 (2000)
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Effect of the limited temporal coherence of the LCLS beam
Scattering factor for a single SASE spike(Gaussian-shaped):
Diffraction pattern is the incoherent sumof multiple SASE spikes.
At LCLS at 8 keV, the particles have to be smaller than 500 nmto achieve atomic resolution
Coherence time ~ maximum time delay of two beams scatteredinto the highest resolution part of the diffraction pattern
Optics Express, Vol. 16 Issue 4, pp.2840-2844 (2008)
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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
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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-
e-
Electron escapes if
)surface(r2
eQE
)center(r
eQE
electron
electron
>
>
Electron trapped if
)surface(r2
eQE
)center(r
eQE
electron
electron
<
<
Molecule with positive net charge Q
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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, 051906 (2004)PRE 77 (2008)
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Damage Dynamics for H48.6C32.9N8.9O8.9S0.7
• Collisional ionization is dominant initially• Heavier atoms get ionized faster• Captured photoelectrons accelerate ionization
0
1x104
2x104
(1,1)(2,0)
(0,0)
(1,0)
(1,2)
(2,1)
(2,2)
(2,3)
(2,4)
(K,L)Carbon
Numberof Atoms
0 40Time (fs) 0 10 20 30 400.0
0.5
1.0
1.5
2.0
R/R0
• Higher-charged outer layers explode faster than inner layers• Inner part of molecule is more strongly ionized
R=50A, τ=40fs, fluence=6x1012 photons/100nm diameter
Time (fs)
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Preliminary estimate of pulse parameters
0 100 200
2
4
6
8
10
1013
1012
3x1012
1011
3x1011
Molecule Radius (A)
0 100 200
9fs6fs5fs
4fs
3fs
2fs
Molecule Radius (A)
1fs
FluenceDetermined by signal required to classifyeach image by angular orientation. (*)
Pulse lengthDetermined by image degradationdue to damage at required fluence.
resolution(Å)
molecule radius (Å)
The maximum pulse length is determined by a competition between signal and damage using the hydro model
(*) Ignores effect of damage on classification.
(Huldt, Szoke and Hajdu, 2004).
Initial LCLS pulse duration: ~70 fs
LCLS number of photon/8 keV pulse: 1012
PRE 71, 061919 (2005)
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Effect of a tamper on explosion dynamics
without tamper with tamper
neutralizedhot core
chargedlayer
tamper (sacrificialcharged layer)
0 5 10 15 200
20
40
60
R (A)
time (fs)0 5 10 15 20
0
20
40
60
R (A)
time (fs)
molecule
Tamper (H2O)
Encapsulating the molecule with a sacrificial layer (e.g. H2O) reduces damage
PRL 98, 198302 (2007)
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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 ofstationary atoms randomly-ionized atoms (on average spatially homogeneous)
• Given average statistical information about the ionization process, one canshow 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
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1 100
5
10
15
20
25
R factor
(%)
number of diffraction patterns
Crystallographic R Factor• 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 - H2O 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)
• 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 photo- electrons 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
Effect of tamper on x-ray diffraction imaging
repair
tamper+repair
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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
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0 5 10 15 20 25 3010
(1,2)
(1,4)
(2,2)
Time (fs)
(2,4)
We have verified the ionization dynamicsin the MD code by comparison to continuum model
(w/out secondary ionization)
C540 Bucky Ball
(K,L) electrons for C in C540
100
500
Numberof
Ions
thick = MDthin = continuum model
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Movie:Molecular Dynamics Simulations of Lysozyme
Irradiated by an LCLS PulsePulse energy = 2.3 mJ (1012 8 keV photons)
● H ● C ● N ● O● S
• 1960 atoms, number of particles in the simulation is <N2>time= 7243.6
• Simulation includes photoionization, Auger decay, and electron impact ionization
• Light H atoms escape first
• 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
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Results of x-ray imaging of biological molecules
• Pulse length of 10 - 50 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
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Conclusions• XFEL’s will open an exciting new field of ultra-short high-intensity x-raymaterial interaction
• A large range of energy densities and physical phenomena can beaccessed
• 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 beamproperties
• Studies of x-ray interaction with biomolecules have help defining thenecessary pulse requirements for future imaging experiments