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Alternative Coherent X-ray SourcesJohn W.G. Tisch
Imperial College London
STI Round-Table Meeting DESY, Hamburg 22-24 June 2004
Outline:• Wavelength ranges• Table-top high intensity lasers•
Strong-field laser-matter interactions• X-rays from solid-target
plasmas• Solid-target High Harmonic Generation• X-ray Lasers•
Relativistic Thomson Scattering• Gas-target High Harmonic
Generation• Brightness Comparison
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Wavelength range under consideration
100nm 10nm 1nm 0.1nm=1Å
Wavelength
Photon Energy10eV 100eV 1keV 10keV
VUV
XUV
Soft X-rays
Hard X-rays
VUV = vacuum ultraviolet
XUV = extreme ultraviolet
HHG
Window cutoff (LiF 104nm)
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Table-top High Intensity lasers have driven x-ray source
development in last 15 years
The CPA principle Oscillator
amplified stretched
pulse
Stretcher
Amplifiers
Compressor
low energy short pulse low energy
stretched pulse
amplified compressed
pulse
versus
Table-top TW CPA laser system Beam Line on NOVA laser, LLNL
TW levels available from kHz table-top
systems focusable to intensities >1018 Wcm-2
(Future: OPCPA?)
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Electron wavepacket
Atomicpotential~1/r
Field-Free Atomic Potential
Laser fieldpotential ~x
Atomic Potential Subject to an Intense Laser Field
Wave packet can tunnel through barrier
Ionisation occurs rapidly by tunnelling
cIE 02 ε=Laser electric field
At I = 3x1016 Wcm-2, E = atomic field
Perturbation theory inadequate for I > ~1013 Wcm-2
short pulse laser
matter
( )K+++= 3)3(1)2()1(0 EEEP χχχε + …)
High-intensity laser-matter interactions
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plasma ion
Inverse Bremsstrahlung (a mechanism for hot plasma
production)
22
22
4λ
ωI
mEe
Ue
p ∝=
Wiggle energy converted to thermal velocity.
The electron wiggle energy in a strong field can be
sizeable.
Up = 10 eV at 1014 Wcm-2
= 1 kev at 1016 Wcm-2Cycle averaged wiggle energy:
Coulomb scattering
But this energy cannot be absorbed by a free electron.
High Harmonic Generation
electron
parent ion
soft X-ray photon
max(hν) = I.P. + 3Up
The wiggle energy of an electron in a strong field can be
absorbed in the presence of an ion
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Possible Targets for high intensity laser-matter
interactions
Solids Microstructures Microdroplets Clusters Molecules
Atoms
10 µm 1 µm 1 - 0.1 µm 2 – 10 nm 5 Å 1 Å
Collisionally dominated
Hot plasmas (kTe > 1 keV)
Copious X-rays
Collisionally dominated
Hot plasmas (kTe > 1 keV)
Copious X-rays
Tunnel Ionisation
ATI
Eion ≈ 0 Eelec
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Short-pulse, high intensity laser-solid interaction
Solid target
B-field
laser
high energyprotons
B-field
B-
field
abso
rption
ablation
energytransport
ionization
fast particlegeneration
& trajectories
Slide courtesy of Karl Krushelnick, Imperial College Plasma
Group
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Line and continuum radiation from hot dense laser-plasma
near thermal continuum
L-shell lines
K-shell lines
Photon Energy
Inte
nsi
ty
hot electrons
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Laser-Plasma X-ray Sources
• Iλ2≤ 1016 Wcm-2µm-2 Thermal + Minority Hot Electrons– Drive
Lasers = table-top ps, fs, kHz rep-rates– Continuum + Line-emission
(thermal and hot electrons) into 2π– ~5% energy conversion into
~1keV (big lasers access ~10keV)– ps pulse durations set by finite
electron transit times
• 1016 Wcm-2µm-2 ≤ Iλ2 ≤ 1018 Wcm-2µm-2 Kα emission– Drive
Lasers = table top fs, >10Hz (kHz becoming feasible)– Kα
emission into 2π– ~10-4 – 10-5 energy conversion into 5-10keV– ~100
fs pulse durations
• Iλ2 ≥ 1018 Wcm-2µm-2 Relativistic electrons– Drive lasers =
facility scale, but table-top feasible (OPCPA)– Relativistic
electron velocities– Deep target penetration– Broadband MeV
emission due to multiple Coulomb Collisions– Partial beam
collimation due to electron self-focusing
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The Kα ultrafast x-ray sourceFully divergent
Monochromatic1 - 8 keV
Duration 100 fs
Flux: 109 ph/shot/str
ENSTA
State of the art in the laser field (for keV x-rays)
A. Rousse et al, Phys. Rev. E 50 (3) 2200 (1994)S. Bastiani et
al, Phys. Rev. E (1996)
Many applications already done (see next slides)
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ENSTA
A. Rousse et al, Nature 2001C. Siders et al Science 2000
Solid-liquid phase transition (0.1 ps)
Rose-Petruck et al, Nature’1999
Strain (10 ps)
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X-dur (γ)XXUVUV-VUV
1 – 100 keV1 – 0.01 nm
> 100 keV< 0.01 nm
25 - 250 eV50 nm – 5 nm
< 10 eV> 100 nm
?Harmonics, HHG, XUV-laser, …
Limitation de la source X Kα
Divergence: L
How to produce aBEAM of x-rays ?
Main limitations of the Kα x-ray source
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X-ray CCD Image~50mrad divergence,
~108 photons/shot
+ + + + + + + + + + + + + ++ + + + + + + + + + + + + +
+ + + + + + + + + + + + + +
Lase
r
Accelerated electron beam
Background electrons of the laser-producedplasma
ENSTA
Betatron source: synchrotron-like x-ray BEAMfrom a laser-gas
target interaction
Wiggling of the electron beam in a ion channel (undulator)
Gas-Jet
PlasmaLaser
Gas jet
X-ray beam
I ~1018Wcm-2
L~1cm
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• High intensity laser is focused on a solid target (intensity
~1020Wcm-2)
• Surface oscillates at vosc~c
• Reflected waveform is modified from sine to ~ sawtooth.
• Reflected spectrum contains veryhigh order harmonics (odd and
even)
• No known mechanism for cut-off(highest harmonics observed are
spectrometer limited)
Incident Pulse
Reflected Pulse (Harmonics)
Oscillating Plasma/Vacuum
interface at vosc~c
HHG from solid targets
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Solid Target HHG results
10-6
10-5
10-4
10-3
5 10 15
Con
vers
ion
effi
cien
cy
Wavelength (nm)
Photons/pulse ~1013 @3.6nmPulse Duration 2π)⇒ brightness
expected to increase to~1026 using shorter fs pulses
(specularemission)
Drive Laser ParametersEnergy: 70 J (on target)Pulse duration:
~700 fsPeak intensity: ~1020 Wcm-2
Pulse contrast: >1011
Data courtesy of Matt Zepf, Queens University Belfast
Red points and curve = dataBlack curve = fit ~n-2.04
Al filter transmission notch
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X-ray Lasers (XRLs)• ASE in extended plasma columns ( λ ~
50-3.56nm ), laser or electrical
discharge pumping• Lasing action between excited states of
highly charged ions
(e.g. Se24+ ~20nm, Ta45+ 4.5 nm)• No cavity, usually single pass
gain (~ 10 cm-1)• Divergence dictated by d/L ratio (typically few
mrad)• High energy (up to mJ), ps pulses in collisional excitation
schemes• Narrow bandwidths → high temporal coherence (λ/∆λ>104
over 4.5-20nm →
Lc>45-200µm)• Transverse coherence fraction ~10-4 (few µm
extrapolated to o/p)• Capillary discharges paving way to high-rep
rate, table-top sources
Slab target 2-20 mm
Driving Laser Line Focus
Plasma column
X-raysX-rays
~few 100µmdiameter
See R.London Phys. Fluids B 5 2707 (1993),
J.Rocca Rev. Sci. Inst 703799 (1999)
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XRL Population Inversion Mechanisms• Collisional Excitation
(elec-ions collisions create population inversion)
– Quasi Steady State population inversion• Facility
scale,~100J/100ps drive laser • Saturated gain down to 5.8nm
(Ni-like Dy)• ~50ps pulse duration, mJ output energies
– Transient Collisional Excitation• 2 drive pulses to achieve
optimum lasing conditions (more efficient)• ~5J drive laser → table
top laser systems• Saturated gain down to 7.3nm (Ni-like Sm)• Few
ps pulses, 0.1mJ output energies
• Recombination Pumping– Upper level populated by 3-body
combination– Demonstrated in H-like, Li-like ions– Better short
wavelength scaling, but lower energies that CE
• Optical Field Ionisation Lasers– fs pulse rapidly ionises
atoms– Pumping into upper level via CE or Recombination– Compact:
table-top, multi-Hz rep-rates
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Relativistic Thomson Scattering• Thomson scattering between TW
laser and MeV electron beam from
accelerator• Scattered laser photons are relativistically
up-shifted to hard x-ray range and
emitted in narrow cone around electron-beam direction• Pulse
duration set by laser transit time through electron bunch (fs-ps)•
5x104 photons in ~300 fs pulse at 30keV (15% b/w) demonstrated
using 90°
Thomson Scattering (ALS) – Schloelein et al. Science 276 236
(1996)• 107-108 photons in 100fs-5 ps pulses at 20-200keV expected
from LLNL
PLEIADES source
MeV electron beam
Focused fs, TW IR laser
Hard x-rays
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HHG in Gas Targets• HHG is the production of high-order
harmonics of the laser frequency
from the strong-field interaction of intense laser pulses with a
gas target.• HHG is a coherent, parametric frequency up-conversion
process
odd harmonics qω1q=3,5,7,..,299+
Gas target (nonlinear medium)atoms, molecules, clusters
1017-1019cm-3
Laser ω1
Focused laser intensity 1013-1016 Wcm-2
Pulses 100s ps to few fs duration
Iq ∝ Ngas2 ⋅ atomic response( ) ⋅ phasematching factor( )
Iq ∝ Ngas2 d qω1( )
2Fq
2
Harmonic signal is due to coherent addition of many atomic
emitters
Cutoff
harmonic order q
Plateau
log
inte
nsity
200nm 2.7nm
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HHG provides an intrinsically coherent, compact soft x-ray
source of unrivalled short pulse duration.
High Order Harmonics Spectrum
Properties of High Harmonic Radiation
• high spatial coherence
• highly directional
• short wavelength (into 2-4nm “water window”)
• ultrafast (shorter than laser pulse –attosecond with few-cycle
laser pulses )
Capillary set-up at JILAHHG set-up at Imperial College
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Simple-man’s model of HHG
laser phase = 0 phase ~ π/2 phase ~ 3π/2 phase ~ 2π
Tunnel ionisation
Acceleration in the laser field
Recombination to ground state
• Valid in the strong-field, low-frequency (IR and near IR
lasers) regime.
• Combines tunnel ionisation with classical motion of electron
in laser field.
3 steps: 1 2 3
picks up k.e. Ea
hνXUV
Harmonic photon energy: hνXUV = Ip + EaMaximum k.e. that can be
gained by electron is Ea = 3.2Up
harmonic photon cutoff energy ~ Ip + 3.2Up(I = Isat)Isat = laser
intensity at which ionisation saturates and HHG is terminated
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High harmonic radiation exhibits high spatial coherence
focusing lens
pulsedgas jet(50 barbackingpressure) spectral
information
laser inλ = 527 nm∆τ = 2 ps
XUV Spectrometer
Soft X-Ray Detector ->Micro-channel plate(CsI coated)
Interferencefringes
Slit pair(27 to 100 µmseparation)
5 cm 180 cm
Experimental Set-up for Young’s Slit Measurement
PRL 77 4756 (1996)
Appl. Phys. 65 313 (1997)
60
40
20
0
position (arb. units)
I = 8 x 10 14 W/cm 2
Fringes for Q15 (35 nm)
Measurements show intrinsicspatial coherence of source, i.e.
actual source size ~ 4x effective incoherent source size.
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With HHG its easy to make mutually coherent soft x-ray
sources
Laser pulse in
τ
τ = 0 fs τ = 25 fs
Lynga et al. PRA 60 4823 (1999)
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Coherent Imaging demonstrated at 30 eV
JILA results: Bartels et al. Science 297 376 (2002)
Footprint of entire set-up incl. laser = 1m x 3.5m
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Applications of gas harmonics• Seeding XRL (Ga XXII)• Plasma
probing
– ne measurements, Theobald et al. PRL 77 298 (1996), PRE 59
3544– Time resolved ne measurements (200 fs res), Salieres et al
PRL 83 5483
(1999)– 2D interferometric probing Descamps et al Opt. Lett. 25
135 (2000)
• Photoionisation spectroscopy– Rare gases Balcou et al. Z.Phys.
D 34 107 (1995)
• Life-time measurements of excited states– He states Larsson et
al. J.Phys.B 28 L53 (1995)
• Ultrafast Chemical Dynamics– Nugent-Glandorf et al.
Rev.Sci.Inst. 73 1875 (2002)
• Surface science– Pump probe photoelectron spectroscopy of
GaAs, Haight and Peale PRL
70 3979 (1993), Rev.Sci.Inst. 65 1853 (1994)• Probe of molecular
alignment
– Lein et al. PRL 88 183904 (2002), PRA 66 R051404 (2002)• + we
have already heard about the remarkable applications in
attosecond
physics… (M. Drescher Tuesday)
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Prospects: more HHG photonsHigher Yields + More average power•
HHG in gas-filled capillary waveguides, Durfee et al. PRL 83
2187 (1999)
– Extended interaction lengths (cms) + improved phase-matching
owing to waveguide dispersion
• Quasi Phase Matching– eg modulated capillaries, Paul et al.
Nature 421 51 (2003)
• High Laser Power + Very Loose Focusing (Takahashi and
co-workers at Riken)
– 20mJ/35 fs 10Hz Ti:S CPA laser– f = 5m lens (b ~ 30cm)–
Residual ∆k from focusing offset again neutral gas
dispersion to achieve phasematching– 4.7µJ/pulse at 62.3nm (Q13
in 0.6 Torr Xe cell, L
~15cm) → 3x1028 Photons s-1 mm-2 mrad-2
• Higher average powers– 100kHz already demonstrated, Lindner et
al. PRA 68
013814 (2003)– Tens of MHz (thin-disc laser) in development
Intensity
z
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Prospects: shorter wavelengths from HHG
Shorter wavelengths (from ions)• Transient phase-matching in
ions from cluster nanoplasmas Tajima et al.
Phys. of Plas. 6 3759 (1999), Tisch PRA 62 041802(R) (2000) +
results from Milchberg group
– Use nanoplasma unusual refractive index properties to overcome
strong plasma dispersion that limits HHG in strongly-ionised
regime
harmonic
Nanoplasmas with plasma background
laser
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Peak Brightness Comparison
100 101 102 103 104 105 106 107 1081E12
1E14
1E16
1E18
1E20
1E22
1E24
1E26
1E28
1E30
1E32
1E34
1E36
Photon Energy (eV)
XFEL
XRL ps,Hz
Solid HHG 500fs,
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Conclusion
• Laser-based x-ray sources will continue to coexist with
accelerator-based sources (cf. the co-existence of table-top and
facility-scale high power lasers).
• XFEL predicted brightness at 1 Angstrom unlikely to be reached
by any other source in foreseeable future…
• But very rapid progress is expected in table-top x-ray sources
over next 5 years, driven by new laser & technological
developments (e.g. Optical Parametric Chirped Pulse Amplification
and gas-filled fibre techniques
• Clear opportunities exisit for scientific and technological
cross-over between XFEL and future laser-based source development,
e.g. seeding with high harmonics, attosecond XFEL pulses using
carrier-envelope stabilised few-cycle laser pulses, etc.