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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
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)
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?)
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
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
Possible Targets for high intensity laser-matter interactions
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
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
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
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)
ENSTA
A. Rousse et al, Nature 2001C. Siders et al Science 2000
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
• 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
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 @3.6nm
Pulse Duration <500 fsBrightness ~1023
Ph/(mm2 mrad2
sec 0.1%BW)
Current performance
⇒ comparable single pulse photon numbers to XFEL⇒ much lower brightness (due to large divergence -> 2π)⇒ brightness expected to increase to~1026 using shorter fs pulses (specularemission)
Data courtesy of Matt Zepf, Queens University Belfast
Red points and curve = dataBlack curve = fit ~n-2.04
Al filter transmission notch
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)
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
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
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
• 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
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
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, <<1Hz
Gas HHG 0.1fs,100kHz
Kα100fs,kHz
Bremsstrahlung (hot elecs) ps,Hz
Thomson 100fs, Hz
Thermal ps,kHz
Betatron(?)
Phase-Matched Gas HHG 35fs,10Hz
Pea
k B
rig
htn
ess
Ph
oto
ns
s-1m
m-2
mra
d-2
in 0
.1%
BW
See also Smith and Key, J.Phys.IV France 11 Pr2-383 (2001)
Te=0.15keV
Te=1.5keV
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.