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

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

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

Solid-liquid phase transition (0.1 ps)

Rose-Petruck et al, Nature’1999

Strain (10 ps)

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

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

• 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 @3.6nmPulse 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

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

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

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

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

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.

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)

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

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)

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

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,

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.