Ultrafast X-ray and Ion

sources

from multi-PW lasers

Matt Zepf

Queen’s University Belfast

Outline

• Ion Acceleration via Radiation Pressure Acceleration

– GeV/u

– Narrow energy distributions

– Attosecond ion bunches

– First demonstration of ‘Light-sail’ regime

• Relativistically Oscillating Mirrors (HHG from Solid Targets)

– Relativistically oscillating mirrors

– Attosecond and zeptosecond pulses

– Extreme brightness with ELI conditions

• Relativistic Mirrors

– Light Sail Regime (thin target)

– Holeboring regime (Semi-infinite target)

I>1019Wcm2

a0>3

Relatvistic electron production

Hot electron propagation

MeV energy, µC charge

Space charge field:

E~Thot/ Debye~MeV/µm=1012V/m

MeV/u protons and ions

Typical targets: Metallic foils

Proton Source: CH Contamination on foil surfaces (typically ~50Å)

THE TNSA Mechanism

(Target Normal Sheath Accleleration))

Target

Typical results

106

107

108

109

1010

1011

Pro

tons/

MeV

40353025201510

Proton Kinetic Energy (MeV)

15 J 42 J 110 J

• Target: 10µm Al

• Temperature

~ 1.8 MeV for 12 J

~ 5 MeV for 85J

• Energy conversion

~2 10-3 for 12 J

~5 10-2 for 85 J

~1 10-1 for 400 J

• Efficiency at 30-35 MeV

hot~10-5-10-4

10 MeV 22 MeV17 MeV

Typical divergence:

30-60°

Beam quality

nm scale surface perturbations are still visible

- irreducible emittance of <0.004 mm mrad

From Cowan et al, PRL 2004

µm scale

virtual source

30-60° divergence

x ~10-3mm x 1 rad ~ 1 mm mrad

Excellent probe for

fields in plasmas

(M. Borghesi et al.)

Plasma

Laser accelerated protons

- an exciting source for the future

• Unique for short pulse duration

– Unrivalled for time resolved probing

– Excellent emittance

• Compared to conventional acceleratorswhat do we need to be competitive?

– Higher average flux

– Narrow angular distribution

– Narrow energy distribution (not simply slicing)

– Higher endpoint energy• 200 MeV protons required for 200mm range in H2O (e.g.

for hadron therapy)

• Fewer particles at higher repetition rate (e.g. hadron therapy)

Beam energy scaling – typically ~I1/2

(Robson, et al., Nature Physics (2006)

500fs scaling: 200 MeV protons requires >4 1021Wcm-2 (>1kJ)

Schreiber Scaling: 200 MeV at 100J, 40fs

Efficiency into >200 MeV around 10-5-10-4 !

Can we do better?

Schreiber et al., PRL (2006)

A novel approach is required -

Radiation Pressure Acceleration (RPA)

Extreme Laser pressure at high intensities:

PL=2I/c=6 Gbar @ 1019Wcm-2

Velocity estimated by momentum conserv.

(accelerated mass balances laser momentum)

niMivi2= vi

2=I/c

vi=(I/ c)1/2

Ei~IRadiation Pressure Accelearation scales faster

than TNSAWilks et al (PRL 92)

Zepf et al., Phys. Plasmas (1996)

30fs

60fs

Relativisitic equations of motion for whole foil acceleration: LIGHT SAIL REGIMEL

~I/ niMiL

Radiation Pressure Acceleration- using circular polarisation (e.g. Robinson, Zepf et al, New J. Phys, 2008)

Ch

arge d

ensity

x

~ I/

At I>1023 Wcm-2RPA dominates over TNSA:

GeV protons with quasi-monoenergetic

distribution for Elaser=10kJ(Simulations by Esirkepov et al., PRL 175003 (2004))

Far beyond ELI first stage…

In the limit of thin foils, extreme intensity: Ep~ GeV

Intermediate intensity - RPA and TNSA coexist

TNSA~I1/2

At 5 1020 Wcm-2 acceleration

due to radiation pressure

becomes comparable to TNSA.

Can we exploit the faster

Emax I scaling?

L.O. Silva et al. PRL 92, 015002 (2004)

RPA

Problem: TNSA decompresses foil during RPA

=> Foil becomes transparent!

Radiation Pressure Acceleration-circular polarisation suppresses TNSA

For I=1021 Wcm2

Circular polarisation suppresses hot electron generation - no TNSA, few -rays

RPA dominates for realistic intensities.

102

100

101

x[µm]

0 5 10

40

20

0

y[µ

m]

200 600 1000 200 600 1000Energy [MeV] Energy [MeV]

Pro

ton

Nu

mb

er (

a.u

.) 105

104

103

102 Pro

ton

Nu

mb

er (

a.u

.) 105

104

103

102

x[µm]80 120 160

x[µm]80 120 160

1.5

1

0.5

-0.5

0

1.5

1

0.5

-0.5

0

106

105

102

101

104

103

px/m

pc

Px/m

pc

RPA – Circ Pol TNSA- Linear

(Robinson et al, NJP 2008)

Foil thickness:100nm150nm

250nm350nm

RPA scaling - a promising route

Time (fs)

0 100 200 300 400 400

500

400

Pro

ton

en

ergy [

MeV

]

300

200

100

0

Co

nversio

n E

fficie

ncy

1

.8

.6

.4

.2

200 MeV predicted in quasi-monoenergetic beam at ~ 1021 Wcm-2

Feasible ELI specfication laser at high repetition rate

Efficiency into 200 MeV peak >60%

Divergence angle: 4°

Quasi-monoenergetic proton beams from RPA

- first experimental demonstration

Data from LIBRA

consortium taken on

Astra GEMINI

Transition to the light-sail regime

I=4 1020Wcm-2

I=9 1020Wcm-2

Ep~I1.75

Ep~I0.8

Light Sail: acceleration to GeV/uat ELI parameters

Stable acceleration to GeV energies shown for I=2…6 10 22 Wcm-2

(Simulations: B. Qiao et al, PRL 2009)

Control of laser or target radial distribution essential for ultimate performance

Solid density bunch

Duration:100nm/c= 0.3 fs

bea

m

dir

ecti

o

n

Fs

cat

Fg

rad

Hi-Rep strategy for complex targets

Wafer target production

Electrostatic Injection

Optical trapping

RPA light sail – the story so far.• RPAwith circular polarisation.

• Low divergence

• High efficiency

• Quasi-Monoenergetic distribution

• Little other radiation (gammas, fast electrons)

• Unique features• Extremly short bunches – attosecond duration

• Solid density bunches (quasi-neutral)

• Synchronisation to optical sources at attosecond level possibl

• Challenges• Maintaining 1D nature during acceleration.

• High repetition, high power lasers to drive accelerator

• GeV accelerator simulation E=0.5- 2kJ, 30 fs

• 200 MeV: E~100J, 60 fs,

Ultrafast probing

Ultrabright laser driven attosecond sources

1) Relativistically Oscillating

Mirrors

Extreme Intensities

Coherent Harmonic Focusing

Attosecond Bunching

Tests of QED using the Relativistically Oscillating Mirror

Relativistically Oscillating Plasma Surfaces

as non-linear medium

Electron kinetic energy = rest mass for a0=(I 2/1.37 1018Wcm-2)1/2=1

Highly relativistic for a0>>1 ( ~a0)

Relativistically Oscillating Surface

Courtesy of G. Tsakiris, MPQ

fout

f in

1 v /c

1 v /c~ 4 2 tout

tin~

1

4 2

Shorter Pulses - Higher Frequencies

The relativistic Doppler effect

=33

ELI laser can generate Relativistic Mirrors

=10fs, =800nm

=2.5as, =2Å,

Plasma

Laser Driven Oscillation

X(t)

Rest

Position

vs

c

t´

Upshifting from an oscillating surface

γs

t´

At these times high harmonics are generated!

s

1

1 vs c2

~T0/ max

1)Upshifting is restricted to a short time ~T0/ max.

2)The upshifted pulse has a duration of O~ / ~ T0/

From Fourier theory, the spectrum must extend to frequencies O~

Predicted pulse duration ~10-19s=100zeptoseconds for =20

A sufficiently intense laser can be used to move electrons in a

target at relativistic velocities.

A sharp edged plasma will act as an oscillating, relativistic mirror.

with = [1+a02/2]1/2

10-30 is possible with latest lasers

Gordienko et al. PRL 93, 115001, 2004

Orders > 1000,

keV harmonics!

asymptotic efficiency ~n-8/3for >>1

Harmonics up to nmax~81/2

Independent of intensity!

Dromey et al., Nature Physics, (2006.)

Experimental data

From Vulcan PW

• Boost due to

– Increased focusability

– Attosecond pulse duration

Enhancement up to the Schwinger limit theoretically possible(Gordienko et al. PRL,94, 103903 (2004)

HHG BOOSTS theoretically achievable intensity.

• How realistic is this in the real world?

– No phase distortions on target.

– No scattering of higher harmonic due to surface

roughness.

Ideal world: focal spot size reduced by n2

FLAT SURFACE

Diffraction limited n= Laser/n

At a0>>1

Laser

Target

PARABOLIC SURFACE

Divergence: =f/D

Diffraction limited focus ~f n/D

Reflected Laser

Focusing

Harmonic

Target Laser

Dynamic effects: Ponderomotive Denting

0 10 20 30x (c/ 0)

0

10

20

30

y (

c/

0)

100 fs

PLASMAVacuum

Laser

Ponderomotive denting due to Gbar

light pressure

f

DHARMONIC

BEAM

Harmonics emitted into constant angle even for initially flat targets

n=Max( Laser/n, f/D)

(from Wilks et al. PRL 1992)

Experimental Results

Wavelength (nm)

An

gle

(m

rad

)

ROM orders

CWE orders

Diffraction limit

I=2 1019Wcm-2

p=50fs

From: Dromey, Zepf, Nature Physics, 5, 146, 2009

500fs, 1020 Wcm-2

50fs, 1019 Wcm-2

Divergence vs harmonic order

Experimental dent vs PIC

GAUSSIAN FOCUS

SUPERGAUSSIAN

WAVEFRONT CONTROL

Preshaped targets (laser machining in situ?)

Pulse duration (5fs -> 2mrad divergence in above expt.)

Intensity distribution (supergaussian)

Divergence and focus control

From Hörlein et al,

EPJ D 55, 475–481 (2009)

Simulations by S. Rykovanov

• Changing roughness does not affect specular reflection –

data consisten with denting only

• Specular reflection observed for initial roughness > n

Is target roughness a limiting factor ?

- diffuse or specular reflection for high orders

Dromey et al, PRL 99, 085001 (2007)

keV harmonics are beamed

• Initial roughness of target is smoothed out

– Simulations by Sergey Rykovanov.

• Focusing to extreme intensities appears feasible

Relativistic plasma dynamics smooth initial roughness

0.01

0.1

1

10

100

1000

10 100 1000 10000

Puls

e d

ura

tio

n (

as

)

nF

Duration of attosecond pulses

n=(21/p-1)nF

Few as pulses

possible <1keV

Zeptosecond@

>1keV

nF

Extremely short pulses are possible

Harmonic efficiency slope as n-p

Harmonics 10 -14 (CWE)

XUV pulse train with ~0.9 fs duration

(in collaboration with MPQ at ATLAS)

Y. Nomura et al., Nature Physics 2009

Attosecond pulse measurement

• Pulse duration reduces

with n:

~n-1

• Diffraction limited

focus:

w0~n-2

Peak intensity of attosecond pulses

• Single harmonic

efficiency:

~n-8/3

• Pulse efficiency (for ~n

harmonics forming the

as pulse)

~n-5/3

34

21

35

max nnn

n

A

EIIntensity increases:

Opens up new regime of high intensity X-ray science

Tool for probing vacuum physics

Coherent superposition of entire spectrum ICHF=a03 I0

Probing vacuum bi-refringence

X-ray

probe

2d

probe

n4

15

d

probe

I

IcritPhase Shift:

Probe wavelength as short as possible (polarised X-ray)

Intensity as high as possible, long interaction length

2

2

2

ellipticity:

• Requires bright polarised X-ray probe

• Requires time synchronisation << laser

• Requires ultra-intense laser >>1024 Wcm-2

• Requires excellent polariser

Practical experiments are challenging

X-ray

probe

Ultraintense laser

(e.g. ELI

Polariser

Reflected Laser

Focusing

Harmonic

Target Laser

Relativistic Mirrors for experimental QED?

-Boosted intensity by CHF

-Harmonics are intrinsic probe of CHF

I[Wcm-2] Length 2

1µm 1022 20 3.8 10-10 1.5 10-19

1nm 1022 20 3.8 10-7 1.5 10-13

1Å 1024 20 0.002 1 10-6

1Å 1026 2 0.02 1 10-4

What magnitude do we expect?

X

X

• Intensity boost via CHF and synchronised short

wavelength probe makes observation feasible

(measurement limit of 2 to date ~ 10-6)

• How well polarised are the harmonics (the high

power laser?)

Summary

• Extreme Intensities beyond current limit

– Coherent Harmonic Focusing

– Attosecond bunching

• Ultrabright attosecond source

• Well suited to tests of QED

• Extreme X-ray physics

Ultrabright laser driven attosecond sources

2) Relativistic Flying Mirrors

Extreme Intensities

Coherent Harmonic Focusing

Attosecond Bunching

Tests of QED using the Relativistically Oscillating Mirror

Relativistic Flying Mirror

Reflectivity:For =const expect upshift of r/ i=4 z

2; NOTE: 4 z2~2 <<4

Coherent reflection: L< (ki z2)-1

Bulanov S V et al PRL 2003; B. Qiao et al, NJP 2009

z

Blowout condition:

a0=60, n/nc=200, z~7; max~100

PIC confirms upshift + Reflectivity (B. Qiao et al.,NJP 2009)

z2~213

Chirp produces broad spectrum

A bright, monochromatic future?

Laser vg=c~vfoil => interaction time >> laser

= (t) => Strong chirp in upshift

Possible solution: secondary foil to reflect drive laser and transmit

Relativistic Mirror

=const => monochromatic upshift

Wu+Meyer-ter-Vehn:

2nd foil enables z= max

(arXiv:1003.1739)

Ultrabright narrowband sources

at 100s of keV are possible

Towards bright coherent -rays

SUMMARY.

• ELI Beamlines facility is ideall placed to exploit emerging laser driven sources

• Radiation Pressure Ion sources

• Ultrashort bunches

• Compact accelarators to GeV/u level

• Science and Medical applications

• Relatvistic mirror sources

• Ultrabright attosecond sources

• Extreme X-ray Intensities possible (extreme source brightness with ELI)

• Towards zeptosecond regime

• Narrowband X-ray and Possibly -ray with Relativistic Flying Mirror

Ultrafast probing

Light-Sail and Holeboring regime

Holeboring Regime

a~I/

Ch

arge d

ensity

x

Light Sail Regime

a~I/

Holeboring acceleration takes place in thick target limit

=> more relaxed requirements

Acceleration depends on local intensity for sufficiently short pulses!

Ultra-short pulses (here 4 cycles) reduce energy requirements.

Gaussian foci acceptable (reduced energy requirements)

From Macchi et al. PRL 2005

Acceleration Propagation phase

Dimensionless holeboring parameter:

Target:

~0.2

Holeboring – I/ scaling

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

1020 1021 1022 1023 1024

70.8 kgm^(-3)30 kgm^(-3)1000 kgm^(-3)

Intensity (Wcm-2)

Low density targets are desirable

Liquid H2, 30 fs pulse

Liquid H2, 30 fs pulse.

Spectral control possible by tailoring pulse shape/density profile: I/ =const.

1021Wcm-2 6×1021Wcm-2

6×1022Wcm-21022Wcm-2

Existing Hydrogen Jets are suitable for Holeboring

Holeboring acceleration.• Desirable features.

• High efficiency

• Quasi-Monoenergetic distribution possible

• Little other radiation (gammas, fast electrons)

• Semi-infinite targets

• Gaussian foci (no 1D requirement)

• Suitable for current high repetition rate technology• For I=6 1022 Wcm-2: E= 5J, 1µm focus, 10fs

=> These are PFS parameters to a achieve 400 MeV/u

• Challenges• Maintaining 1D nature during acceleration.

• High repetition, high power lasers to drive accelerator

• GeV accelerator simulation E=0.5- 2kJ, 30 fs

• 200 MeV: E~100J, 60 fs,

Summary - Protons

• Laser accelerated protons (TNSA)– Excellent beam emittance

– Unique beam characteristics – excellent short pulse ion source

– ps temporal resolution of electric field evolution

– But:Slow scaling to high beam energies, broad spectrum

• RPA schemes

– Highly desirable beam qualities with circular polarisation.• Low divergence

• High efficiency

• Quasi-Monoenergetic distribution

• Little other radiation (gammas, fast electrons)

– Potentially ideal for medical applications.

– Excellent laser beam control is essential for light sail

– Relaxed operating conditions for holeboring regime

• High repetition, high energy beams appear within reach.

Plasma Mirror

- an ultra-fast optical switch

A low reflectivity surface (i.e. a piece of glass with AR coating can operate as

~100fs rise time optical switch:

- Illuminate with Imax> plasma formation threshold

- prepulse sees Rcold<10-2

-main pulse sees Rplasma~60-80%

- contrast enhancement: Rplasma/Rcold~ 100

Disadvantages: Energy loss, new PM required every shot.

A

A

A

Plasma

Mirror

Advantage: Interaction with near-perfect plasmas surfaces is possible.

Experimental set-up

15Mev

0

5

10

15

20

25

30

35

40

0 500 1000 1500 2000 2500

5Mev 10Mev

Detector depth [µm]

Proton energy loss

En

erg

y lo

ss [K

eV

/µm

]

0

5

10

15

20

25

30

35

0 5 10 15 20 25

layer Alayer B

RCF layer number

Pro

ton

en

erg

y [M

eV

]

Layered detector stacks give 2D energy resolved beam images

Detector Stack

Quasi-monoenergetic proton beams from RPA

- first experimental demonstration

0

2 1012

4 1012

6 1012

8 1012

1 1013

4 8 12 16 20

Pro

ton

s/M

eV

/sr

Energy [MeV]

-1 1013

0

1 1013

2 1013

3 1013

4 1013

5 1013

6 1013

7 1013

0 5 10 15 20 25

Pro

ton

s/M

eV

/sr

Energy [MeV]

Astra Gemini results

LIBRA consortium

Beam quality - spectral

1ps

E

t 100ps

E

t200ps

Typical conventional accelerator @20MeV E/E=10-4 :

E t~2keV*10ns=20 10-6 eV s

Laser accelerator: E t~10 MeV*1ps=10 10-6 eV s

Comparable to conventional accelerator

After phasespace rotator

Circular polarisation

- suppression of foil heating and expansion

Copious hot electrons

TNSA

Gamma ray production

No hot electrons

Suppressed TNSA (foil expansion)

Fewer gamma rays

Simulations by Sergey Rykovanov, MPQ (published

Reflected waveform contains attosecond bursts of harmonics every at

every -spike

t

a)

Laserf

IFT

c)

f

I

d)

FT

t

I

e)

t

E

b)

E

t

Many cycle interaction

Attosecond pulse generation

For a few cycle pulse the highest harmonics are only generate in one cycle

-> isolated attosecond pulse

t

IFT

e)

t

a)

f

I

d)

f

IFT

c)

E

t

t

E

b)

Few cycle interaction

Attosecond pulses by spectral filtering

Removing optical harmonics + fundamental changes wave

from from saw-tooth to individual as-pulses

from (G. D. Tsakiris et al.,New J. Phys. 8, 19(2006)

0

0.2

0.4

0.6

0.8

1

-15 -10 -5 0 5 10 15

CircularLinear

|E|

Time [fs]

Electric field does not oscillate for circular polarisation –

light pressure becomes quasi-static

P~I/c~E2/c