Andreas Henig

J. Schreiber, D. Kiefer, D. Jung, R. Hörlein, P. Hilz, K. Allinger, J. Bin, S. G. Rykovanov, H.-C. Wu, X. Q. Yan, V. Liechtenstein,

J. Meyer-ter-Vehn, T. Tajima, F. Krausz, D. Habs

K. Flippo, C. Gautier, L. Yin, B. J. Albright, S. Letzring, R. Johnson, T. Shimada, J. Fernandez, B. M. Hegelich

S. Steinke, T. Sokollik, M. Schnürer, P. V. Nickles, W. Sandner

M. Geissler, K. Markey, M. Zepf

opaque target

hot electrons generate quasi-static E-field (TV/m)

ions are accelerated from the back surface (MeV/u)

large transverse spreading of E-field (100s of m)

energy transfer to electrons spatially separated from ion acc.

stationary instead of co-propagating acceleration field

exponential spectrum (low conversion efficiency to highest energetic ions)

But:

ASE pedestal / prepulses preceding the main peak

target thickness limited by heating / shock breakout

optimum target thickness decreases with ASE duration

M. Kaluza et al., Phys. Rev. Lett. 93, 045003 (2004) What would be the optimum for

a nearly background-free pulse?

for high intensities / ultra-thin targets, the laser will either directly (thickness d < skin depth ls) or eventually

(depletion of electron density / relativistic transparency) penetrate the foil

volumetric energy transfer to foil electrons ↔ reduced total absorption

empirical law for optimum areal density opt:

opt / ncr 3 + 0.4 (I/I1)1/2

I1 = 1.368 1018 W/cm2 ( m/ )2

at I = 7 1019 W/cm2, ne/ncr = 660, = 1053 nm:⇒ dopt 10 nmEsirkepov et al, Phys. Rev. Lett. 96, 105001 (2006)

Proton maximum energies from Al/H+ double-layer targetvs. areal density and intensity I (from 2D PIC simulations)

thickness 2 nm - 60 nm

bulk density (2.7±0.3) g/cm3 (75 % sp3)

damage threshold: 1011 W/cm2 @ 500 fs, 108 W/cm2 @ 1.2 ns

high percentage of sp3 bonds gives diamond-like properties

high radiation and heat resistence

high tensile strength

50 nm DLC foil - proton beam profile

14 MeV43°

20 MeV30°

25 MeV18°

1053 nm700 fs FWHM80 Jps-contrast ~ 10-8

/4 waveplate for polarisation adjustmentKDP crystal,3 mm thickness, 200 mm

intensity at focus~ 7 1019 W/cm2

RCF stackproton beam profile

Thomsonparabolaspectrometer

double plasma mirrorreflectivitytot ~ 50 - 60%40 - 50 J energy throughputresulting ps-contrast ~ 10-12

ultrathin foil targetsdiamond-like carbon (DLC) foils50% sp3, high mechanical stability, radiation hardness10 - 50 nm thickness

1 mm

CR39 / image plates

simple analytical model derived to predict the instantaneous laser intensity at burn-through time (i.e. I (t1))

relativistic transparency / BOA regime:highest ion energies generated for max. I (t1)

Etarget = 90 J540 fs FWHMIpeak 2 1020 W/cm2

new OPA-based frontend resulting in high contrast on target without plasma mirrors, increased peak intensity of Ipeak 2 1020 W/cm2

optimum thickness increases as expected, highest ion energies at 58 nm:

C6+ max. energy:0.5 GeV

advanced analytical model to predict ion energies:

data spanning over wide range of target thicknesses

beginning: target opaque, TNSA-like acceleration

t1: ne / (ncr ) ~ 1, target becomes relativistically transparent

short period of strong ion acceleration until t2: ne / ncr ~ 1

highest ion energies observed at 8.5° angle orthogonal to laser polarization

0 10 20 30 40 50 600

1

2

3

4

5

6

7

8

9

10

11

12

13

14

Ion

Cut

-Off

Ene

rgy

[MeV

/am

u]

target thickness [nm]

exp. H+ (a0=3.6)

sim. H+ (a0=3.6)

exp. C6+ (a0=3.6)

sim. C6+ (a0=3.6)

exp. H+ (a0=5.0)

exp. C6+ (a0=5.0)

- - - H+

theo. TNSA

(cp. Andreev et al.)

10 Hz Ti:sapph system, 45 fs FWHM, 0.7 J on target after DPM (contrast ~ 10-12)peak intensity up to 5.3 1019 W/cm2 (a0 = 5), linear polarization

5 nm DLC (a0 = 5):

13 MeV protons71 MeV C6+ ions

4

5

6

7

Carbon C6+

protons

104

105

106

107

10 20 30 40 50 60 70 80 900energy (MeV)

part

icle

num

ber

(MeV

-1 m

sr-1)

Carbon C6+

protons

optimum thickness just below skin depth: dopt < ls = c / p

J. Fuchs et al., Nature Physics (2006)

TNSA- scaling according to fluid-model predictions:

J. Fuchs et al., Nature Physics (2006)

TNSA- scaling according to fluid-model predictions:

much increased maximum energy and conversion efficiency:

0 10 20 30 40 50 400 600 800 10000

1

2

3

4

5

6

7

8

9

10

11

12

13

co

nv

ers

ion

eff

icie

ncy

(%

)

target thickness (nm)

H+ (E > 2MeV)

C6+

(E > 5MeV)

sim C6+

sim H+

0,0 0,2 0,4 0,6 0,8 1,00,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0,40

0,45

0,50

an

gle

[d

eg

]

an

gle

[ra

d]

PIC

FIT -0.34 x + 0.34

normalized energy

0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

RCF

0 2 4 6 8 10 121

10

100

1000p p ( p)

pro

ton

@1

14

nsr

(E

= 3

%)

DPM & DLC 5nm (CE = 1.6%)

no DPM 1μm Titan (CE = 0.13%)

1.6% into protons10.5% into C6+ ions

5 nm DLC (a0 = 5): 1 m Ti (a0 = 7):

0.13 % into protons

First experiment showing the correlation of relativistic transparency enabled laser penetration and enhanced ion energies using nm-scale DLC foils and a DPM-setup at TRIDENT

World record 0.5 GeV carbon ions generated at upgraded TRIDENT, sufficient for fast ignition in ICF

13 MeV protons and 71 MeV C6+ from a small-scale, high rep. rate Ti:sapph laser system with pulse energies as low as 0.7 J

But: so far only monotonically decaying ion energy distributions

Many potential applications (such as IBT or FI-ICF) require quasi-monoenergetic spectrum

Route towards intrinsically monochromatic ion beam generation needed

whole target gets accelerated, light sail driven by radiation pressure = intensity / c

whole target gets accelerated, light sail driven by radiation pressure = intensity / c

Photo courtesy The Planetary Society

small intensities (0.14 W/cm )“infinite” acceleration times (years)

JFL Simmons et al, Am J Phys 1993 (Marx Nat 1966 )

whole target gets accelerated, light sail driven by radiation pressure = intensity / c

Photo courtesy The Planetary Society

small intensities (0.14 W/cm )“infinite” acceleration times (years)

JFL Simmons et al, Am J Phys 1993 (Marx Nat 1966 )

high power (1015 W)high intensities (>1021 W/cm )short acceleration times (10’s, 100’s fs)ultrathin foils required5 nm, 2 m focus: 5J rest energy

Klimo et al, Phys. Rev. ST Accel. Beams 11, 031301 (2008)

electron spectrum

1D PIC simulation, 32nm foil thickness, 100fs pulse duration (sin ), 1.5 1020 W/cm

electron spectrum ion density ion density

1D PIC simulation, 150nm foil thickness, 64fs pulse duration (sin ), 2 1021 W/cm

Robinson, Zepf et al., New J. Phys. 10, 013021 (2008)

proton spectrum proton spectrum

linear polarization circular polarization

1D PIC simulations (by Sergey Rykovanov)

linear polarization circular polarization

strong electron heating

foil explosion

electrons remain cold

ballistic neutral foil acceleration

formation of compressed, highly dense electron layer

characteristic loops in ion phase space

key to success: suppression of electron heating

opt / ncr = (I/I1)1/2optimum condition:

a) b)

z z

ni

ni

ne

ne

EzEz

c)

0 0 z0ld ld + lc

ne0

Ez0

nec

electrondepletion

layer

compressedelectron

layer

pz

ld + lc

ionphasespace

evolution

t1

t2

t3

0 5 10 15 2520 30 35 40 50target thickness (nm)

45

15

10

5

0

max

. ion

ene

rgy

(MeV

/ u) H+

C6+

lin.

lin. circ.

circ.

elec

tron

num

ber

(a. u

.)

energy (MeV)

lin. circ.

0.5 1.0 2.01.5 2.5 3.0

1010

109

electronspectrum

d = 5.3 nm

maximum ion energy vs. target thickness

electron spectrum at optimum thickness

optimum thickness d = 5.3 nm consistent with condition opt / ncr = (I/I1)1/2

circular polarization results in significant reduction of electron heating

0 10 20 30 5040 60 70 80 90energy (MeV)

part

icle

s (M

eV-1 m

sr-1) 107

106

105

carbon C6+

protonscarbon C6+

protons

linearpolarization

circularpolarization

0 10 20 30 5040 60 70 80 90energy (MeV)

part

icle

s (M

eV-1 m

sr-1) 107

106

105

circularpolarization

0 10 20 30 5040 60 70 80 90energy (MeV)

1

10

100

part

icle

s (a

. u.)

t = 45 fst = 221 fs

linearpolarization

0 10 20 30 5040 60 70 80 90energy (MeV)

1

10

100

part

icle

s (a

. u.)

carbon C6+t = 45 fst = 221 fscarbon C6+

experiment

simulationsimulation

experiment

0

0.02

0.04

0.06

0.08

circularpolarization

3.0 3.1 3.2 3.3 3.4 3.5x (λ)

p x (m

i c )

carbon C6+

t = 45 fs

0

0.02

0.04

0.06

0.08

3.0 3.2 3.4 3.6 3.8 4.0x (λ)

p x (m

i c )

linearpolarization

carbon C6+

t = 45 fs

measured proton and C6+ spectra

0 10 20 30 5040 60 70 80 90energy (MeV)

part

icle

s (M

eV-1 m

sr-1) 107

106

105

carbon C6+

protonscarbon C6+

protons

linearpolarization

circularpolarization

0 10 20 30 5040 60 70 80 90energy (MeV)

part

icle

s (M

eV-1 m

sr-1) 107

106

105

circularpolarization

0 10 20 30 5040 60 70 80 90energy (MeV)

1

10

100

part

icle

s (a

. u.)

t = 45 fst = 221 fs

linearpolarization

0 10 20 30 5040 60 70 80 90energy (MeV)

1

10

100

part

icle

s (a

. u.)

carbon C6+t = 45 fst = 221 fscarbon C6+

experiment

simulationsimulation

experiment

0

0.02

0.04

0.06

0.08

circularpolarization

3.0 3.1 3.2 3.3 3.4 3.5x (λ)

p x (m

i c )

carbon C6+

t = 45 fs

0

0.02

0.04

0.06

0.08

3.0 3.2 3.4 3.6 3.8 4.0x (λ)

p x (m

i c )

linearpolarization

carbon C6+

t = 45 fs

measured proton and C6+ spectra

Hegelich et al. (TNSA): 20 J laser pulse energy

0 10 20 30 40 50 60 70 80 90

105

106

107

energy (MeV)

part

icle

s (M

eV1 m

sr1 )

Hegelich et al., Nature 2006 Henig et al., PRL 2009

Henig et al. (RPA): 0.7 J laser pulse energy

40 times increased conversion efficiency

comparison with TNSA, Hegelich et al., Nature 439, 441 (2006):

0 10 20 30 5040 60 70 80 90energy (MeV)

part

icle

s (M

eV-1 m

sr-1) 107

106

105

carbon C6+

protonscarbon C6+

protons

linearpolarization

circularpolarization

0 10 20 30 5040 60 70 80 90energy (MeV)

part

icle

s (M

eV-1 m

sr-1) 107

106

105

circularpolarization

0 10 20 30 5040 60 70 80 90energy (MeV)

1

10

100

part

icle

s (a

. u.)

t = 45 fst = 221 fs

linearpolarization

0 10 20 30 5040 60 70 80 90energy (MeV)

1

10

100

part

icle

s (a

. u.)

carbon C6+t = 45 fst = 221 fscarbon C6+

experiment

simulationsimulation

experiment

0

0.02

0.04

0.06

0.08

circularpolarization

3.0 3.1 3.2 3.3 3.4 3.5x (λ)

p x (m

i c )

carbon C6+

t = 45 fs

0

0.02

0.04

0.06

0.08

3.0 3.2 3.4 3.6 3.8 4.0x (λ)

p x (m

i c )

linearpolarization

carbon C6+

t = 45 fs

measured proton and C6+ spectra

2D PIC results for C6+

C6+ spectrum at different times

C6+ phase space

2 3 4 5 6 7x (λ)

linearpolarization

electrondensity

circularpolarization

electrondensity

8

-4

-2

0

2

4

y (λ

)

2 3 4 5 6 7x (λ)

8

-4

-2

0

2

4

y (λ

)

0

5

10

15

20

25

0

5

10

15

20

25

linearpolarization

carbondensity

2 3 4 5 6 7x (λ)

8

-4

-2

0

2

4

y (λ

)

circularpolarization

carbondensity

2 3 4 5 6 7x (λ)

8

-4

-2

0

2

4

y (λ

)

0

1

2

3

4

0

1

2

3

4

linear polarization circular polarization

strong expansion driven by hot electrons

ballistic acceleration of whole focal volume

Gaussian focal spot causes foil to bend

Strong electron heating sets in due to oblique incidence on walls

Rapid foil expansion / termination of RPA

Potential solutions: increased focal spot size

higher laser intensities

pre-curved targets

circularpolarization

electrondensity

2 3 4 5 6 7x (λ)

8

-4

-2

0

2

4

y (λ

)

0

5

10

15

20

25

collimated ion beam from back surface (TNSA)

divergent ion beam from front surface

TNSA ion now accelerated into 4

highly directed shock accelerated ion beam

vs /c = aL / 2 (me /mi)1/ 2(nc /ni)

1/ 2shock speed:

First experimental demonstration of radiation pressure acceleration (RPA) to become the dominant ion acceleration mechanism when using nm-thin foil targets and circular polarization

Highly promising route towards intrinsically mono-energetic, dense ion beams at large conversion efficiencies

For the moment, keeping the monochromaticity by suppressing electron heating over longer times remains a demanding challenge

MAP biomedical beam line is currently being set up at MPQ

magnetic chicane will be used as interim energy filter