ENHANCED LASER-DRIVEN

PROTON ACCELERATION

IN MASS-LIMITED TARGETS

Jan Psikal

1 Department of Physical Electronics, Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague

2 Centre Lasers Intenses et Applications, CNRS – CEA – Universite Bordeaux 1, Talence, France

7th DDFI workshop, Prague

PhD student at

3.5. – 6.5.2009

in collaboration with …

S. Ter-Avetisyan, S. Kar

J. Limpouch, O. KlimoFaculty of Nuclear Sciences and Physical Engineering, CTU Prague, Czechia

V. Tikhonchuk, E. D’Humieres CELIA, Universite Bordeaux 1 - CNRS – CEA, Talence, France

Department of Physics and Astronomy, The Queen’s University of Belfast, UK

A. Andreev

J. Fuchs and others LULI, Ecole Polytechnique – CNRS – CEA - UPMC, Palaiseau, France

Vavilov State Optical Institute, St. Petersburg, Russia

Laser-driven proton acceleration

2) electric fields 1012-1013 V/m (104 times higher than in conventional accelerators)

3) ion acceleration

Target normal sheath acceleration (TNSA) mechanism

1) electron heating by a short and intense laser pulse

two temperature electron distribution – cold and hot electrons

hot electrons are cooled down and protons originated from water or hydrocarbon surface contaminants are accelerated

Possible applications and their requirements

- proton radiography of laser interactions (already used)

applications employing laser-driven proton beams requires improvement of the beam parameters in several areas:

- oncological hadrontherapy and medical physics

- neutron source and isotope production

- fast ignition of inertial confinement fusion

1) increase of maximum energy

2) increase of laser-to-proton conversion efficiency

3) reduction of the beam divergence

4) reduction of the ion energy spread (i.e., monoenergetic beams)

Electron recirculation in thin foils

Y. Sentoku et al., Phys. Plasmas 10, 2009 (2003)

A. J. Mackinnon et al., Phys. Rev. Lett. 88, 215006 (2002)

decreasing foil thickness increasing hot electron density (due to recirculation and lower transverse electron beam spread) higher accelerating electric fields higher ion acceleration efficiency (maximum and total proton energies)

Electron recirculation in mass-limited foils

Lp > Ds Lp spatial length of laser pulse

Ds transverse size of foil

Experiments with limited mass foilslaser pulse of duration 350 fs, =529 nm, I21019Wcm-2 m2, beam width FWHM = 6 m is incident (incidence angle 45º) on a thin Au foil (thickness 2 m) with reduced target transverse surface area down to 50 80 m2

S. Buffenchoux et al., Phys. Rev. Lett., submitted

laser

Magneticspectrometer

RCF with hole

0

5

10

15

0.001 0.01 0.1 1 10

Au 2 µm thickAu 2 µm thick + 10 µm thick 0.1 n

c nanofoam upfront

Surface (mm²)

constant thickness

variable surface

(a)

(b)0.01

0.1

1

10

0.001 0.01 0.1 1 10

Au 2 µm thickAu 2 µm thick + 10 µm thick0.1 n

c nanocloth upfront

Surface (mm²)

Experiments with limited mass foils

• enhancement of maximum proton energy

S. Buffenchoux et al., Phys. Rev. Lett., submitted

• strong enhancement of laser-to-proton conversion efficiency

with decreasing target surface (and constant foil thickness)

• reduced ion beam divergence

(a) azimuthally averaged angular proton dose profiles, extracted from films corresponding to E/Emax~0.

(b) FWHM of angular transverse proton beam profiles

2D3V PIC simulations

Simulation parameters: To decrease high computational demands, the laser pulse duration and foil surface (e.g. transverse foil size in 2D case) are reduced in our simulations. Nevertheless, the ratio of the transverse foil size to the spatial length of the pulse is similar (approx. 0.6 for smaller foil – 5080 m2 in the experiment - and 2.4 for larger foil - 200300 m2 in the experiment).

laser pulse duration 40 (=2 fs) is incident on target from 35 to 75, intensity I=3.41019 W/cm2, beam width (FWHM) 7, =600 nm, target density 20nc composed of protons and electrons

1) smaller foil 202

L < Ds/vetrans

2) larger foil 802

L > Ds/vetrans vetrans c

Ds transverse foil size, vetrans transverse velocity of hot electrons

Time evolution of electron energy spectra – smaller foil

Time evolution of electron energy spectra – larger foil

Proton energy spectra characteristics

maximum proton energies are in agreement with experiment

higher accelerating electric field is sustained for a longer time as hot electrons are reflected back from foil edges

proton conversion efficiency – difficult to determine from numerical simulations (which protons to take into account?)protons emitted from the central part of the foil – 3.5% for larger vs. 5.5% for smaller foil – which is the difference 60% (but at least 400% in experiment!)

How to explain the discrepancy in the conversion efficiency?

maximum proton energy

total energy (e.g., conversion efficiency)

PIC code 2D 3D

transverse foil size

20 20 20

80 80 80

foil “surface” 20 400

80 6400

conversion efficiency is overestimated in 2D, we expect higher difference in hot electron density3D approach is necessary

P. Mora, Phys. Rev. Lett. 90,

185002 (2003)

J. Schreiber et al., Phys. Rev. Lett. 97, 045005 (2006)

Mutual interaction of two ion species

• A thin layer of protons at the rear surface of the target is accelerated by a strong electric field. Heavy ions are accelereted somewhat later because of their inertia. They shield the sheath electric field for other protons from deeper layers and also interact with earlier accelerated protons.

• The fastest protons are futher accelerated by electrons, the slower (close to heavy ion front) are accelerated by heavy ion front which acts like a piston and are decelerated by Coulomb explosion of the fastest protons at the same time.

proton phase space proton energy spectra

Mutual interaction of two ion species

Simulation parameters: laser pulse is incident perpendicularly on foils, plasma composed of protons and C4+ ions in ratio 1:1

1) smaller foil 202

2) larger foil 802quasimonoenergetic

feature in proton energy spectra is observed for smaller foil

Z1=4, Z2=1, A1=12, A2=1

1.6 MeVV. T. Tikhonchuk et al. Plasma Phys. Control. Fusion 47, B869 (2005)

Why we do not observe this modulation in proton energy spectra for larger foil?

overall energy spectra:

larger surfacesmaller surface

Experimental results

S. Kar, private communication

parameters: perpendicular incidence, pulse duration 5 ps, focal spot 4 m, intensity 31020 Wcm-2

1.E+05

1.E+06

1.E+07

1.E+08

0 5 10 15 20 25

Proton Energy [MeV]

dN

/dE

[a

rb. u

nit

]

50x50x2 um Cu-TP2-IP1

50x50x2 um Cu-TP1-IP1

500x500x20 um Cu-TP1-IP1

500x500x20 um Cu-TP1-IP2

50x50x2 um Cu-TP1-IP2

Conclusions

• reduced transverse sizes of a thin foil lead to hot electron recirculation from foil edges, which enhances laser-to-proton conversion efficiency and maximum proton energy

• to observe appropriate scaling of the conversion efficiency, 3D simulations have to be used

• strong modulations in proton energy spectra (dips and peaks) could be observed in foils with transverse sizes of about several times of the laser focal spot size

Thank you for your attention