1. ENHANCED LASER-DRIVEN PROTON ACCELERATION IN MASS-LIMITED TARGETS
Jan Psikal
PhD student at
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
3.5. –

2. in collaboration with …
Faculty of Nuclear Sciences and Physical Engineering, CTU Prague, Czechia
J. Limpouch, O. Klimo
CELIA, Universite Bordeaux 1 - CNRS – CEA, Talence, France
V. Tikhonchuk, E. D’Humieres
Department of Physics and Astronomy, The Queen’s University of Belfast, UK
S. Ter-Avetisyan, S. Kar
LULI, Ecole Polytechnique – CNRS – CEA - UPMC, Palaiseau, France
J. Fuchs and others
Vavilov State Optical Institute, St. Petersburg, Russia
A. Andreev

3. Laser-driven proton 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
2) electric fields V/m (104 times higher than in conventional accelerators)
3) ion acceleration
hot electrons are cooled down and protons originated from water or hydrocarbon surface contaminants are accelerated

4. Possible applications and their requirements
- proton radiography of laser interactions (already used)
- oncological hadrontherapy and medical physics
- neutron source and isotope production
- fast ignition of inertial confinement fusion
applications employing laser-driven proton beams requires improvement of the beam parameters in several areas:
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)

5. Electron recirculation in thin foils
Y. Sentoku et al., Phys. Plasmas 10, 2009 (2003)
A. J. Mackinnon et al., Phys. Rev. Lett. 88, (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)

6. Electron recirculation in mass-limited foils
Lp > Ds
Lp spatial length of laser pulse
Ds transverse size of foil

7. Experiments with limited mass foils
laser pulse of duration 350 fs, =529 nm, I21019Wcm-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

8. Experiments with limited mass foils
with decreasing target surface (and constant foil thickness)
enhancement of maximum proton energy
strong enhancement of laser-to-proton conversion efficiency
reduced ion beam divergence
S. Buffenchoux et al., Phys. Rev. Lett., submitted
azimuthally averaged angular proton dose profiles, extracted from films corresponding to E/Emax~0.
FWHM of angular transverse proton beam profiles

9. 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 – 5080 m2 in the experiment - and 2.4 for larger foil - 200300 m2 in the experiment).
laser pulse duration 40 (=2 fs) is incident on target from 35 to 75, intensity I=3.41019 W/cm2, beam width (FWHM) 7, =600 nm, target density 20nc composed of protons and electrons
1) smaller foil 202
L < Ds/vetrans
2) larger foil 802
L > Ds/vetrans
vetrans  c
Ds transverse foil size, vetrans transverse velocity of hot electrons

10. Time evolution of electron energy spectra – smaller foil

11. Time evolution of electron energy spectra – larger foil

12. 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!)

13. How to explain the discrepancy in the conversion efficiency?
maximum proton energy
P. Mora, Phys. Rev. Lett. 90, (2003)
J. Schreiber et al., Phys. Rev. Lett. 97, (2006)
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 density
3D approach is necessary

14. Mutual interaction of two ion species
proton phase space
proton energy spectra
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.

15. 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 202
2) larger foil 802
quasimonoenergetic feature in proton energy spectra is observed for smaller foil
Z1=4, Z2=1, A1=12, A2=1 
 1.6 MeV
V. T. Tikhonchuk et al. Plasma Phys. Control. Fusion 47, B869 (2005)

16. Why we do not observe this modulation in proton energy spectra for larger foil?
smaller surface
larger surface
overall energy spectra:

17. Experimental results
parameters: perpendicular incidence, pulse duration 5 ps, focal spot 4 m, intensity 31020 Wcm-2
S. Kar, private communication

18. 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

19. Thank you for your attention