Laser Ion Acceleration: from present to intensities achievable at ELI-Beamlines

Transcription

1 Laser Ion Acceleration: from present to intensities achievable at ELI-Beamlines J. Limpouch a,b, J. Pšikal a,b, O. Klimo a,b, J. Vyskočil a,b, J. Proška a,f. Novotný a, L.Štolcová a,b, M. Květoň a a Czech Technical University in Prague, Faculty of Nuclear Sciences and Physical Engineering, Břehová 7, Praha 1, Czech Republic and b Institute of Physics of the Academy of Sciences of the Czech Republic, Na Slovance 2, Praha 8, Czech Republic

2 Contents Target Normal Sheath Acceleration efficiency improvement Foils with monolayer of microspheres on the front side Grid targets thin foils with relief on the front side Impact of microstructure on the foil rear side angular spectrum Laser interactions with foils at intensities > W/cm 2 Linear vs. circular polarization, normal vs. oblique incidence, role of target material Transition to RPA (radiation pressure acceleration) for linear polarization for thin foils via hole boring regime Conclusions

3 TNSA acceleration Increase TNSA efficiency thin foils or reduced mass targets recirculation of hot electrons, important parameter surface to volume ratio absorption efficiency target surface modification velvet, microspheres, snowflakes etc.

4 Microspheres target production a) b) Aluminium holder with drilled holes The holder overcoated with PET (Mylar) foil a) top view; b) side view Deposition of floating monolayer on the Mylar foil Monolayer of self-assembled PS spheres of diameter 535 nm on water surface

5 Optimization of microsphere size Optimum microsphere diameter for laser absorption and maximum proton energy is close to laser wavelength. According to theory, which does not include hot electron recirculation effects, maximum proton energy scales like E max T hot ln 2 (n hot ), while the energy transformation efficiency scales like T hot n hot. 20 fs laser pulse, W/cm 2, LP, normal incidence The foil thickness is much smaller than the spatial length of the laser pulse (cτ) In this case, n hot must be replaced by n hot /d, where d is the foil thickness. Hot electron recirculation is more important for energy transformation efficiency than for the maximum ion energy. SEM image (microsphere 530 nm)

6 Thin grid targets - production Interference lithography and thermal imprinting Positive photoresist spin-coated on glass Computer model of desired microstructure -> exposure PET foil To enable mechanical embossing, the microstructures were transferred into a metal stamp using nickel electroplating

7 Thin grid targets - simulations The foil thickness 0.9 μm with and without microspheres deposited on the surface or grating embossed into the foil. The target consists of 1:1 mixture of H + and C 4+ ions. Microsphere diameters 0.47 and 0.94 μm. Grating periodicity 1.6 μm and depth 0.25 and 0.5 μm. Resonance angle for surface plasmon wave excitation k sw = k 0 (sinq + l / a), for our conditions k k 0 and for a= 2l, the optimum angle is 30. (M. Raynaud et al., Phys. Plasmas 14 (2007), ) Laser pulse duration 40 fs, intensity W/cm 2, p-polarization, incidence angle is 10 or 30 The grating structure is better. Absorption increased 6 times vs. flat foil. The maximum proton energy in gratings is increased 2.5 times in comparison with flat foil and 1.4 times in comparison with microspheres.

8 Target comparison at higher I 0.5 PW pulse (15 J, 30 fs) focused to 3 mm (FWHM) Gaussian focal spot maximum intensity W/cm 2 (a 0 = 58), l = 800 nm, linear polarization, normal incidence Plastic (C 6+ H + ) target of density ~1 g/cm 3 Comparison of 1 mm-thick foil with monolayer of microspheres of Ø530 nm on the same foil and with grating of depth 500 nm and periodicity 1.6 mm on the same foil Highest proton energy is achieved for grating target, proton energy for microsphere target is lower mainly due increased effective thickness Proton spectra for various targets Proton density and longitudinal electric field for grating target fs after laser pulse end

9 Microstructure at the foil rear side Microstructure on the foil rear side may influence angular divergence of the proton/ion beam (K. Takahashi et al., Phys. Plasmas 17 (2010) ) 0.5 PW pulse (15 J, 30 fs) focused to 3 mm (FWHM) Gaussian focal spot maximum intensity W/cm 2 (a 0 = 58), l = 800 nm, LP, normal incidence Plastic foil (C 6+ H + ) of density ~1 g/cm 3, 1 mm thick with distance 3.4 mm between wings 1 mm long, 0.4 mm wide The angular divergence of protons with energy > 30 MeV is ~20 for planar foil, while this decreased to ~2 when the focus center is between the wings If the focus is across a wing, most protons are accelerated at angles ± 10 Computational geometry Angular spectrum of protons with energy >30 MeV

10 Foil acceleration at 1 PW level 1 PW pulse (30 J, 30 fs) focused to 3 mm (FWHM) Gaussian focal spot maximum intensity W/cm 2 (a 0 = 81), l = 800 nm Polyethylene foils (C 6+ H + 2) of density ~1 g/cm 3 (n e = 200 n c ) Thin (200 nm) and thick (1 mm) foil, comparison with pure hydrogen foils (200 n c ) Comparison of linear and circular polarization Proton energy spectrum (all forward directions) Laser absorption efficiency H plasma linear pol. 36% 25% CH 2 plasma circular pol. 21% 14% LP higher absorption and higher maximum proton energy Thin (200 nm thick foil), normal incidence CH 2 target has lower hole boring speed and lower deformation leads to lower absorption

11 Normal vs. oblique incidence 0.5 PW pulse (15 J, 30 fs) focused to 3 mm (FWHM) Gaussian focal spot maximum intensity W/cm 2 (a 0 = 58), l = 800 nm, Linear Polarization Polyethylene foil (C 6+ H + 2) of density ~1 g/cm 3 (n e = 200 n c ), thin (200 nm) foil Comparison of normal and oblique (30 ) incidence Thick foil (1mm) - maximum proton energies are 85 MeV (normal) and 75 MeV (obl.) Proton energy distribution (forward half-solid angle) Direction of accelerated protons from target normal axis (ε p >30 MeV) Proton maximum energy higher for normal incidence, angle shifted for oblique

12 Thin versus thick foil 1 PW pulse (30 J, 30 fs) focused to 3 mm (FWHM) Gaussian focal spot maximum intensity W/cm 2 (a 0 = 81), l = 800 nm Linear polarization, normal incidence Polyethylene foils (C 6+ H + 2) of density ~1 g/cm 3 (n e = 200 n c ) Thin (200 nm) and thick (1 mm) foil Classical hole boring distance is ~600 nm Proton density at 100 fs after laser maximum 200 nm 1 mm Thin foil laser produced hole Thick foil dense region preserved

13 Proton phase space - 1 mm foil 0.09 c Rear side acceleration (TNSA), rear side protons have maximum energy 0.09 c 0.14 c 0.16 c Protons from the focus centre 1l wide area c 0.20 c c 0.27 c

14 Proton phase space nm foil Front side acceleration dominant, whole foil accelerated in focal spot centre 0.09 c 0.14 c 0.22 c Protons from the focus centre 1l wide area 0.31 c 0.36 c 0.40 c

15 Electric field and density thin foil 30 fs, W/cm 2 (a 0 = 81), l = 800 nm, C 6+ H + 2, ~1 g/cm 3, 200 nm Longitudinal electric field at focal axis high peak RPA and lower TNSA peak merge after front side protons reach the initial rear side Charge density profiles at the focal axis - Electron density is 2x original at 22 fs - At 34 fs nearly all protons ahead of C t = 22 fs (laser maximum) 30 t = 34 fs Initial density profile t=0 fs

16 Energy spectra thin vs thick foil 30 fs, W/cm 2 (a 0 = 81), l = 800 nm, C 6+ H + 2, ~1 g/cm 3, 200 nm Spectra of proton and C 6+ ions from central 3 mm wide strip At laser pulse end at ~40 fs - 85% of protons are ahead of C 6+ ions for thin foil, while for thick foil it is only 7% of protons, number of accel. protons is more 2 higher for thin foil Monoenergetic features in proton distribution are seen at the laser pulse end, but high energy tail is formed quickly by Coulomb explosion Proton energies are higher for thin foil -175 MeV vs. 115 MeV 200 nm 1 mm

17 Conclusions For intensities below ~10 20 W/cm 2,TNSA acceleration prevails and its efficiency may be increased by micostructures on the front side Two types of foils with microstructures on the front side were developed foil with a monolayer of microspheres grid target foil with embossed profile Acceleration enhancement and existence of optimum spheres size proven experimentally in GIST (D. Margarone et al., PRL 109 (2012) ) Simulations show bigger enhancement by grid targets Structures on the rear side may decrease angular divergence For high intensities of order W/cm 2 and linear polarization, TNSA acceleration prevails only if the target thickness is greater than hole boring distance For thin foils, new regime with dominant RPA is described (published recently by others B. Qiao et al., PRL 108 (2012), ) The hole boring velocity v b ~ (I/r) 1/2, so the boundary thickness is d, proton energy e ~ I or higher (Light Sail e ~ I 2 b I( ) / r d ) Proton energies are higher for thin foils, monoenergetic features are transient and part of protons post-accelerated by Coulomb explosion Proton energies of order 200 MeV achievable for 1 PW laser

18 Thank you for attention