Laser-plasma interactions simulations, theory and experiments J. Limpouch, O. Klimo, J. Pšikal

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1 Laser-plasma interactions simulations, theory and experiments J. Limpouch, O. Klimo, J. Pšikal Czech Technical University in Prague, Faculty of Nuclear Sciences and Physical Engineering Department of Physical Electronics Břehová 7, Prague 1, Czech Republic Colaboration: R. Liska, M. Kuchařík, P. Váchal, J. Proška, F. Novotný External collaboration: D. Margarone, V.T. Tikhonchuk, S. Kawata, S. Gus kov

2 Syllabus Czech Technical University in Prague Department of Physical Electronics Computational Physics Group Computational capabilities Fluid simulations, interactions of ns laser pulses, foams 1D PIC code and K-α generation PIC studies of laser corona interactions relevant to shock ignition Ion acceleration studies (RPA, TNSA, clusters) Simulations relevant to previous and current experiment at GIST

3 Czech Technical University in Prague Founded in 1707 (polytechnic since 1806) Approximately students 8 faculties civil engineering, machinery engineering, electrical eng., architecture, transport eng., biomedical eng., information technology and Faculty of Nuclear Sciences and Physical Engineering students, research oriented Department of Mathematics, Physics, Nuclear Reactors, Dosimetry, Nuclear Chemistry, Material Science, Solid State Engineering, and Department of Physical Electronics (largest) teaching and scientific staff ~40, research budget ~ 0.5 M$/year (info on

4 Department of Physical Electronics Solid state lasers and applications, laser satellite ranging Diffractive and non-linear optics X-ray tomography and x-ray optics (with Rigaku branch) Nanotechnology Computational physics (6 members + students) Capillary discharge for x-ray laser pumping Max. voltage 40 kv, C =15 nf Pulse length ns halfperiod, ~ 500 ns full Capillary length cm Ablation filled, gas filled

5 Computational capabilities Fluid code 2D Arbitrary Lagrangian-Eulerian (ALE) code (planar and cylindrical version) Particle-In-Cell 1D3V relativistic electromagnetic code with collisions and ionization written in C++ and parallelized in MPI with dynamic load balancing 2D3V relativistic electromagnetic code with ionization written in Fortran and parallelized in OpenMP 2D3V relativistic electromagnetic code with collisions written in C++ and parallelized in MPI with dynamic load balancing 3D3V relativistic electromagnetic code written in C++ and parallelized in MPI with moving window is being developed by one student Monte Carlo PENELOPE code for electron, photon and positron transport in matter

6 Fluid simulation ns interactions ALE code is Lagrangian code with rezoning (mesh untangling and improvement) and remapping (conservative interpolation of conservative quantities to the new mesh) ALE eliminates problems with grid in 2D and 3D Part of grid (a) start, (b) Lagrangian 0.5 ns, (c) ALE 0.5 ns Impact of laseraccelerated disk on bulk target, right crater formation after 30 ns

7 Laser interactions with undercritical foams Developed analytical model for laser driven heating+ionization wave propagation in undercritical homogeneous plasma. Developed analytical model for the wave propagation in foam depending on the fractal number α of the foam structure (α = 1 meaning closed cells and α = 0.5 open cells). Spatial profiles of electron temperature (a) and mean ion charge (b) in the ionization wave in neon of the density 5 mg/cm 3 in time 25, 50, 100 a 150 ps for the laser pulse intensity W/cm 2 at the wavelength λ=0.35 μm. Profiles from numerical simulation (full line) are compared with the analytical model (dashed line). (c) The temporal dependence of the ionization front velocity from the model with the fractal number α in comparison with the LIL experiment. Foam C 15 H 20 O 6 with the density 6 mg/cm 3 and cell size 1 2 μm, λ = μm, I= W/cm 2. Foams are used in experiments for smoothing of laser beams (Depierreux et al., PRL 2009), for long corona preparation for SBS, SRS studies relevant to shock ignition (Depierreux et al., PPCF 2011) and radiative transport studies (O. Rosmej et al., NIMA 2011)

8 K-α generation in fs laser-solid interactions K-α studies devoted to X-ray crystallography with subpicosecond temporal resolution For experiments at NTT BRL Tokyo (dr. Nakano) and later for experiments in Max-Born Institute, Berlin (dr. Zhavoronkov) I = W/cm 2, 120 fs Left up scheme of simulation PIC + MC Left down Al K α pulse 200 fs for NTT experim. Right Mox Born experiment 45 fs laser pulse Up critical surface moves against laser Down mean ion charge for copper

9 Laser-corona interactions in Shock Ignition Theobald et al. PoP 2008 Decoupling of target compression and ignition Higher gain/yield for a given laser drive energy Same laser using pulseshaping capabilities Lower implosion velocity better hydro stability Less sensitive to hot electrons & fuel preheat Conventional ICF limited by parametric instabilities to Iλ W/cm 2 SI hot inhomogeneous large scale plasma and high laser pulse intensity Example corresponding to the HiPER target Threshold condition fulfilled everywhere (in simulations with high intensity) Growth rates almost constant T e = 5keV, T i =1keV Collisional damping negligible, Landau damping very strong kλ D SRS activity above all as absolute instability around n c /4 resonance

10 Spectrally resolved reflectivity (1D PIC) SBS takes place first, later saturates at due to density perturbations SRS excited at n c /4 (growing as absolute instability) and producing ω 0 /2 backscattered light SRS excited at n c /16 resonance point for wave backscattered at n c /4 producing ω 0 /4 forward scattered light Raman cascade could continue if scattered light exceeds threshold condition mainly O. Klimo, collaboration with V. Tikhonchuk, CELIA, Bordeaux Reflectivity and spectral energy density

11 SI Cavitation in corona Absolute instability grows locally in time pond. force expels electrons, ions undergo Coulomb explosion cavitation trapped em. field Cavitation at n c /16 much more pronounced Cavities spread due to spectral Ion density 1D simulation above, 2D below spreading of light Cavities stable for 10s of picoseconds in 1D, in 2D they appear and disappear continuously Cavitation mechanism converts em. field energy into kinetic energy Small portion of laser pulse propagates beyond Conclusion: collective effects responsible for absorption, rather than ib

12 SI Laser intensity dependence Reflectivity Spectra of electrons After transient stage, reflectivity saturates at 40% almost independent of I. For W/cm 2 same physics but different repartition between the collisional and collisionless processes absorption mostly collisional. Collisional absorption leads to heating of thermal electrons from 2.5 kev at the lowest intensity to 3 kev and to 4 kev for higher intensities. With increasing laser intensity 70% and 93% of absorbed energy is contained in hot electrons. Hot electron temperature does not strongly depend on I kev

13 Ion acceleration by fs laser pulses quasineutral acceleration in thin solid foils electrons are heated by the laser (TNSA) or shifted towards the target interior by ponderomotive force (RPA) but not removed from the target non quasineutral acceleration in clusters electrons are partially but rapidly removed from the cluster, making the cluster positively charged small clusters Coulomb explosion larger clusters Coulomb explosion together with ambipolar (hydrodynamic) expansion

14 Circular x linear polarization 1D PIC parameters: laser I= W/cm 2, 100 fs, λ=800 nm foil C 6+ ions, cm -3, 32 nm O. Klimo et al., Phys. Rev. ST-AB 11, (2008) Circular polarization (CP) Linear polarization (LP) RPA (radiation pressure acceleration) missing 2ω0 ponderomotive force Sheath (TNSA) acceleration, here break-out afterburner (BOA) regime

15 Ion acceleration by CP laser Figures from 1D PIC: laser I= W/cm 2, 80 fs (rect. profile), λ=800 nm laser incident from left, foil C 6+ ions, cm -3, 200 nm Ion acceleration takes place on the front side of the foil (the local electric field strength in TV/m is plotted by the gray color transition) Ballistic evolution helps to make the acceleration process stable after the whole foil is accelerated. t=10 fs t=25 fs Ballistic evolution in the average velocity frame

16 Monoenergetic ion beam in RPA Very high contrast + very thin foil + circular polarization = the whole foil may be accelerated as compact block of quasineutral plasma Circular polarization + no preplasma + normal incidence = production of very fast electrons suppressed ion acceleration at the front side of the foil The foil is accelerated as a whole and all ion species attain the same velocity, heavier ions absorb the most of energy The acceleration process is well described by the momentum transfer from the laser pulse to the foil (non-relativistic expression) E ion ~ I L 2 It works like acceleration of solid mirror by laser The acceleration efficiency is 2β η where β = v i /c = 1 + β Flat-top profile of laser beam important to suppress resonance absorption at the edges of laser spot to avoid explosion of ion bunch

17 Ion velocity and ion spectrum

18 RPA suited for high intensities Solid density bunch Duration:100nm/c= 0.3 fs Stable acceleration to GeV energies shown for I= Wcm -2, H-foil of n e = 100 n c, d = 350 nm (Simulations: B. Qiao et al, PRL 2009) Experimental verification partially Henig et al., PRL 103, (2009) intensity 5x10 19 W/cm 2, also M. Zepf - Astra-Gemini > 3x10 20 W/cm 2, not yet convincing

19 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.

20 TNSA very thin foils Electron recirculation improves acceleration efficiency Very thin foils if target thickness L < pulse length/2 a few μm s electron recirculation The thinner foil the better electron slowing down and lateral losses minimized 100 nm and thinner foils ultrahigh contrast needed double plasma mirror CEA Saclay Diamond-like carbon (DLC) foils (2-100 nm), 13 MeV protons (1.6 %), 71 MeV carbons (10%), max for 5 nm thick foil for laser 1.2 J, 45 fs, W/cm 2, MBI Berlin Steinke et al. T. Ceccotti et al.

21 Reduced mass targets Lateral energy losses from focal region are suppressed laser pulse of duration 350 fs, λ=529 nm, Iλ Wcm -2 μm 2, beam width FWHM = 6 μm is incident (incidence angle 45º) on a thin Au foil (thickness 2 μm) with reduced target transverse surface area down to μm 2 S. Buffechoux, J. Psikal et al., Phys. Rev. Lett (2010) constant thickness variable surface (a) RCF with hole Magnetic spectrometer laser Au 2 µm thick Au 2 µm thick + 10 µm thick 0.1 n c nanofoam upfront 10 1 Au 2 µm thick Au 2 µm thick + 10 µm thick 0.1 n c nanocloth upfront 5 (b) Surface (mm²) Surface (mm²) 1 10

22 Influence of surface structure Absorption enhancement for high contrast laser pulses

23 Monolayer of microspheres on foil There is no big difference between structure shapes. We proposed using thin foil covered by monolayer of closely packed polystyrene spheres of various size. Can be prepared by self assembly at water/air interface. Proposed target are quite simple for fabrication and optimization one free parameter. SEM image Optimum microsphere diameter for laser absorption and maximum proton energy is close to laser wavelength. Hot electron recirculation is more important for energy transformation efficiency than for the maximum ion energy.

24 2D simulations for GIST 100 TW experiment Mylar foil 900 nm thick without/with monolayer of polystyrene microspheres of diameter 266, 530 and 900 nm Laser W/cm 2, 30 fs, spot Ø 6 μm (in simulations 2.6 μm) Total proton energy (protons > 0.5 MeV), in shot comparison normalized on foil value, absolute acceleration efficiency in simulations is η = 5 % for 530 nm spheres Max. proton energy ~30 MeV for GIST-PW

25 3D PIC simulations Very very preliminary (MSc. thesis of J. Vyskočil) Longitudinal E field Ion density in various moments (section in plane where spheres do not touch 9 spheres, normal incidence, plane wave, periodic conditions in lateral directions Absorption by pure 544 nm foil 18%, absorption 46% with 528nm spheres

26 PW thin double layer foils Thin double layer foils 15 nm thick higher Z material (average ion of Si 3 O 4 n e = 320 n c ) + 10 nm proton layer (n i = 30 n c same as proton density in plastics Compared with foil of C 6+ H + of n e = 320 n c, 25 nm thick Laser I = W/cm2, 30 fs, spot FW at exp( 2) = 6λ Up normal incidence, double layer left longitudinal x transverse proton velocity; middle proton spectrum and right comparison of proton spectra in 5 cone around normal with that for plastic foil Lower right angle of incidence 30, double layer foil

27 Plans for future Interpretation of PW APRI GIST experiment Ion acceleration experiment (T. Ceccotti) is scheduled for this May at 100 TW laser at CEA, Saclay, France it will use microsphere layer + foils as in GIST and foils with profiled surface proposed by French team Collaboration with ELI-Beamlines team in preparatory experiments design and interpretation Theoretical studies of ion acceleration by fs laser pulses Studies of interaction physics relevant to shock ignition Fluid simulations of nanosecond laser-plasma interactions Studies of laser interactions with undercritical foams

28 Thank you for attention I would like to invite you to 10 th Direct Drive and Fast Ignition Workshop Prague, May 27-30, that we organize with the support of ESF RNP SILMI as traditional meeting of mainly theoreticians on advanced ICF schemes