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We present a novel 2D reduced numerical model for stimulated Raman scattering (SRS) in laser fusion plasmas in which envelope equations for the electromagnetic fields are coupled to a hybrid description of the electron species. Specifically, the electron distribution is split between a bulk part described by a Zakharov-like linear model and a kinetic tail discretized using a particle-in-cell-like (PIC) scheme. By avoiding to sample the bulk-electron distribution, this approach greatly reduces the numerical cost of SRS simulations compared with PIC codes, while still being able to describe the nonlinear evolution of the electron tail and trapping-related kinetic phenomena. First, our model is shown to reproduce accurately the linear Landau damping of an infinitesimal electron plasma wave (EPW) whose phase velocity falls into the tail of the electron distribution. Then, applying it to the simulation of the trapped-particle modulational instability of a large-amplitude EPW, results comparable to those of previously published 2D Vlasov simulations are obtained. Finally, we simulate the excitation of kinetic backward SRS from a single strong laser speckle (λ = 0.527 μ m , I = 10 16 W cm − 2 ) in an underdense ( n e = 0.036 n c ) plasma, which drives an EPW with wavenumber k λ D ≈ 0.34. The model predictions fairly agree with the results of a PIC simulation regarding the kinetic saturation mechanisms (i.e., trapped-particle instabilities), and with experimental data and Vlasov simulations related to the frequency shift of nonlinear EPWs. For this SRS simulation, we estimate that our hybrid model is over an order of magnitude less costly than an equivalent PIC simulation due to the lower particle count. [ABSTRACT FROM AUTHOR]

Multi-petawatt laser systems will open up a novel interaction regime mixing collective plasma and quantum electrodynamic processes, giving rise to prolific generation of gamma-ray photons and electron–positron pairs. Here, using particle-in-cell simulations, we investigate the physics of the interaction of a 1024 W cm−2 intensity, 30 fs duration, circularly polarized laser pulse with a long deuterium plasma at classically overcritical electron density (1022 cm−3). We show that radiative trapping of the plasma electrons causes a high-density (∼5 × 1023 cm−3), quasineutral electron–ion bunch to form inside the laser pulse. This phenomenon is accompanied by up to ∼40% energy conversion efficiency of the laser into gamma rays. Moreover, we find that both the radiation-modified Laplace force and the longitudinal electric field exerted on the positrons created by the multiphoton Breit–Wheeler process can accelerate them to GeV-range energies. We develop a theoretical model, the predictions of which provide a good match to the simulation results. Finally, we address the influence of the ion mass, showing that the laser absorption and positron acceleration is enhanced with deuterons compared to protons. [ABSTRACT FROM AUTHOR]

We numerically investigate the impact of Coulomb collisions on the ion dynamics in high-Z, solid density caesium hydride and copper targets, irradiated by high-intensity (), ultrashort (∼10 fs), circularly polarized laser pulses, using particle-in-cell simulations. Collisions significantly enhance electron heating, thereby strongly increasing the speed of a shock wave launched in the laser-plasma interaction. In the caesium hydride target, collisions between the two ion species heat the protons to ∼100−1000 eV temperatures. However, in contrast to previous work (A E Turrell et al 2015 Nat. Commun. 6 8905), this process happens in the upstream only, due to nearly total proton reflection. This difference is ascribed to distinct models used to treat collisions in dense/cold plasmas. In the case of a copper target, ion reflection can start as a self-amplifying process, bootstrapping itself. Afterwards, collisions between the reflected and upstream ions heat these two populations significantly. When increasing the pulse duration to 60 fs, the shock front more clearly decouples from the laser piston, and so can be studied without direct interference from the laser. The shock wave formed at early times exhibits properties typical of both hydrodynamic and electrostatic shocks, including ion reflection. At late times, the shock is seen to evolve into a hydrodynamic blast wave. [ABSTRACT FROM AUTHOR]

Two-dimensional particle-in-cell simulations are performed to study laser-induced proton acceleration from solid-density targets in the presence of laser-generated preformed plasma. The preplasma generation and hydrodynamics are described using a one-dimensional Lagrangian code. The electron acceleration mechanism is shown to depend on the plasma scale length, exhibiting a transition from jvector ×Bvector heating to standing wave heating as smoother and smoother profiles are considered. Accordingly, the relativistic electron temperature and the cutoff proton energy are found to increase with the preplasma characteristic length. [ABSTRACT FROM AUTHOR]

We report on two-dimensional simulations of parametric instabilities excited by a single, intense (2 × 1016 W cm−2), wavelength-sized laser speckle immersed in a nonuniform CH plasma close to the quarter-critical density. A first set of simulations is performed using a Zakharov-type reduced model. This newly developed simulation tool, which we present in detail, describes Stimulated Raman Scattering (SRS), Two-Plasmon Decay (TPD), Stimulated Brillouin Scattering, and secondary decays involving ion acoustic waves. Because of the high speckle intensity considered, strong electron plasma waves (EPWs) are driven via TPD, whose beating induces fast-collapsing ion cavities. Yet, Zakharov equations are notoriously incapable of modeling collapse arrest due to their neglect of kinetic processes dissipating energy from small-scale EPW packets, thus resulting in nonphysical density evolution. Transit-time damping is such a process, and we show that its inclusion in the reduced model allows for a self-consistent description of all phases of the collapse. The accuracy of our model is checked against a second simulation, performed using a particle-in-cell (PIC) code run under similar conditions. Good qualitative agreement is obtained, particularly in regard to the cavities' dynamics. Still, an excessive SRS reflectivity is predicted by the reduced model, which we ascribe to missing kinetic effects. The influence of electron trapping and heating on the decay of EPWs and saturation of SRS is assessed based on the PIC simulation. [ABSTRACT FROM AUTHOR]

Fast electron scattering on plasma ions due to stimulated Bremsstrahlung is investigated and modeled. Comparison with Coulomb scattering suggests that stimulated Bremsstrahlung scattering can be dominant in low-density, radiation-driven plasmas, provided that the radiation spectrum has a sufficiently high brightness temperature in the neighborhood of the plasma frequency. While stimulated Bremsstrahlung scattering cannot be easily observed in laboratory plasmas due to their small size, it should operate in large-scale astrophysical plasmas, such as those met in the flaring solar corona. The effect of the solar microwave radiation on fast-electron scattering is evaluated through a parameterized flaring corona model. We find that stimulated Bremsstrahlung greatly enhances the fast-electron scattering frequency in the flare magnetic loop, leading the transport of deka-keV electrons to occur in the diffusion regime, characterized by significant precipitation rates. This prediction is consistent with the interpretation of the above-loop-top hard x-ray and microwave emissions from the X3.1 flare of August 24, 2002. Our analysis indicates that stimulated Bremsstrahlung may play an essential role in the dynamics of fast electrons trapped in solar flare loops. [ABSTRACT FROM AUTHOR]

A theoretical and numerical analysis is carried out for turbulent collisionless shocks mediated by the ion-Weibel instability during high-velocity plasma collisions. We develop a simple model based on the coalescence dynamics of the ion current filaments, which predicts the spatio-temporal evolution of the magnetic fluctuations formed in the upstream plasma region. From comparison with particle-in-cell simulations, our model is shown to correctly capture the magnetic-field and ion-beam properties during the early-time shock propagation. [ABSTRACT FROM AUTHOR]

We present a particle-in-cell simulation of the generation of a collisionless strong shock in a dense plasma driven by an ultra-intense, plane-wave laser pulse. A linear theory analysis, based on a multi-waterbag model of the particle distributions, highlights the role of the laser-heated electrons in triggering the Weibel-like instability causing shock formation. It is demonstrated that the returncurrent electrons play a major role in the instability development as well as in the determination of the saturated magnetic field. By contrast, the ions are found of minor importance in driving the instability and the magnetic field fluctuations responsible for their isotropization. Finally, we show that a Weibel-mediated shock can also be generated by a focused laser pulse of large enough spot size. [ABSTRACT FROM AUTHOR]

We present a predictive model of the nonlinear phase of the Weibel instability induced by two symmetric, counter-streaming ion beams in the non-relativistic regime. This self-consistent model combines the quasilinear kinetic theory of Davidson et al. [Phys. Fluids 15, 317 (1972)] with a simple description of current filament coalescence. It allows us to follow the evolution of the ion parameters up to a stage close to complete isotropization, and is thus of prime interest to understand the dynamics of collisionless shock formation. Its predictions are supported by 2-D and 3-D particle-in-cell simulations of the ion Weibel instability. The derived approximate analytical solutions reveal the various dependencies of the ion relaxation to isotropy. In particular, it is found that the influence of the electron screening can affect the results of simulations using an unphysical electron mass. [ABSTRACT FROM AUTHOR]

Isochoric heating of solid-density matter up to a few tens of eV is of interest for investigating astrophysical or inertial fusion scenarios. Such ultra-fast heating can be achieved via the energy deposition of short-pulse laser generated electrons. Here, we report on experimental measurements of this process by means of time- and space-resolved optical interferometry. Our results are found in reasonable agreement with a simple numerical model of fast electron-induced heating. [ABSTRACT FROM AUTHOR]

We present an exact calculation of the power spectrum of the electromagnetic fluctuations in a relativistic equilibrium plasma described by Maxwell-Jüttner distribution functions. We consider the cases of wave vectors parallel or normal to the plasma mean velocity. The relative contributions of the subluminal and supraluminal fluctuations are evaluated. Analytical expressions of the spatial fluctuation spectra are derived in each case. These theoretical results are compared to particle-in-cell simulations, showing a good reproduction of the subluminal fluctuation spectra. [ABSTRACT FROM AUTHOR]

An improved Monte Carlo collisional scheme modeling both elastic and inelastic interactions has been implemented into the particle-in-cell code CALDER [E. Lefebvre et al., Nucl. Fusion 43, 629 (2003)]. Based on the technique proposed by Nanbu and Yonemura [J. Comput. Phys. 145, 639 (1998)] allowing to handle arbitrarily weighted macro-particles, this binary collision scheme uses a more compact and accurate relativistic formulation than the algorithm recently worked out by Sentoku and Kemp [J. Comput. Phys. 227, 6846 (2008)]. Our scheme is validated through several test cases, demonstrating, in particular, its capability of modeling the electrical resistivity and stopping power of a solid-density plasma over a broad parameter range. A relativistic collisional ionization scheme is developed within the same framework, and tested in several physical scenarios. Finally, our scheme is applied in a set of integrated particle-in-cell simulations of laser-driven fast electron transport. [ABSTRACT FROM AUTHOR]

A novel numerical modeling of field ionization in PIC (Particle In Cell) codes is presented. Based on the quasistatic approximation of the ADK (Ammosov Delone Krainov) theory and implemented through a Monte Carlo scheme, this model allows for multiple ionization processes. Two-dimensional PIC simulations are performed to analyze the cut-off energies of the laser-accelerated carbon ions measured on the UHI 10 Saclay facility. The influence of the target and the hydrocarbon pollutant composition on laser-accelerated carbon ion energies is demonstrated. [ABSTRACT FROM AUTHOR]

The interest in relativistic beam-plasma instabilities has been greatly rejuvenated over the past two decades by novel concepts in laboratory and space plasmas. Recent advances in this long-standing field are here reviewed from both theoretical and numerical points of view. The primary focus is on the two-dimensional spectrum of unstable electromagnetic waves growing within relativistic, unmagnetized, and uniform electron beam-plasma systems. Although the goal is to provide a unified picture of all instability classes at play, emphasis is put on the potentially dominant waves propagating obliquely to the beam direction, which have received little attention over the years. First, the basic derivation of the general dielectric function of a kinetic relativistic plasma is recalled. Next, an overview of two-dimensional unstable spectra associated with various beam-plasma distribution functions is given. Both cold-fluid and kinetic linear theory results are reported, the latter being based on waterbag and Maxwell-Jüttner model distributions. The main properties of the competing modes (developing parallel, transverse, and oblique to the beam) are given, and their respective region of dominance in the system parameter space is explained. Later sections address particle-in-cell numerical simulations and the nonlinear evolution of multidimensional beam-plasma systems. The elementary structures generated by the various instability classes are first discussed in the case of reduced-geometry systems. Validation of linear theory is then illustrated in detail for large-scale systems, as is the multistaged character of the nonlinear phase. Finally, a collection of closely related beam-plasma problems involving additional physical effects is presented, and worthwhile directions of future research are outlined. [ABSTRACT FROM AUTHOR]

Abstract: Implicit particle-in-cell codes offer advantages over their explicit counterparts in that they suffer weaker stability constraints on the need to resolve the higher frequency modes of the system. This feature may prove particularly valuable for modeling the interaction of high-intensity laser pulses with overcritical plasmas, in the case where the electrostatic modes in the denser regions are of negligible influence on the physical processes under study. To this goal, we have developed the new two-dimensional electromagnetic code ELIXIRS (standing for ELectromagnetic Implicit X-dimensional Iterative Relativistic Solver) based on the relativistic extension of the so-called Direct Implicit Method [D. Hewett, A.B. Langdon, Electromagnetic direct implicit plasma simulation, J. Comput. Phys. 72 (1987) 121–155]. Dissipation-free propagation of light waves into vacuum is achieved by an adjustable-damping electromagnetic solver. In the high-density case where the Debye length is not resolved, satisfactory energy conservation is ensured by the use of high-order weight factors. In this paper, we first derive the electromagnetic direct implicit method as a simplified Newton scheme. Its linear properties are then investigated through numerically solving the relation dispersions obtained for both light and plasma waves, accounting for finite space and time steps. Finally, our code is successfully benchmarked against explicit particle-in-cell simulations for two kinds of physical problems: plasma expansion into vacuum and relativistic laser–plasma interaction. In both cases, we will demonstrate the robustness of the implicit solver for crude discretizations, as well as the gains in efficiency which can be realized over standard explicit simulations. [Copyright &y& Elsevier]

Collisionless beam-plasma instabilities are expected to play a crucial role during the early phase of the relativistic electron transport in the Fast Ignition scheme. This Letter presents a theoretical study of these instabilities in a two-dimensional geometry, highlighting the role of unstable modes propagating obliquely to the beam direction. The main features identified through a linearized analysis in a very general kinetic framework are examined by means of a particle-in-cell simulation. Good agreement between the two approaches is observed in the linear phase. Beam trapping is found to account for the nonlinear wave saturation. [ABSTRACT FROM AUTHOR]

It is generally considered that the linear filamentation instability encountered when two counter streaming electron beams interpenetrate is purely transverse. Exact and approximated results are derived in the relativistic fluid approximation showing that within some parameter range, filamentation can be indeed almost longitudinal with [formula], where γb is the relativistic factor of the beam. Temperature effects are then evaluated through relativistic kinetic theory and yield even fewer transverse filamentation modes. In the cold case, the transverse approximation overestimates the growth rate by a factor ∝γb. [ABSTRACT FROM AUTHOR]

Abstract: The most unstable modes encountered in relativistic electron beam plasma interaction are found for oblique wave vectors. We characterize these modes within the framework of the linear theory and explore temperature effects. Comparisons with 2D PIC simulation show a good agreement with the theory. [Copyright &y& Elsevier]

Using one-dimensional, long-timescale particle-in-cell simulations, we study the processes of ion acceleration from the interaction of ultraintense ( 10 20 W cm − 2 ), ultrashort (30 fs) laser pulses with near-critical, nonuniform gas targets. The considered initially neutral, nitrogen gas density profiles mimic those delivered by an already developed noncommercial supersonic gas shock nozzle: they have the generic shape of a narrow (20 μ m wide) peak superimposed on broad (∼1 mm, ∼ 180 μ m scale length), exponentially decreasing ramps. While keeping its shape constant, we vary its absolute density values to identify the interaction conditions leading to collisionless shock-induced ion acceleration in the gas density ramps. We find that collisionless electrostatic shocks (CES) form when the laser pulse is able to shine through the central density peak and deposit a few 10% of its energy into it. Under our conditions, this occurs for a peak electron density between 0.35 n c and 0.7 n c . Moreover, we show that the ability of the CES to reflect the upstream ions is highly sensitive to their charge state and that the laser-induced electron pressure gradients mainly account for shock generation, thus highlighting the benefit of using sharp gas profiles, such as those produced by shock nozzles. [ABSTRACT FROM AUTHOR]

Using high-resolution, two-dimensional particle-in-cell simulations, we investigate numerically the mechanisms of terahertz (THz) emissions in sub-micrometer-thick carbon solid foils driven by ultraintense (∼ 10 20 W cm − 2 ), ultrashort (30 fs) laser pulses at normal incidence. The considered range of target thicknesses extends down to the relativistic transparency regime that is known to optimize ion acceleration by femtosecond laser pulses. By disentangling the fields emitted by longitudinal and transverse currents, our analysis reveals that, within the first picosecond after the interaction, THz emission occurs in bursts as a result of coherent transition radiation by the recirculating hot electrons and antenna-type emission by the shielding electron currents traveling along the fast-expanding target surfaces. [ABSTRACT FROM AUTHOR]

The formation of collisionless shocks mediated by the ion Weibel instability is addressed theoretically and numerically in the nonrelativistic limit. First, the model developed in C. Ruyer et al., Phys. Plasmas 22, 032102 (2015) for the weakly nonlinear ion Weibel instability in a symmetric two-stream system is shown to be consistent with recent experimental and simulation results. Large-scale kinetic simulations are then performed to clarify the spatiotemporal evolution of the magnetic-field and plasma properties in the subsequent strongly nonlinear phase leading to shock formation. A simple analytical model is proposed which captures the simulation results up to a point close to ion isotropization. Electron screening effects are found important in the instability dynamics, so that numerical simulations using a nonphysical electron mass should be considered with caution. [ABSTRACT FROM AUTHOR]

The nonlinear evolution of current filaments generated by the Weibel-type filamentation instability is a topic of prime interest in space and laboratory plasma physics. In this paper, we investigate the stability of a stationary periodic chain of nonlinear current filaments in counterstreaming pair plasmas. We make use of a relativistic four-fluid model and apply the Floquet theory to compute the two-dimensional unstable eigenmodes of the spatially periodic system. We examine three different cases, characterized by various levels of nonlinearity and asymmetry between the plasma streams: a weakly nonlinear symmetric system, prone to purely transverse merging modes; a strongly nonlinear symmetric system, dominated by coherent drift-kink modes whose transverse periodicity is equal to, or an integer fraction of the unperturbed filaments; a moderately nonlinear asymmetric system, subject to a mix of kink and bunching-type perturbations. The growth rates and profiles of the numerically computed eigenmodes agree with particle-in-cell simulation results. In addition, we derive an analytic criterion for the transition between dominant filament-merging and drift-kink instabilities in symmetric two-beam systems. [ABSTRACT FROM AUTHOR]

We present novel structured targets capable of collimating laser-generated fast electrons through dense plasmas. The proposed targets are made of narrow high- and low-Z filaments leading to a transversely modulated electrical resistivity profile. When featuring a spatially decreasing density, these targets permit both to guide the fast electrons and reduce their angular dispersion. The principle of our target design is explained by a theoretical model. Two-dimensional particle-in-cell simulations are performed to demonstrate its efficiency. [ABSTRACT FROM AUTHOR]

We present a self-consistent semianalytical model of the relativistic plasma minor, based on the exact computation of the laser-driven electron surface oscillations within the cold-fluid approximation. Valid for arbitrary solid densities, laser incidence angle, and a large set of laser intensities (1018-1021 W/cm2), the model unravels different regimes of harmonic generation. In particular, it is found that efficient conversion of p-polarized laser pulses into high-order harmonics well above the plasma frequency requires either high laser intensities, low plasma densities, or incidence angles larger than a threshold value. This critical angle corresponds to a transition between a regime where the electron surface dynamics is mostly governed by the laser J x B force and a "cyclotron Brunei" regime, where electrons perform many cyclotron gyrations when moving into the vacuum. Under conditions relevant to current laser experiments, the latter regime gives rise to nonmonotonic variations of the harmonic yield with the laser field. Our predictions are supported by an extensive parametric study performed with highly resolved one-dimensional particle-in-cell simulations. [ABSTRACT FROM AUTHOR]

In this article, we use the general theory derived in Paper I [M. Tacu and D. Bénisti, Phys. Plasmas 29, 052108 (2022)] in order to address several long-standing issues regarding nonlinear electron plasma waves (EPWs). First, we discuss the relevance and practical usefulness of stationary solutions to the Vlasov–Poisson system, the so-called Bernstein–Greene–Kruskal modes, to model slowly varying waves. Second, we derive an upper bound for the wave breaking limit of an EPW growing in an initially Maxwellian plasma. Moreover, we show a simple dependence of this limit as a function of k λ D , with k being the wavenumber and λD the Debye length. Third, we explicitly derive the envelope equation ruling the evolution of a slowly growing plasma wave, up to an amplitude close to the wave breaking limit. Fourth, we estimate the growth of the transverse wavenumbers resulting from wavefront bowing by solving the nonlinear, nonstationary, ray tracing equations for the EPW, together with a simple model for stimulated Raman scattering. [ABSTRACT FROM AUTHOR]

Following a recent work by Sanz et al. [Phys. Rev. E 85, 046411 (2012)], we elaborate upon a one-dimensional model describing the interaction between an ultra-intense, normally incident laser pulse and an overdense plasma. The analytical solutions of the reflected laser field, the electrostatic field, and the plasma surface oscillation are obtained within the cold-fluid approximation. The high-order harmonic spectrum is calculated from the exact solution of the plasma surface oscillations. In agreement with particle-in-cell simulations, two regimes of harmonic generation are predicted: for moderately relativistic laser intensities, or high plasma densities, the harmonic spectrum is determined by the discontinuity in the derivative of the reflected field when the electron plasma boundary oscillates across the fixed ion boundary. For higher intensities, the electron plasma boundary is confined inside the ion region and oscillates at relativistic velocities, giving rise to a train of reflected attosecond pulses. In both cases, the harmonic spectrum obeys an asymptotic ω-4 scaling. The acceleration of electrons and the related laser absorption efficiency are computed by a test particle method. The model self-consistently reproduces the transition between the 'anomalous skin effect' and the 'J × B' heating predicted by particle-in-cell simulations. Analytical estimates of the different scalings are presented. [ABSTRACT FROM AUTHOR]

In this paper, we derive coupled envelope equations modeling the growth of stimulated Raman scattering (SRS) in a multi-dimensional geometry and accounting for nonlinear kinetic effects. In particular, our envelope equations allow for the nonlinear reduction of the Landau damping rate, whose decrease with the plasma wave amplitude depends on the rate of side-loss. Account is also made of the variations in the extent of the plasma wave packet entailed by the collisionless dissipation due to trapping. The dephasing between the electron plasma wave (EPW) and the laser drive, as well as the self-focussing of the plasma wave, both induced by the EPW nonlinear frequency shift, are also included in our envelope equations. These equations are solved in a multi-dimensional geometry using our code dubbed BRAMA, whose predictions regarding the evolution of Raman reflectivity as a function of the laser intensity are compared against previously published particle in cell results, thus illustrating the ability of BRAMA simulations to provide the correct laser threshold intensity for SRS as well as the right order of magnitude of Raman reflectivity above threshold. [ABSTRACT FROM AUTHOR]

Two-dimensional particle-in-cell simulations were performed to study the channeling in long (>500μm) underdense plasmas of long duration (>10 ps), relativistic-intensity (I=1018-20 W/cm2) laser pulses. We describe five different types of channeling behaviors, and the corresponding ranges of plasmas and laser parameters are given. In all of these cases, self-corrective mechanisms come into play, which help straighten the channel provided that the laser pulse is long enough to push the plasma ahead. High-quality channels are observed when ξ=(n/nc(1+a02/2)-0.5)1.22πW0/λa0<0.2, where nc is the critical density, a0 is the vacuum vector potential, W0 is the waist of the laser pulse, and λ is its wavelength. We also define a method to measure the channeling velocity without ambiguity, and we establish scaling laws. It is then possible to use them to predict the channel front position in an inhomogeneous plasma, such as the coronal plasma of a fast ignition target, and to deduce the energy needed to reach the critical density. Our scaling laws indicate that the required laser energy is 50 times higher when using a laser with I=1020 W/cm2 than with I=1018 W/cm2. Our predictions are compared with a simulation of the laser propagation through a mm-long exponential plasma. [ABSTRACT FROM AUTHOR]

A technique developed to measure in time and space the dynamics of the electron populations resulting from the irradiation of thin solids by ultraintense lasers is presented. It is a phase reflectometry technique that uses an optical probe beam reflecting off the target rear surface. The phase of the probe beam is sensitive to both laser-produced fast electrons of low-density streaming into vacuum and warm solid density electrons that are heated by the fast electrons. A time and space resolved interferometer allows to recover the phase of the probe beam sampling the target. The entire diagnostic is computationally modeled by calculating the probe beam phase when propagating through plasma density profiles originating from numerical calculations of plasma expansion. Matching the modeling to the experimental measurements allows retrieving the initial electron density and temperature of both populations locally at the target surface with very high temporal and spatial resolution (∼4 ps,6 μm). Limitations and approximations of the diagnostic are discussed and analyzed. [ABSTRACT FROM AUTHOR]

Beam-plasma instabilities are a key physical process in many astrophysical phenomena. Within the fireball model of Gamma ray bursts, they first mediate a relativistic collisionless shock before they produce upstream the turbulence needed for the Fermi acceleration process. While non-relativistic systems are usually governed by flow-aligned unstable modes, relativistic ones are likely to be dominated by normally or even obliquely propagating waves. After reviewing the basis of the theory, results related to the relativistic kinetic regime of the poorly-known oblique unstable modes will be presented. Relevant systems besides the wellknown electron beam-plasma interaction are presented, and it is shown how the concept of modes hierarchy yields a criterion to assess the proton to electron mass ratio in Particle in cell simulations. [ABSTRACT FROM AUTHOR]

Abstract: We have analyzed the acceleration of laser-generated protons, produced at the rear surface of flat-cone targets irradiated by an ultra-intense (I∼5×1019 W/cm2) short (400fs) laser pulse. We used different target sizes and shapes in order to find the optimum target layout. We find that for targets with a too narrow cone structure, the production of the hot electrons, driving the proton acceleration, is located prior to the accelerating rear surface of the target, resulting in a reduced maximum proton energy. [Copyright &y& Elsevier]

We study numerically the mechanisms of proton acceleration in gas–foil targets driven by an ultraintense femtosecond laser pulse. The target consists of a near-critical-density hydrogen gas layer of a few tens of microns attached to a $2\ \mathrm {\mu }$ m-thick solid carbon foil with a contaminant thin proton layer at its back side. Two-dimensional particle-in-cell simulations show that, at optimal gas density, the maximum energy of the contaminant protons is increased by a factor of $\sim$ 4 compared with a single foil target. This improvement originates from the near-complete laser absorption into relativistic electrons in the gas. Several energetic electron populations are identified, and their respective effect on the proton acceleration is quantified by computing the electrostatic fields that they generate at the protons' positions. While each of those electron groups is found to contribute substantially to the overall accelerating field, the dominant one is the relativistic thermal bulk that results from the nonlinear wakefield excited in the gas, as analysed recently by Debayle et al. (New J. Phys., vol. 19, 2017, 123013). Our analysis also reveals the important role of the neighbouring ions in the acceleration of the fastest protons, and the onset of multidimensional effects caused by the time-increasing curvature of the proton layer. [ABSTRACT FROM AUTHOR]

Using particle-in-cell simulations, we demonstrate an improvement of the target-normal-sheath acceleration (TNSA) of protons in non-periodically nanostructured targets with micron-scale thickness. Compared to standard flat foils, an increase in the proton cutoff energy by up to a factor of two is observed in foils coated with nanocones or perforated with nanoholes. The latter nano-perforated foils yield the highest enhancement, which we show to be robust over a broad range of foil thicknesses and hole diameters. The improvement of TNSA performance results from more efficient hot-electron generation, caused by a more complex laser–electron interaction geometry and increased effective interaction area and duration. We show that TNSA is optimized for a nanohole distribution of relatively low areal density and that is not required to be periodic, thus relaxing the manufacturing constraints. [ABSTRACT FROM AUTHOR]

We present a Monte Carlo implementation of Bremsstrahlung, Bethe-Heitler, and Coulomb Trident processes into the particle-in-cell (PIC) simulation framework. In order to address photon emission and electron-positron pair productions in a wide range of physical conditions, we derive the Bremsstrahlung and Bethe-Heitler cross sections taking account of screening effects in arbitrary ionized plasmas. Our calculations are based on a simple model for the atomic Coulomb potential that describes shielding due to both bound electrons, free electrons, and ions. We then detail a pairwise particle interaction algorithm suited to weighted PIC plasma simulations, for which we perform several validation tests. Finally, we carry out a parametric study of photon and pair production during high-energy electron transport through micrometric solid foils. Compared to the zero-dimensional model of Myatt et al. [Phys. Rev. E 76, 066409 (2009)], our integrated one-dimensional simulations pinpoint the importance of the electron energy losses resulting from the plasma expansion. [ABSTRACT FROM AUTHOR]

Powerful nanosecond laser-plasma processes are explored to generate discharge currents of a few 100 kA in coil targets, yielding magnetostatic fields (B-fields) in excess of 0.5 kT. The quasi-static currents are provided from hot electron ejection from the laser-irradiated surface. According to our model, which describes the evolution of the discharge current, the major control parameter is the laser irradiance I las λ las 2 . The space-time evolution of the B-fields is experimentally characterized by high-frequency bandwidth B-dot probes and proton-deflectometry measurements. The magnetic pulses, of ns-scale, are long enough to magnetize secondary targets through resistive diffusion. We applied it in experiments of laser-generated relativistic electron transport through solid dielectric targets, yielding an unprecedented 5-fold enhancement of the energy-density flux at 60 μm depth, compared to unmagnetized transport conditions. These studies pave the ground for magnetized high-energy density physics investigations, related to laser-generated secondary sources of radiation and/or high-energy particles and their transport, to high-gain fusion energy schemes, and to laboratory astrophysics. [ABSTRACT FROM AUTHOR]

We investigate the target normal sheath acceleration of protons in thin aluminum targets irradiated at a relativistic intensity by two time-separated ultrashort (35 fs) laser pulses. When the full-energy laser pulse is temporally split into two identical half-energy pulses, and using target thicknesses of 3 and 6 μm, we observe experimentally that the second half-pulse boosts the maximum energy and charge of the proton beam produced by the first half-pulse for time delays below ∼0.6–1 ps. Using two-dimensional particle-in-cell simulations, we examine the variation of the proton energy spectra with respect to the time-delay between the two pulses. We demonstrate that the expansion of the target front surface caused by the first pulse significantly enhances the hot-electron generation by the second pulse arriving after a few hundreds of fs time delay. This enhancement, however, does not suffice to further accelerate the fastest protons driven by the first pulse once three-dimensional quenching effects have set in. This implies a limit to the maximum time delay that leads to proton energy enhancement, which we theoretically determine. [ABSTRACT FROM AUTHOR]

We investigate the electron heating induced by a relativistic-intensity laser pulse propagating through a near-critical plasma. Using particle-in-cell simulations, we show that a specific interaction regime sets in when, due to the energy depletion caused by the plasma wakefield, the laser front profile has steepened to the point of having a length scale close to the laser wavelength. Wave breaking and phase mixing have then occurred, giving rise to a relativistically hot electron population following the laser pulse. This hot electron flow is dense enough to neutralize the cold bulk electrons during their backward acceleration by the wakefield. This neutralization mechanism delays, but does not prevent the breaking of the wakefield: the resulting phase mixing converts the large kinetic energy of the backward-flowing electrons into thermal energy greatly exceeding the conventional ponderomotive scaling at laser intensities>1021 W cm-2 and gas densities around 10%of the critical density.We develop a semi-numerical model, based on the Akhiezer–Polovin equations, which correctly reproduces the particle-in-cell-predicted electron thermal energies over a broad parameter range. Given this good agreement, we propose a criterion for full laser absorption that includes field-induced ionization. Finally, we show that our predictions still hold in a two-dimensional geometry using a realistic gas profile. [ABSTRACT FROM AUTHOR]

We experimentally investigate the fast (< 1 ps) isochoric heating of multi-layer metallic foils and subsequent high-pressure hydrodynamics induced by energetic electrons driven by high-intensity, high-contrast laser pulses. The early-time temperature profile inside the target is measured from the streaked optical pyrometry of the target rear side. This is further characterized from benchmarked simulations of the laser-target interaction and the fast electron transport. Despite a modest laser energy (< 1 J), the early-time high pressures and associated gradients launch inwards a strong compression wave developing over ≳ 10 ps into a ≈ 140 Mbar blast wave, according to hydrodynamic simulations, consistent with our measurements. These experimental and numerical findings pave the way to a short-pulse-laser-based platform dedicated to high-energy-density physics studies. [ABSTRACT FROM AUTHOR]

In this paper, we investigate, both experimentally and numerically, the backward stimulated Raman scattering (SRS) excited collectively by two laser pulses. The experiments have been carried out at the LULI facility using two co-propagating 1-µm wavelength, 1.5-ps duration laser pulses focused in a preformed underdense plasma. A particular emphasis is laid on the configuration where the pulses are focused side-by-side, with a lateral distance of 80-90 µm, but not simultaneously. It is experimentally demonstrated that a weak-intensity speckle, ineffective when fired alone in a preformed plasma, yields a significant SRS-induced reflectivity if launched a few picoseconds after a strong one. The data have been obtained by using both highly space-time resolved Thomson diagnostics and space-resolved SRS reflectivity measurements. By choosing either parallel or orthogonal polarizations for the two laser pulses, our experiments shed light on the role of either electrostatic or electromagnetic seeding in enhancing SRS from weak-intensity speckles. A major finding is that seeding operates over unexpectedly long times (15-20 ps under our experimental conditions). Similar results are obtained in lower-density plasmas, or when the weak pulse is smoothed by a random phase plate, thus leading to multiple speckle interaction, while the strong pulse is focused within the speckle pattern. The data are discussed with the help of particle-in-cell numerical simulations, which confirm the destabilizing effect of the strong pulse over the weak one after a short transient time. [ABSTRACT FROM AUTHOR]

We report on the first self-consistent numerical study of the feasibility of laser-driven relativistic pair shocks of prime interest for high-energy astrophysics. Using a QED-particle-in-cell code, we simulate the collective interaction between two counterstreaming electron-positron jets driven from solid foils by shortpulse (∼60 fs), high-energy (∼100 kJ) lasers. We show that the dissipation caused by self-induced, ultrastrong (>106 T) electromagnetic fluctuations is amplified by intense synchrotron emission, which enhances the magnetic confinement and compression of the colliding jets. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the saturation mechanisms of backward stimulated Raman scattering (BSRS) induced by nonlinear kinetic effects. In particular, we stress the importance of accounting for both the nonlinear frequency shift of the electron plasma wave and the growth of sidebands, in order to understand what stops the coherent growth of Raman scattering. Using a Bernstein-Greene-Kruskal approach, we provide an estimate for the maximum amplitude reached by a BSRS-driven plasma wave after the phase of monotonic growth. This estimate is in very good agreement with the results from kinetic simulations of stimulated Raman scattering using both a Vlasov and a Particle in Cell code. Our analysis, which may be generalized to a multidimensional geometry, should provide a means to estimate the limits of backward Raman amplification or the effectiveness of strategies that aim at strongly reducing Raman reflectivity in a fusion plasma. [ABSTRACT FROM AUTHOR]

Fast ignition (FI) inertial confinement fusion is a variant of inertial fusion in which DT fuel is first compressed to high density and then ignited by a relativistic electron beam generated by a fast (<20�ps) ultra-intense laser pulse, which is usually brought in to the dense plasma via the inclusion of a re-entrant cone. The transport of this beam from the cone apex into the dense fuel is a critical part of this scheme, as it can strongly influence the overall energetics. Here we review progress in the theory and numerical simulation of fast electron transport in the context of FI. Important aspects of the basic plasma physics, descriptions of the numerical methods used, a review of ignition-scale simulations, and a survey of schemes for controlling the propagation of fast electrons are included. Considerable progress has taken place in this area, but the development of a robust, high-gain FI ‘point design’ is still an ongoing challenge. [ABSTRACT FROM AUTHOR]

Laser plasma acceleration of electrons has progressed along with advances in laser technology. It is thus expected that the development in the near-future of multi-PW-class laser and facilities will enable a vast range of scientific opportunities for laser plasma acceleration research. On one hand, high peak powers can be used to explore the extremely high intensity regime of laser wakefield acceleration, producing for example large amounts of electrons in the GeV range or generating high energy photons. On the other hand, the available laser energy can be used in the quasi-linear regime to create accelerating fields in large volumes of plasma and study controlled acceleration in a plasma stage of externally injected relativistic particles, either electrons or positrons. In the frame of the Centre Interdisciplinaire de la Lumière EXtrême (CILEX), the Apollon-10P laser will deliver two beams at the 1PW and 10PW levels, in ultra-short ( ) pulses, to a target area dedicated to electron acceleration studies, such as the exploration of the non-linear regimes predicted theoretically, or multi-stage laser plasma acceleration. [ABSTRACT FROM AUTHOR]