[Show abstract][Hide abstract] ABSTRACT: The knowledge of the absolute energy distributions of the particles emitted from a clinical accelerator is important for the evaluation of Monte Carlo simulations developed for treatment planning. In this paper, an original approach is presented which allows to measure the absolute energy distribution of the electron beam delivered by a Varian 21Ex medical accelerator. The electron beam was characterized at the isocenter with calibrated image plates covering the exit window of a magnetic spectrometer. The characteristics of the electron beam emitted from an effective source have been inferred from the measurements using the Geant4 Monte Carlo code. The contribution of direct electrons to the absolute depth-dose curve in a water phantom is estimated.
[Show abstract][Hide abstract] ABSTRACT: The shock ignition scheme is an alternative approach, which aims to achieve ignition of fusion reactions in two subsequent steps: first, the target is compressed at a low implosion velocity and second, a strong converging shock is launched during the stagnation phase and ignites the hot spot. In this paper we describe the major elements of this scheme and recent achievements concerning the laser-plasma interaction, the crucial role of hot electrons in the shock generation, the shock amplification in the imploding shell and the ignition conditions.
No preview · Article · Jan 2016 · Plasma Physics and Controlled Fusion
[Show abstract][Hide abstract] ABSTRACT: A new deterministic method for calculating the dose distribution in the electron radiotherapy field is presented. The aim of this work was to validate our model by comparing it with the Monte Carlo simulation toolkit, GEANT4. A comparison of the longitudinal and transverse dose deposition profiles and electron distributions in homogeneous water phantoms showed a good accuracy of our model for electron transport, while reducing the calculation time by a factor of 50. Although the Bremsstrahlung effect is not yet implemented in our model, we propose here a method that solves the Boltzmann kinetic equation and provides a viable and efficient alternative to the expensive Monte Carlo modeling.
[Show abstract][Hide abstract] ABSTRACT: The electronic model is widely used for electron transport studies in a hot collisional plasma. However, the moment extraction of the electron–electron collision operator from the kinetic collision operator, for this angular moments model, is challenging and some approximations are required. In this work a characterisation of the electron–electron and electron–ion collision operators is given and the electron plasma transport coefficients are derived. It is shown that in the high Z limit the electronic model and the Fokker–Planck–Landau equation coincide in the case of near equilibrium. Also, in general, the electron–electron collision operator proposed for the electronic model recovers accurate electron transport plasma coefficients.
No preview · Article · Dec 2015 · Physica A: Statistical Mechanics and its Applications
[Show abstract][Hide abstract] ABSTRACT: The dynamics of self-generated magnetic B-fields produced following the interaction of a high contrast, high intensity (I > 1019 W cm−2) laser beam with thin (3 μm thick) solid (Al or Au) targets is investigated experimentally and numerically. Two main sources drive the growth of B-fields on the target surfaces. B-fields are first driven by laser-generated hot electron currents that relax over ∼10–20 ps. Over longer timescales, the hydrodynamicexpansion of the bulk of the target into vacuum also generates B-field induced by non-collinear gradients of density and temperature. The laser irradiation of the target front side strongly localizes the energy deposition at the target front, in contrast to the target rear side, which is heated by fast electrons over a much larger area. This induces an asymmetry in the hydrodynamicexpansion between the front and rear target surfaces, and consequently the associated B-fields are found strongly asymmetric. The sole long-lasting (>30 ps) B-fields are the ones growing on the target front surface, where they remain of extremely high strength (∼8–10 MG). These B-fields have been recently put by us in practical use for focusing laser-accelerated protons [B. Albertazzi et al., Rev. Sci. Instrum. 86, 043502 (2015)]; here we analyze in detail their dynamics and structure.
No preview · Article · Dec 2015 · Physics of Plasmas
[Show abstract][Hide abstract] ABSTRACT: A model providing an accurate estimate of the charge accumulation on the surface of a metallic target irradiated by a high-intensity laser pulse of fs-ps duration is proposed. The model is confirmed by detailed comparisons with specially designed experiments. Such a model is useful for understanding the electromagnetic pulse emission and the quasistatic magnetic field generation in laser-plasma interaction experiments.
Full-text · Article · Nov 2015 · Physical Review E
[Show abstract][Hide abstract] ABSTRACT: We investigate the interaction of trains of femtosecond microjoule laser
pulses with dielectric materials by means of a multi-scale model. Our
theoretical predictions are directly confronted with experimental observations
in soda-lime glass. We show that due to the low heat conductivity, a
significant fraction of the laser energy can be accumulated in the absorption
region. Depending on the pulse repetition rate, the material can be heated to
high temperatures even though the single pulse energy is too low to induce a
significant material modification. Regions heated above the glass transition
temperature in our simulations correspond very well to zones of permanent
material modifications observed in the experiments.
Full-text · Article · Nov 2015 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: The next generation of laser facilities will routinely produce relativistic particle beams from the interaction of intense laser pulses with solids and/or gases. Their modeling with Particle-In-Cell (PIC) codes needs dispersion-free Maxwell solvers in order to properly describe the interaction of electromagnetic waves with relativistic particles. A particular attention is devoted to the suppression of the numerical Cherenkov instability, responsible for the noise generation. It occurs when the electromagnetic wave is artificially slowed down because of the finite mesh size, thus allowing for the high energy particles to propagate with super-luminous velocities. In the present paper, we show how a slight increase of the light velocity in the Maxwell's equations enables to suppress this instability while keeping a good overall precision of calculations.
[Show abstract][Hide abstract] ABSTRACT: We present a formulation of the model of laser-plasma interaction (LPI) at hydrodynamical scales that couples the plasma dynamics with linear and nonlinear LPI processes, including the creation and propagation of high-energy electrons excited by parametric instabilities and collective effects. This formulation accounts for laser beam refraction and diffraction, energy absorption due to collisional and resonant processes, and hot electron generation due to the stimulated Raman scattering, two-plasmon decay, and resonant absorption processes. Hot electron (HE) transport and absorption are described within the multigroup angular scattering approximation, adapted for transversally Gaussian electron beams. This multiscale inline LPI-HE model is used to interpret several shock ignition experiments, highlighting the importance of target preheating by HEs and the shortcomings of standard geometrical optics when modeling the propagation and absorption of intense laser pulses. It is found that HEs from parametric instabilities significantly increase the shock pressure and velocity in the target, while decreasing its strength and the overall ablation pressure.
Full-text · Article · Oct 2015 · Physical Review E
[Show abstract][Hide abstract] ABSTRACT: Quasi-static magnetic-fields up to 800 T are generated in the interaction of intense laser pulses (500 J, 1 ns, 10(17) W cm(-2)) with capacitor-coil targets of different materials. The reproducible magnetic-field peak and rise-time, consistent with the laser pulse duration, were accurately inferred from measurements with GHz-bandwidth inductor pickup coils (B-dot probes). Results from Faraday rotation of polarized optical laser light and deflectometry of energetic proton beams are consistent with the B-dot probe measurements at the early stages of the target charging, up to t approximate to 0.35 ns, and then are disturbed by radiation and plasma effects. The field has a dipole-like distribution over a characteristic volume of 1 mm(3), which is consistent with theoretical expectations. These results demonstrate a very efficient conversion of the laser energy into magnetic fields, thus establishing a robust laser-driven platform for reproducible, well characterized, generation of quasi-static magnetic fields at the kT-level, as well as for magnetization and accurate probing of high-energy-density samples driven by secondary powerful laser or particle beams.
[Show abstract][Hide abstract] ABSTRACT: Hot electrons created in laser plasma interaction at laser intensities 1 - 10 PW cm - 2 in shock ignition scheme can deposit their energy in the shell of the target, augmenting the strength of the ignitor shock. Here, we present a model that describes the effect of the spatial profile of fast electron energy deposition on the dynamics of shock wave formation. A criterion of a strong shock formation is obtained for an arbitrary electron beam distribution function. It is shown that the time and the position of the shock formation are defined by the electron average stopping range, while the strength of the shock decreases as the width of electron energy distribution increases. The latter feature is explained by the fast electron target preheat. The conclusions of theoretical model are confirmed in numerical simulations. The pressure, the strength of the shock, and the efficiency of shock generation are calculated for different electron distributions with the same average stopping range.
No preview · Article · Oct 2015 · Physics of Plasmas
[Show abstract][Hide abstract] ABSTRACT: An exact analytic solution is found for the steady-state distribution function of fast electrons with an arbitrary initial spectrum irradiating a planar low-Z plasma with an arbitrary density distribution. The solution is applied to study the heating of a material by fast electrons of different spectra such as a monoenergetic spectrum, a step-like distribution in a given energy range, and a Maxwellian spectrum, which is inherent in laser-produced fast electrons. The heating of shock- and fast-ignited precompressed inertial confinement fusion (ICF) targets as well as the heating of a target designed to generate a Gbar shock wave for equation of state (EOS) experiments by laser-produced fast electrons with a Maxwellian spectrum is investigated. A relation is established between the energies of two groups of Maxwellian fast electrons, which are responsible for generation of a shock wave and heating the upstream material (preheating). The minimum energy of the fast and shock igniting beams as well as of the beam for a Gbar shock wave generation increases with the spectral width of the electron distribution.
No preview · Article · Sep 2015 · Journal of Experimental and Theoretical Physics
[Show abstract][Hide abstract] ABSTRACT: Optical materials can be structured by laser pulses to get new material functionalities in various scientific area going from photonics to medicine. For instance, wave guides, nano-gratings, emergence of nonlinear optical properties for data storage , cutting and welding of materials are applications of great interest. Structuration driven by a train of laser pulses is strongly emerging due to its advantages: table top laser facility, very well controlled structuration with energy deposition accuracy in the nJ range by adjusting the number of pulses, etc. The material structuration due to pulse-to-pulse cumulative effects should be deeply understood to design specific structures. This may be achieved by modelling the main physical processes and their possible coupling. Briefly, each laser pulse first induces photo-ionization and heats the conduction electrons which can then transfer their energy to the lattice. That leads to a local increase in the material temperature together with heat diffusion and thermally-activated ions migration on longer timescales. Since the laser pulse is partially absorbed, the electron dynamics and the pulse propagation are closely coupled. Due to the low heat diffusion coefficient of dielectric materials, the laser energy may be accumulated in the absorption region, leading to high temperatures even if the single pulse energy is too low to induce itself any significant material modification. A general modelling including all the above-mentioned processes will be presented, including the two following applications of interest.