•Inverse ray tracing allows for fast and low noise 3-D field computations.•Efficient parallelization is achieved with decoupled hydrodynamics and laser grids.•Cross beam energy transfer is ...efficiently implemented with inverse ray-tracing.•Etalon Integral methods remove free parameters in laser field computations.
A novel approach to efficiently model 3-D laser plasma interactions at fluid scales is presented. This method, implemented in the IFRIIT propagation code developed at CELIA, relies on inverse ray tracing to compute laser fields at arbitrary locations in a plasma. This enables to describe the fields at high order in space compared to standard forward ray tracing approaches. In addition, inverse ray tracing enables the use of etalon integral methods to reconstruct caustic fields and greatly speeds up calculations of cross-beam energy transfer by decoupling the ray amplitude and ray phase calculations. A comparison of the inverse and forward methods for 3-D calculations of fields in presence or not of cross-beam energy transfer illustrates the significant advantages of the inverse method. Conversely, while the inverse method is well suited to most spherical plasma profiles, it currently cannot treat concave profiles or target holders. The coupling of IFRIIT with the 3-D ASTER radiative hydrodynamics code developed at the Laboratory for Laser Energetics is then presented. ASTER and IFRIIT resolve their respective equations on separate grids which communicate through interpolation. As such, IFRIIT uses a dedicated laser grid adapted to the computations at play, which also allows to use different parallelization methods for both codes: block decomposition for the hydrodynamics versus domain duplication for the laser. Applications to direct-drive implosions for inertial confinement fusion are presented, for which a geodesic icosahedron grid is implemented in IFRIIT. The performances of the ASTER/IFRIIT coupling are demonstrated by conducting simulations of cryogenic implosions performed on the OMEGA laser system, in presence of various sources of 3-D effects; laser port geometry, cross-beam energy transfer, beam imbalance and target mis-alignment. Comparison with neutron data, measured through bang-time, for a cryogenic implosion experiment shows an excellent agreement for the laser-plasma coupling.
We describe the development of a 3D Monte-Carlo model to study hot-electron transport in ionized or partially ionized targets, considering regimes typical of inertial confinement fusion. Electron ...collisions are modeled using a mixed simulation algorithm that considers both soft and hard scattering phenomena. Soft collisions are modeled according to multiple-scattering theories, i.e., considering the global effects of the scattering centers on the primary particle. Hard collisions are simulated by considering a two-body interaction between an electron and a plasma particle. Appropriate differential cross sections are adopted to correctly model scattering in ionized or partially ionized targets. In particular, an analytical form of the differential cross section that describes a collision between an electron and the nucleus of a partially ionized atom in a plasma is proposed. The loss of energy is treated according to the continuous slowing down approximation in a plasma stopping power theory. Validation against Geant4 is presented. The code will be implemented as a module in 3D hydrodynamic codes, providing a basis for the development of robust shock ignition schemes and allowing more precise interpretations of current experiments in planar or spherical geometries.
We describe two numerical investigations performed using a 3D plasma Monte-Carlo code, developed to study hot-electron transport in the context of inertial confinement fusion. The code simulates the ...propagation of hot electrons in ionized targets, using appropriate scattering differential cross sections with free plasma electrons and ionized or partially ionized atoms. In this paper, we show that a target in the plasma state stops and diffuses electrons more effectively than a cold target (i.e., a target under standard conditions in which ionization is absent). This is related to the fact that in a plasma, the nuclear potential of plasma nuclei has a greater range than in the cold case, where the screening distance is determined by the electronic structure of atoms. However, in the ablation zone created by laser interaction, electrons undergo less severe scattering, counterbalancing the enhanced diffusion that occurs in the bulk. We also show that hard collisions, i.e., collisions with large polar scattering angle, play a primary role in electron beam diffusion and should not be neglected. An application of the plasma Monte-Carlo model to typical shock ignition implosions suggests that hot electrons will not give rise to any preheating concerns if their Maxwellian temperature is lower than 25–30 keV, although the presence of populations at higher temperatures must be suppressed. This result does not depend strongly on the initial angular divergence of the electron beam set in the simulations.
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