R. Ramis

Ignition conditions for inertial confinement fusion targets with a nuclear spin-polarized DT fuel

The nuclear fusion cross-section is modified when the spins of the interacting nuclei are polarized. In the case of deuterium?tritium it has been theoretically predicted that the nuclear fusion cross-section could be increased by a factor ??=?1.5 if all the nuclei were polarized. In inertial confinement fusion this would result in a modification of the required ignition conditions. Using numerical simulations it is found that the required hot-spot temperature and areal density can both be reduced by about 15% for a fully polarized nuclear fuel. Moreover, numerical simulations of a directly driven capsule show that the required laser power and energy to achieve a high gain scale as ??0.6 and ??0.4 respectively, while the maximum achievable energy gain scales as ?0.9.

Irradiation uniformity of directly driven inertial confinement fusion targets in the context of the shock-ignition scheme

In direct drive inertial confinement fusion (ICF) the uniformity of the irradiation of the capsule still represents a crucial issue. The quality of the capsule irradiation in the context of the shock-ignition (SI) scheme has been assessed numerically. Schemes characterized by different directions of irradiation associated with a single laser beam or a bundle of laser beams have been considered. Beam imperfections as power imbalance and pointing errors have been taken into account and show that the focal spot that minimizes the root-mean-square deviation depends on these beam imperfections. We discuss the advantages provided by laser facilities accounting for a large number (up to a few thousand) of beamlets. Preliminarily results concerning the use of the Laser-Megajoule facility associated with a SI scheme will be discussed.

Fast ignition induced by shocks generated by laser-accelerated proton beams

Fast ignition (FI) of a deuterium?tritium target compressed to a density of 500?g?cm?3 by the energy deposition of two laser-accelerated proton beams is studied by two-dimensional (2D) and three-dimensional (3D) numerical simulations. The first proton beam has an annular radial profile while the second beam is cylindrical. Both beams are characterized by a super-Gaussian profile in radius. A 3D-hydrodynamic study has been performed to identify a way to generate a nearly annular energy deposition by using a discrete number of cylindrical beams. It has been found that the energy deposited by the first proton beam can modify the density and temperature of the plasma before the arrival of the second beam allowing ignition in a zone not directly irradiated by the beams. Thus, differently from the classical FI concept, fuel ignition is not a direct consequence of plasma heating by the particle beam. Indeed, ignition occurs as a result of the synergetic action of the shocks generated by proton energy deposition.

A 3 MJ OPTIMIZED HOHLRAUM TARGET FOR HEAVY ION INERTIAL CONFINEMENT FUSION

Abstract A preliminary design for a HIDIF target is presented. The one-dimensional analysis of an igniting NIF-Beryllium-DT capsule is matched to 2-D heavy-ion-driven-hohlraum numerical simulations. Pulse shaping, compression symmetry and other features are optimized to get ignition conditions with minimum energy requirements. A Rayleigh–Taylor instability capsule analysis is also performed.

European fusion target work

Abstract Target concepts developed in the context of the HIDIF project (Heavy Ion Driven Inertial Fusion) are presented. Target design has evolved, from the beginning of the study in 1993, in parallel with the evolution of accelerator parameters. Two different phases can be distinguished: (a) up to 1998 the goal was just demonstration of ignition with heavy ions, (b) from 1998 target and driver are being upgraded to high-gain designs able to be used for energy production. Analytical models and numerical tools developed in this context are summarized, and future research directions are discussed.

Nonlinear theory of the ablative Rayleigh-Taylor instability

Here, a model for the nonlinear Rayleigh–Taylor instability (RTI) of a steady ablation front based on a sharp boundary approximation is presented. The model includes the effect of mass ablation and represents a basic tool for investigating many aspects of the nonlinear ablative RTI relevant to inertial confinement fusion. The single mode analysis shows the development of a nonlinear exponential instability for wave numbers close to the linear cutoff. Such a nonlinear instability grows at a rate faster than the linear growth rate and leads to saturation amplitudes significantly larger than the classical value 0.1λ. We also found that linearly stable perturbations with wave numbers larger than the linear cutoff become unstable when their initial amplitudes exceed a threshold value. The shedding of long wavelength modes via mode coupling is much greater than predicted by the classical RTI theory. The effects of ablation on the evolution of a front of bubbles is also investigated and the front acceleration is computed.

Enhancement of laser to x-ray conversion by a double-foil gold target

A novel double-foil configuration is proposed to improve the laser to x-ray conversion efficiency from laser irradiating a solid target. One-dimensional radiation hydrodynamic simulations show that the total x-ray conversion efficiency for the double-foil target is as high as 54.7%, which has a 10% improvement compared with the normal target. The improvement is mainly due to the enhanced soft x-ray emissions. Influences of the target geometry parameters on the x-ray conversion efficiency are investigated. Detailed energy distributions and the individual contributions of the two foils to the thermal and kinetic energy terms are presented. It is found that the main energy terms are mostly determined by the first foil, and the enhancement of radiation is attributed to the lower ion kinetic energy of the double-foil target.

Two-dimensional simulation of heavy ion hohlraum targets

Abstract A preliminary design of a hohlraum driven by heavy ions has been analysed. The hohlraum is based on using foams of different densities to form a cavity surrounding the fuel capsule. The parameters of the beams have been provided by the HIDIF study group. The fuel capsule and pulse shaping have been taken from NIF. Doping of the ablator with intermediate-Z materials has also been studied. Results pertaining to the efficiency of the hohlraum, the ion pulse, the symmetry and the energy necessary to get ignition are presented.

Three-Dimensional Simulations of Cylindrical Target Implosion Imaging Using Laser-Driven Proton Source

Many experiments, based on the road map of the European High Power laser Energy Research facility project, were performed to study fast electron transport in compressed matter in the context of fast ignition approach to inertial confinement fusion. The generation of high intensity beams from laser-matter interaction extends the possibility to use protons as a diagnostic to image imploding targets in these experiments. The analysis of experimentally obtained proton images requires a careful analysis and accurate numerical simulations using both hydrodynamic and Monte Carlo (MC) codes. An experiment has been performed in 2008 at Rutherford Appleton Laboratory to study fast electron propagation in cylindrical imploding targets illuminated by four laser pulses. In this paper, we present new simulation results in 3-D geometry. Three-dimensional density map is generated by running the 3-D version of the MULTI code. Proton radiography images are then simulated using the MC code MCNPX.