A Computational Study of the Stagnation Phase in Inertial Confinement Fusion: Hotspot Energetics, Diagnostics, and Burn Propagation

Abstract

This thesis studies stagnating ICF capsules. The work is theoretical, with numerical modelling carried out primarily using a 3D multi-physics hydrocode, Gorgon. Simulations of NIF-relevant ICF stagnations are generated from the output of a 1D Hydra NIF post-shot simulation of shot N120205, with an inherently 3D perturbation imposed in the velocity field to generate asymmetries. The simulations are used to develop a detailed phenomenology of the effect of perturbations from spherical symmetry on the energy balance of the hotspot at stagnation. The main effect is an increase in the fluid velocity at the hotspot edge before the time of peak temperature, which results in early thermalisation of kinetic energy and additional compression early in time. The increase of the perturbed target’s surface area above 4πR^2 is found to increase the thermal conduction rate. An inefficiency of thermalisation of kinetic enegy is caused by asymmetries in the ram pressure. The combination of these effects makes it possible for the yield to be reduced to experimental levels in the simulations purely by perturbation of an all-DT system. In order to study the energy balance, an accurate analytic model of the hotspot energy source terms in unperturbed targets is developed by taking advantage of the self-similar temperature profile the hotspot converges upon. This is an improvement upon 0D models such as that of Widner [1]. In particular, the thermal conduction loss is significantly lower when the thermal conductivity at the hotspot edge is used, and the ‘ideal ignition temperature’ is found to be lower when the ion and electron species are in equilibrium. The diagnostic signatures of hotspot asymmetries are investigated by generating synthetic diagnostic data from the 3D simulations. It is found that stagnation-phase perturbations of mode number l≳6 with amplitudes large enough to reduce the yield to experimental levels are not visible on the currently available NIF diagnostics, or are able to alias as perturbations of lower mode number. Comparisons are also made between the P0 as measured from X-ray images with the physical hotspot radius, and the neutron spectrum width with the burn-averaged and mass-averaged hotspot temperatures, including the effects of motion Doppler broadening. The effect of alpha particle transport on the burn process is investigated using the 1D Lagrangian code Medusa by comparing an accurate model of the alpha particle species with models that make various assumptions about the particle kinetics. The fraction of fusion energy deposited within the burning region χ is related empirically to the properties of the hotspot, which allows alpha particle transport to be included as a loss term in the energy balance model. Alpha transport in 3D perturbed hotspots is investigated by post-processing Gorgon data, which finds that perturbed targets have significantly increased fusion energy loss relative to equivalent unperturbed targets.