Modelling ignition and burn in pre-magnetised inertial confinement fusion experiments

Abstract

Ignition was first obtained in an Inertial Confinement Fusion (ICF) experiment in August 2021, a major milestone allowing the possibility of high energy gain through burn propagation. Use of external magnetic fields, applied primarily to reduce thermal losses, could increase hotspot temperature and ease requirements for ignition but have the potential to inhibit burn propagation. In this work, radiation-magnetohydrodynamics simulations carried out using the code Chimera are used to investigate the effect of a pre-imposed magnetic field on capsule dynamics, ignition and burn.

Modelling of recent `symcap' experiments with applied axial fields of up to 26T shows enhancement of ion temperature by 33% and of fusion yield by 1.8. Implosion shape is elongated along the direction of applied field due to suppression of pre-heat at shock fronts. Anisotropic density and temperature profiles are observed in the hotspot. A scan over field magnitude shows that current experiments are close to the limit of performance enhancement.

A study of magnetised burn propagation in an idealised planar model identifies three regimes of magnetised burn suppression: (1) thermal conduction driven; (2) alpha transport driven; and (3) fully suppressed. Extended-MHD effects at the burn front are important, particularly the Nernst term, in field transport close to the burn front, and a possible mechanism for the formation of a self-insulating layer is proposed due to a rapid growth in the Hall parameter.

Simulations of the ignition shot N210808 with an applied 40T axial field show a ~ 50% reduction in fusion performance and clear indication of burn suppression perpendicular to field lines. Implosion shape is degraded by the field, and anisotropic conduction causes significant modification to the rate of ablation during stagnation. Yield is degraded through both suppression of burn and the impact of the field during hotspot formation.