A theoretical model for low-mode asymmetries in ICF implosions

Abstract

Low-mode asymmetry is known to be a main source of yield degradation in implosion experiments performed at the National Ignition Facility at the Lawrence Livermore National Laboratory. In this paper, we present a theoretical model of the deceleration phase to investigate low-mode asymmetries, which is derived by considering the main fuel layer to be composed of thin shell pieces and neglecting the interaction between these pieces in the longitudinal direction. The model is able to characterize the evolution of low-mode asymmetries and assess the corresponding performance degradation, as validated numerically using the radiation hydrodynamics code LARED-S. The deceleration phases of implosions modulated separately by P2 (in Legendre polynomials) asymmetries in the shell mass, shell velocity, and hot-spot radius are studied using this model. It is found that asymmetries in the shell velocity and hot-spot radius have more pronounced effects than shell mass asymmetry on capsule distortion, resulting in greater yield degradation. The results obtained using this model indicate that yield degradation is mainly caused by the increase in residual kinetic energy at stagnation time, which is identical for all three types of asymmetries.