Quantifying Low-Mode Directional Coupling Effects on ICF Implosion Performance

Low-mode asymmetries, particularly spherical harmonic modes ℓ = 1 and ℓ = 2, play a dominant role in limiting performance in inertial confinement fusion (ICF) implosions. In OMEGA laser facility, these long-wavelength perturbations arise from imperfections in target misalignment, beam pointing errors, and laser drive imbalance. They couple nonlinearly to degrade compression and reduce neutron yield. In this work, we present a quantitative study of low-mode directional coupling effects and their impact on fusion yields and neutron-inferred apparent ion temperatures. Using DEC3D deceleration-phase hydrodynamic simulations, an angular dependence of fusion yield degradations caused by modes 1 and 2 directional coupling was observed. The low-mode directional coupling coefficients were parametrized by a perturbation model. Experimental results from a dedicated low-mode interaction campaign on OMEGA will also be presented. In these experiments, the effects of low modes ℓ = 1 and ℓ = 2 interaction on fusion yields and ion temperatures were investiaged, where modes ℓ = 1 and ℓ = 2 were intentionally seeded with controlled orientations. The measured apparent ion temperatures and inferred flows are compared with the predictions of 3D simulations. These results offer new insights into low-mode coupling mechanisms in ICF implosion asymmetries.