“m=4” azimuthal x-ray asymmetry in high-yield implosions on the NIF

Since 2022, several ignition experiments on the National Ignition Facility (NIF) have achieved yields of 1 MJ or more. On all these shots, the x-ray emission of the compressed target shows a pronounced and repeatable four-fold "cloverleaf" feature as seen from the north pole, instead of a single bright spot from the hot spot self-emission usually seen on every experiment until then. The pattern is attributed to the four-fold symmetry the NIF laser's "inner cone" of beams being imprinted into the compressed target, leading to eight localized regions of thinner shell at bang time (four on the upper half of the shell, and four on the lower half). When ignition happens, the pressure from the igniting hot spot bursts through these thinner regions, leading to expanding shock waves that generate the four-fold x-ray feature observed from the pole. This presentation will describe how the suite of x-ray and neutron diagnostics on NIF as well as 3D simulations made it possible to understand the process. We will present recent experiments aimed at mitigating this asymmetry by using crossed-beam energy transfer or by adjusting the power balance between cones of NIF beams in the foot of the laser pulse. The impact of this asymmetry on ignition performance will be discussed.