Diagnosing inertial confinement fusion ignition

Fusion ignition by inertial confinement requires compression and heating of the fusion fuel to temperatures in excess of 5 keV and densities exceeding hundreds of g/cc. In August 2021 this scientific milestone was surpassed at the National Ignition Facility (NIF), when the Lawson criterion for ignition was exceeded generating 1.37MJ of fusion energy from a target driven by 1.9MJ of laser energy. On the NIF, ICF research primarily uses laser indirect drive in which the fusion capsule is surrounded by a high-Z enclosure (“Hohlraum”) used to convert the directed laser energy into symmetric x-ray drive on the capsule. Precise measurements of the plasma conditions, x-rays, -rays and neutrons produced is key to understanding the pathway to higher performance. The paper discusses the diagnostics and measurement techniques developed to understand these experiments, focusing on three main topics: (1) key diagnostic developments for achieving igniting plasmas, (2) novel signatures related to thermonuclear burn and (3) advances to diagnostic capabilities in the igniting regime with a perspective toward developments for Inertial Fusion Energy (IFE).