First results from the time resolved opacity spectrometer on the National Ignition Facility (invited)

Radiative opacity plays a central role in determining stellar structure, with iron contributing significantly to the solar opacity, particularly near the radiation-convection boundary. Current atomic models of opacity remain a major source of uncertainty, factoring into discrepancies between standard solar models and helioseismic observations. Agreement with models could be attained if key opacity values were 10%-50% larger, motivating high-precision experimental benchmarks at stellar interior conditions. The Opacity-on-National Ignition Facility campaign measures iron opacity via x-ray transmission through samples heated in a hohlraum. Previous measurements using the time-integrated opacity spectrometer (OpSpec) have been limited to temperatures <160 eV and electron densities ∼1021-1022 cm-3. Higher temperatures are achievable, but measurements were precluded by strong background emission. To address this limitation, a new time-resolved spectrometer, OpSpecTR, was developed. OpSpecTR employs an elliptically bent crystal and a grazing-incidence x-ray mirror to achieve high spectral purity, and a small crossover slit to minimize background. Time-resolved spectra in the 1-2 keV range are recorded on three ns-gated hCMOS detectors, simultaneously measuring transmitted, backlighter, and self-emission signals required for opacity extraction. Nanosecond-scale gating isolates the backlighter pulse and reduces background contributions by up to 70%. Spectral resolving power is also improved ∼2× relative to the time-integrated instrument. The first results from OpSpecTR presented herein demonstrate significantly improved fidelity in opacity measurements under high-temperature conditions, advancing experimental constraints on stellar opacity models.