Sub-keV design for the National Ignition Facility's soft x-ray Opacity Spectrometer (OpSpec) and expansion plans for time-resolved measurements. - Wallace MS; Heinmiller JM; Dutra EC; Knight RA; Heeter RF; Opachich YP; Buscho J; Fontes CJ; Max DA; Emig JA; Posadas R; Ayers J; et al. (The Review of scientific instruments, 2022)