Progress Extending Opacity Measurements on the National Ignition Facility (NIF) to Higher X-ray Energies (1.8-2.9 keV)*

Measurements of X-ray opacity of hot dense plasmas are needed to help resolve discrepancies in solar and other stellar models.¹ Previous measurements at the Sandia Z Facility suggest opacities up to 2x higher than theory for the iron L-band, particularly for temperatures >160 eV and electron densities > 10²² e^-/cm³.¹ Initial NIF results tend to support the Z Facility data,² though uncertainties need to be reduced,³ raising the question whether the discrepancies might extend to higher X-ray energies. The NIF Opacity Spectrometer has recently been modified to measure X-ray energies from 1.8-2.9 keV, in addition to the usual 1.0-2.0 keV band.⁴ This talk presents the first NIF data using a "dual band" spectrometer configuration to record X-ray absorption spectra from 1.0-2.9 keV from samples in LTE at temperatures above 140 eV, including data from magnesium-oxide/silicon dioxide samples. Prospects for further experiments will be discussed. 

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   3) R. F. Heeter, et al., ATOMS 6, 57 (2018)
   
   4) M.S. Wallace, R.F. Heeter et al., Rev. Sci. Instrum. 92, 035108 (2021)