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Accurate models of X-ray absorption and re-emission in partly stripped ions are necessary to calculate the structure of stars, the performance of hohlraums for inertial confinement fusion and many other systems in high-energy-density plasma physics. Despite theoretical progress, a persistent discrepancy exists with recent experiments at the Sandia Z facility studying iron in conditions characteristic of the solar radiative–convective transition region. The increased iron opacity measured at Z could help resolve a longstanding issue with the standard solar model, but requires a radical departure for opacity theory. To replicate the Z measurements, an opacity experiment has been designed for the National Facility (NIF). The design uses established techniques scaled to NIF. A laser-heated hohlraum will produce X-ray-heated uniform iron plasmas in local thermodynamic equilibrium (LTE) at temperatures
${\geqslant}150$
eV and electron densities
${\geqslant}7\times 10^{21}~\text{cm}^{-3}$
. The iron will be probed using continuum X-rays emitted in a
${\sim}200$
ps,
${\sim}200~\unicode[STIX]{x03BC}\text{m}$
diameter source from a 2 mm diameter polystyrene (CH) capsule implosion. In this design,
$2/3$
of the NIF beams deliver 500 kJ to the
${\sim}6$
mm diameter hohlraum, and the remaining
$1/3$
directly drive the CH capsule with 200 kJ. Calculations indicate this capsule backlighter should outshine the iron sample, delivering a point-projection transmission opacity measurement to a time-integrated X-ray spectrometer viewing down the hohlraum axis. Preliminary experiments to develop the backlighter and hohlraum are underway, informing simulated measurements to guide the final design.
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