Control and diagnosis of temperature, density, and uniformity in x-ray heated iron/magnesium samples for opacity measurements^a)

Experimental tests are in progress to evaluate the accuracy of the modeled iron opacity at solar interior conditions, in particular to better constrain the solar abundance problem [S. Basu and H. M. Antia, Phys. Rep. 457, 217 (2008)]. Here, we describe measurements addressing three of the key requirements for reliable opacity experiments: control of sample conditions, independent sample condition diagnostics, and verification of sample condition uniformity. The opacity samples consist of iron/magnesium layers tamped by plastic. By changing the plastic thicknesses, we have controlled the iron plasma conditions to reach (1) T_e = 167 ± 3 eV and n_e = (7.1 ± 1.5)× 10²¹ cm⁻³, (2) T_e = 170 ± 2 eV and n_e = (2.0 ± 0.2) × 10²² cm⁻³, and (3) T_e = 196 ± 6 eV and n_e = (3.8 ± 0.8) × 10²² cm⁻³, which were measured by magnesium tracer K-shell spectroscopy. The opacity sample non-uniformity was directly measured by a separate experiment where Al is mixed into the side of the sample facing the radiation source and Mg into the other side. The iron condition was confirmed to be uniform within their measurement uncertainties by Al and Mg K-shell spectroscopy. The conditions are suitable for testing opacity calculations needed for modeling the solar interior, other stars, and high energy density plasmas.