Ionization and temperature measurements in warm dense copper using x-ray absorption spectroscopy

Abstract

We detail experimental results inferring ionization and temperature for warm dense copper plasmas at several times solid density (15 to 25 g/cm${}^3$) and temperatures of 10 to 21 eV. Experiments performed at the OMEGA Laser Facility generate uniform warm dense matter conditions via symmetric shock compression of a buried copper layer. The plasma is probed with a laser-generated x-ray source to collect the K-shell x-ray absorption spectrum. Fitting bound-bound absorption contributions from constituent charge states of copper provides an estimated $\overline{Z}$ of approximately 4 to 7 for these warm dense copper plasmas. We find that these partially ionized plasmas have K-edge shifts of 12 to 30 eV and bound-bound resonance 1s$\to$3p absorption shifts of 4 to 26 eV with respect to the cold K-edge. This study provides necessary experimental data to improve ionization and opacity models in the warm dense matter regime.

12 Pages, 9 Figures, In preparation with Physical Review Research