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

We detail experimental results inferring ionization and temperature for warm dense copper plasmas at several times solid density (15–25 <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:msup> <a:mrow> <a:mi mathvariant="normal">g</a:mi> <a:mo>/</a:mo> <a:mi>cm</a:mi> </a:mrow> <a:mn>3</a:mn> </a:msup> </a:math> ) and temperatures of 10–21 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 <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mover> <c:mi>Z</c:mi> <c:mo>¯</c:mo> </c:mover> </c:math> of approximately 4–7 for these warm dense copper plasmas. We find that these partially ionized plasmas have K-edge shifts of 12–30 eV and bound-bound resonance <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:mrow> <d:mn>1</d:mn> <d:mi>s</d:mi> <d:mo>→</d:mo> <d:mn>3</d:mn> <d:mi>p</d:mi> </d:mrow> </d:math> absorption shifts of 4–26 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.