Measurements of Unequilibrated Ions in Hot, Solid-Density Plasmas Via X-Ray Lineshapes

Ion-electron equilibration drives plasma dynamics but is challenging to model at high plasma densities, where thermalization is fast and correlation effects are significant. Experimental benchmarks in this regime have been rare due to a lack of precision diagnostics for electron or ion temperatures at high density. We present simultaneous measurements of electron and ion temperatures in solid-density titanium based on high-resolution x-ray emission spectra and newly applied advances in Stark lineshape modeling. The measurements imply an ion temperature significantly lower than the electron temperature, despite short Spitzer equilibration times. X-ray lineshapes are emitted from buried layer targets heated with a short-pulse, high-intensity laser. Titanium tracer layers heat to 1 keV electron temperatures, as inferred from x-ray line intensity ratios, while remaining at solid density due to the high temporal contrast of the laser. Observed x-ray lineshapes are double-peaked and redshifted, both of which are hallmarks of Stark broadening that are influenced differently by the electron density and the ion temperature. A new lineshape model that abandons the common assumption of dipole interactions for electron broadening can model both redshifts and line broadening self consistently. Comparing the data with this model strongly constrains both the ion temperature (700 eV) and the electron density (1.2×10 24 cm -3 ). Measured plasmas are near solid density and support a significant temperature differential between hot electrons and cooler ions. The lack of equilibration suggests that either (1) the equilibration rate is substantially slower than theorized or (2) ion heat transport plays an important role in the system.