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 densities. We present simultaneous measurements of electron and ion temperatures in solid-density, highly ionized Ti based on high-resolution x-ray Stark lineshapes; analysis implies that ions are significantly cooler than electrons. Foils with buried Ti layers are heated with a high-intensity laser with extreme temporal contrast. The localized Ti regions emit x-ray lineshapes that are both double-peaked and redshifted; these features, both hallmarks of Stark broadening, are differently affected by electron density and ion temperature. The data is interpreted with a new lineshape code that abandons the common assumption of dipole interactions for the electron broadening; comparing the data with this model strongly constrains both the ion temperature and the electron density. The electron temperature is independently constrained by x-ray line ratios. 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 the Spitzer equation predicts or (2) ion heat transport plays an important role in the system.