Measuring Femtosecond Collisional Ionization Rates in Solid-Density Plasmas

The rate at which atoms and ions within a plasma are further ionized by collisions with free electrons is a fundamental parameter that dictates the dynamics of plasma systems at intermediate and high densities. While collisional ionization rates are well known experimentally in a few dilute systems, similar measurements for non-ideal plasmas at densities approaching or exceeding those of solids remain elusive. Here we illustrate a spectroscopic method capable of measuring rates of collisional ionization dynamics in solid-density plasmas by clocking them to Auger recombination processes. We have recently employed this technique on the LCLS X-ray free-electron laser at SLAC and will present the first experimental results for optically-thin, solid-density magnesium plasmas at peak temperatures exceeding 200 eV. \\[4pt] [1] S.M. Vinko {\it et al.}, Nature {\bf 482}, 59 (2012). \\[0pt] [2] S.M. Vinko {\it et al.}, Nature Communications {\bf 5}, 6397 (2015).