Creation of optically-thin solid-density plasmas using LCLS

The advent of X-ray free-electron-lasers such as LCLS provides the capability to truly isochorically heat solid-density matter on femtosecond time-scales [1]. K-shell emission from such plasmas has provided new information on ionization potential depression [2] and collisional ionisation rates [3]. However, in previous work the targets were 1-$\mu$m thick, resulting in high-opacity on the K-shell transitions. We report here results of a detailed study of K-shell emission from exactly solid-density Mg plasmas with thicknesses ranging from 500 down to 25 nm -- just over 100 atoms across. A curve-of-growth analysis exhibits text-book behavior, and confirms peak optical depths for the thinnest targets well below unity, in excellent agreement with simulations. The rich data-set provides information on line-widths, collisional dynamics, and radiation transfer in solid density plasmas. \\[4pt] [1] S.M. Vinko {\it et al.}, Nature, {\bf 482}, 59 (2012)\\[0pt] [2] O. Ciricosta {\it et al.}, Phys. Rev. Lett., {\bf 109}, 065002 (2012)\\[0pt] [3] S.M. Vinko {\it et al.}, Nat. Comm., {\bf 6}, 6397 (2015)