Temperature Measurements of Solid-Density Germanium Plasmas Created with an X-Ray Free-Electron-Laser

We have used the focused femtosecond x-ray output from LCLS at photon energies of 1400 eV and intensities of order 10$^{17}$ Wcm$^{-2}$ to isochorically heat sub-micron thick foils of Ge to temperatures between 150-200 eV. L-shell X-Rays emitted from the solid-density plasma were recorded using a Bragg crystal spectrometer. An analysis of the bound-free recombination emission, including both its slope and relative intensity as a function of total energy in the FEL beam, allows for an accurate determination of the peak temperature of the plasma. Simulations using atomic-kinetics calculations show that the system cools due to the emission of blackbody radiation on a timescale of several picoseconds, and that the time-integrated recombination emission is heavily gated towards the peak temperature within the system, rapidly decaying on timescales shorter than the estimated inertial confinement time.