High energy density matter generation using a focused soft-X-ray laser for volumetric heating of thin foils

We report on time-resolved transmission measurements of a 21.2 nm X-ray laser through thin Al and polyimide foils in the intensity range 10¹⁰-10¹² Wcm^-2. Absolute time-integrated and relative timedependent transmission measurements indicate significant differences with cold opacity data, due to the heating, leading to partially ionized states with temperatures of 0.5-20 eV. The transmission data are compared with simulations that qualitatively reproduce the time-dependent transmission measurements and demonstrate the importance of including detailed atomic absorption processes in the warm dense matter regime.