Target density dependence of plasma parameters and heating mechanisms of porous foams irradiated by a laser-driven hohlraum x-rays source

We present an experimental investigation into the radiative heating of boron-doped carbon-hydrogen-oxygen (CHO) foam targets by x-ray fluxes generated from laser-driven gold hohlraum. Time-integrated x-ray transmission and emission spectra were measured. The spectra exhibit a 26 eV blackbody radiation source background with a pronounced absorption profile and characteristic emission lines from the CHO plasma. The absorption profile roughly ranging from 8 to 13 nm arises from partially ionized carbon and oxygen ions. The plasma parameters, such as temperature, ionization degree, free electron density, coupling parameter, and degeneracy, were determined based on the characteristic emission lines intensity analysis. As the foam density increases, the x-ray absorption efficiency rises that is experimentally proved by the larger absorption profile, while the plasma electron temperature decreases, and the free electron density increases. The radiative heating mechanisms transferred from volumetric x-ray heating in an optically thin medium at low densities like 2 mg/cm3 to radiation heat wave propagation at higher densities like 5 and 10 mg/cm3. The electronic coupling parameter increases to 0.17, and the degeneracy decreases to 30 for the foam with a mass density of 10 mg/cm3, indicating a transition toward a weakly coupled, partially degenerate plasma state. This work provides critical insights into mass density dependence of radiative heating mechanisms and plasma conditions evolution of high-energy-density systems.