Does laser-driven heat front propagation depend on material microstructure?*

We showed earlier that the laser-driven heat front propagation velocity in low-density Ti-silica aerogel and TiO$_{2}$ foam targets was slower than that simulated with a 2D radiation-hydrodynamics code incorporating an atomic kinetics model in non-LTE and assuming initially homogeneous material (F. P\'{e}rez, et al., Physics of Plasmas 21, 023102, 2014). Some theoretical models suggest that the heat front is slowed over what it would be in a homogeneous medium by the microstructure of the foam. In more recent experiments with Cu-loaded carbon nanotube foam, however, we find the opposite behavior; that is, the simulations under-predict the measured heat-front velocity. We present details of the Cu foam experiments and comparisons with simulations, and then discuss implications for models of heat-front slowing in foams of a more-recent gas vs. foam comparison experiment. F. P\'{e}rez presents the design and results of this comparison experiment in a companion presentation.