Optical Measurements of Preheated Polystyrene and Aluminum Layers

We present optical measurements of polystyrene and aluminum layers modified by ionizing radiation within a 100-ps time scale. The experiments were performed at the OMEGA Laser Facility using high-energy, 100-ps, 5 $\times $ 10$^{14}$ W/cm$^{2}$ laser pulses impinging onto a 40-\textit{$\mu $}m plastic ablator in order to generate the preheat radiation. Two temporal resolving optical diagnostics, a velocity interferometer for any reflector (VISAR), and a temperature-calibrated streaked optical pyrometer (SOP) measure the change of the optical properties and the temperature increase of samples mounted on the target's back side. Preheating prior to the arrival of a shock front is observed for the aluminum and plastic layers. Within the laser interaction time, a strong absorption and a frequency shift of the optical probe laser reflecting from the samples are measured with VISAR. The SOP measurement indicates temperatures of up to $\sim $4 eV at the target's back side prior to the shock front arrival. The experimental results are compared to one-dimensional hydrodynamic simulations with the code \textit{LILAC}. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.