Strong Shock Waves in Highly Porous Materials

Abstract

Strong shock waves traversing through porous materials occur in a broad range of applications. Foamed plastics are a popular material in many high-energy-density physics (HEDP) experiment due to the low, tunable density, and being easily machinable. Shocked foams are also of interest in equation of state (EOS) studies due to the ability to change the initial density to obtain different shocked states. Finally, the effect of porosity on asteroid collisions is needed to understand the cratering mechanism. This dissertation presents experimental and theoretical work related to strong shock waves in highly porous materials.

A platform to study shocked foams on the OMEGA EP laser system was developed in the first of a series of shot days. The imaging x-ray Thomson spectrometer (IXTS) was the main diagnostic and the measurement provided information on the compression, shock front location, temperature, and ionization, making it a potentially powerful diagnostic for equation of state measurements. The first shot day demonstrated the ability to perform the x-ray Thomson scattering technique on OMEGA EP. A second shot day improved the target design and obtained good quality data with a 150 mg/cc carbon foam. The experimental data was compared to Rankine-Hugoniot calculations of commonly used carbon EOS tables. The findings from this experiment suggest that the carbon EOS table over predicts the compression of the shocked carbon foam.

The theoretical aspect of this dissertation describes the pore closure in highly porous materials due to a strong shock wave. Many previous models of pore collapse due to shock waves are in the low-pressure regime where the pore is crushed as a response to the shock wave. This dissertation presents a simple 1-D pore heating model where thermal radiation from the shock can penetrate deep into the porous material and cause heating of the pore walls. As the pore walls heat up, they start to expand and fill in the pores. This work suggests that there may be enough time for the pores to close prior to the arrival of the shock in conditions of interest to HEDP experiments.