Session K6: HED/WDM III

Measurements of the Shock Release Of Quartz and Paralyene-N

The shock and release properties of Quartz and hydrocarbons are important to high energy density (HED) research and inertial confinement fusion (ICF) science. The bulk of HED material research studies single shock or multiple shock conditions. The challenge with measuring release properties is unlike shocks which have a single interface from which to measure the properties, the release establishes gradients in the sample. The streaked x-ray imaging capability of the NIKE laser allow the interface between quartz and CH to be measured during the release, giving measurements of the interface velocity and CH density. Here, we present experimental results from the NIKE laser where quartz and parylene-N are shock compressed to high pressure and temperature and the release state is measured through x-ray imaging. The shock state is characterized by shock front velocity measurements using VISAR and the release state is characterized by using side-on streaked x-ray radiography   

Study of molecular carbon-hydrogen bond dissociation during shock compression

Advancements in theory and experiment show that chemical interactions in warm dense mixtures play a non-negligible role in the high-temperature and high-pressure properties of a molecular compound. For example, recent work on polystyrene has observed features suggestive of molecular dissociation - non-linear ``kinks" are evident in the material's Hugoniot [Barrios et al.], consistent with CH bond breaking. The assumption used in linear mixing models, that species are chemically inert, breaks down in warm dense mixtures. At the Institute for Shock Physics, we are developing the necessary capabilities to perform high-repetition-rate experiments needed to map out chemical-reaction features along a material's Hugoniot. Initially, we plan to benchmark our work to the data taken by Barrios et al., by reproducing the observed kink in the polystyrene Hugoniot. We then extend this capability to explore polypropylene, CH${}_2$, where we expect to observe multiple kink features - representative of the disassociation of multiple CH bonds. \\ \\ M. Barrios et al. Phys. Plasmas 17, 056307 (2010)   

Generation of cylindrically convergent shockwaves in water on the MACH facility

We report on the first experiments utilizing MACH facility at Imperial College London to explode copper wire arrays in water, generating extremely symmetric, cylindrical convergent shockwaves. The experiments were carried out with 10mm diameter arrays consisting of 60 x 130µm wires, and currents >500kA were achieved despite the high inductance load. Laser backlit framing images and streak photography of the implosion showed a highly uniform, stable shockwave that travelled towards the axis at velocities up to ~7.5kms-1. For the first time, imaging of the shock front has been carried at radii < 0.5mm, and there is strong evidence that even at radii <0.1mm the shock front remains stable, resulting in a convergence ratio of 50:1. 2D hydrodynamic simulations that match the experimentally obtained implosion trajectory suggest pressures of >1Mbar are produced within 10µm of the axis, with water densities 3gcm-3 and temperatures of many 1000s of Kelvin. The results represent a significant step in the application of the technique to drive different material samples, and calculations of scaling the technique to larger pulsed power facilities are presented.   

Laser-driven Mach waves for gigabar-range shock experiments

Mach reflection offers possibilities for generating planar, supported shocks at higher pressures than are practical even with laser ablation. We have studied the formation of Mach waves by algebraic solution and hydrocode simulation for drive pressures at much than reported previously, and for realistic equations of state. We predict that Mach reflection continues to occur as the drive pressure increases, and the pressure enhancement increases monotonically with drive pressure even though the ``enhancement spike'' characteristic of low-pressure Mach waves disappears. The growth angle also increases monotonically with pressure, so a higher drive pressure seems always to be an advantage. However, there are conditions where the Mach wave is perturbed by reflections. We have performed trial experiments at the Omega facility, using a laser-heated halfraum to induce a Mach wave in a polystyrene cone. Pulse length and energy limitations meant that the drive was not maintained long enough to fully support the shock, but the results indicated a Mach wave of 25-30 TPa from a drive pressure of 5-6 TPa, consistent with simulations. A similar configuration should perform well at the NIF, and a Z-pinch driven configuration may be possible.