Session V5: LS Large Scale Experiments II

Simultaneous unfolding of compression and opacity from time-resolved radiography

Radiographs of symmetric objects can be analyzed to give the spatial variation of attenuation, as in the Abel inversion of an axisymmetric object. If the opacity is known, the mass density can be derived from the attenuation. The space- and time-variation of density is needed to make equation of state (EOS) measurements by radiography, e.g. by measuring the speed and compression of a shock. However, in our experiments using hohlraum drive at the National Ignition Facility (NIF) to perform EOS measurements at gigabar pressures with spherically-converging shocks, the opacity may vary by an order of magnitude because of ionization. We have developed a new algorithm to simultaneously deduce the compression and opacity of the sample given time-resolved radiographs with a Lagrangian location behind the shock, such as the edge of the sample. This approach relies on spatial integration to deduce the opacity in the region just behind the shock from the difference between the known and apparent mass. We assume that the change in opacity is dominated by shock-heating, so that subsequent variations, as shocked material is either released or compressed further, are negligible or can be accounted for by a model. We used this algorithm to analyze our NIF data on the Hugoniot of CH at 10-40 TPa.   

Analysis of data from shockless compression experiments to multi-megabar pressure

Quasi-isentropic, shockless ramp-wave experiments promise accurate equation-of-state (EOS) data in the solid phase at relatively low temperatures and multi-megabar pressures. In this range of pressure, isothermal diamond-anvil techniques have limited pressure accuracy due to reliance on theoretical EOS of calibration standards, thus accurate quasi-isentropic compression data would help immensely in constraining EOS models. Multi-megabar shockless compression experiments using the Z Machine at Sandia as a magnetic drive with stripline targets have been performed on a number of solids. New developments will be presented in the analysis of data from these experiments using the single-sample inverse Lagrangian approach, including topics such as 2-D and magneto-hydrodynamic (MHD) effects and uncertainty quantification. Results will be presented for selected metals, with comparisons to independently developed EOS.   

Ramp compression of a metallic liner driven by a shaped 5 MA current on the SPHINX machine

SPHINX is a 6MA, 1-$\mu $s Linear Transformer Driver operated by the CEA Gramat (France) and primarily used for imploding Z-pinch loads for radiation effects studies. Among the options that are currently being considered for improving the generator performances, there is a compact Dynamic Load Current Amplifier (DLCM). A method for performing magnetic ramp compression experiments, without modifying the generator operation scheme, was developed using the DLCM to shape the initial current pulse. We present the overall experimental configuration chosen for these experiments, based on electrical and hydrodynamic simulations. Initial results obtained over a set of experiments on an aluminum cylindrical liner, ramp-compressed to a peak pressure of 23 GPa, are presented. Details of the electrical and Photonic Doppler Velocimetry (PDV) setups used to monitor and diagnose the ramp compression experiments are provided. Current profiles measured at various locations across the system, particularly the load current, agree with simulated current profile and demonstrate adequate pulse shaping by the DLCM. The liner inner free surface velocity measurements agree with the hydrocode results obtained using the measured load current as the input. Higher ramp pressure levels are foreseen in future experiments with an improved DLCM system.   

A new pulsed power facility for isentropic compression experiments

A new pulsed power facility has been commissioned at Imperial College as part of the Institute for Shock Physics. The facility, based around the 2MA MACH - Mega Ampere Compression and Hydrodynamics - generator, is dedicated towards exploring ramp loading of material samples with pressure up to $\sim$100KBar. Here we present details of the facility, including its suite of diagnostics. Initial strip line experiments will be discussed, including simulations of the strip line behavior. Finally we will discuss future work on the machine, including novel load ideas to significantly increase pressures, new diagnostics and expansion of the facility to include external high magnetic fields and intense laser pulses. This work was supported by the Institute of Shock Physics, funded by AWE Aldermaston, and the EPSRC.   

Observation of H/He Demixing Under Deep Jovian Planetary Conditions

Giant gas planets, such as Jupiter, Saturn and most of the exoplanets discovered so far, consist mostly of hydrogen and helium. A major source of influence for their interior models is the possibility of demixing for warm dense hydrogen/helium mixtures. As proposed 30 years ago by Salpeter and Stevenson, H/He phase separation should completely change the interior structure and the evolution of the planets when it happens (sometimes pictured as a He rain). We will present our experimental approach to observe this separation by making a high pressure experiment on earth. It is based on the concept of laser shock in diamond anvil cells. This has been first applied successfully to determine the equation of state of warm dense helium and warm dense hydrogen. It will be shown that a pre-compression of 4.0 GPa is necessary to reach the thermodynamic conditions of deep Saturn. A new target design has been developed for that. Experiments have been performed by using 6 KJ of the OMEGA laser facility.   

The Science of Dynamic Compression at the Mesoscale and the Matter-Radiation Interactions in Extremes (MaRIE) Project

A scientific transition is underway from traditional observation and validation of materials properties to a new paradigm where scientists and engineers design and create materials with tailored properties for specified functionality. Of particular interest are the regimes of materials' response to thermomechanical extremes including materials deforming under imposed strain rates above the quasi-static range (i.e. $>10^{-3}$ s$^{-1}$), material subjected to imposed shocks, but also material response to static, high-pressures. There is a need for the study of materials at the ``mesoscale,'' the scale at which sub-granular physical processes and inter-granular organization couple to determine microstructure, crucially impacting constitutive response at the engineering macroscale. For these reasons Los Alamos is proposing the MaRIE facility as a National User Facility to meet this need. In particular, three key science challenges will be identified: Link material microstructure to macroscopic behavior under dynamic deformation conditions; Make the transition from observation and validation to prediction and control of dynamic processes; and Develop the next generation of diagnostics, dynamic drivers, and predictive models to enable the necessary, transformative research.