Publications

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The intense magnetic field generated in the 20 MA Z-machine is used to accelerate metallic flyer plates to high velocity for the purpose of generating strong shocks in equation of state experiments. We present results pertaining to experiments in which a 0.085 cm thick Al flyer plate is magnetically accelerated across a vacuum gap into a quartz target. Peak magnetic drive pressures up to 4.9 Mbar were produced, which yielded a record 34 km/s flyer velocity without destroying it by shock formation or Joule heating. Two-dimensional MHD simulation was used to optimize the magnetic drive pressure on the flyer surface, shape the current pulse to accelerate the flyer without shock formation (i.e., quasi-isentropically), and predict the flyer velocity. Shock pressures up to 11.5 Mbar were produced in quartz. Accurate measurements of the shock velocity indicate that a fraction of the flyer is at solid density when it arrives at the target. Comparison of measurements and simulation results yields a consistent picture of the flyer state at impact with the quartz target.

The intense magnetic field produced by the 20 MA Z accelerator is used as an impulsive pressure source to accelerate metal flyer plates to high velocity for the purpose of performing plate impact, shock wave experiments. This capability has been significantly enhanced by the recently developed pulse shaping capability of Z, which enables tailoring the rise time to peak current for a specific material and drive pressure to avoid shock formation within the flyer plate during acceleration. Consequently, full advantage can be taken of the available current to achieve the maximum possible magnetic drive pressure. In this way, peak magnetic drive pressures up to 490 GPa have been produced, which shocklessly accelerated 850 {micro}m aluminum (6061-T6) flyer plates to peak velocities of 34 km/s. We discuss magnetohydrodynamic (MHD) simulations that are used to optimize the magnetic pressure for a given flyer load and to determine the shape of the current rise time that precludes shock formation within the flyer during acceleration to peak velocity. In addition, we present results pertaining to plate impact, shock wave experiments in which the aluminum flyer plates were magnetically accelerated across a vacuum gap and impacted z-cut, {alpha}-quartz targets. Accurate measurements of resulting quartz shock velocities are presented and analyzed through high-fidelity MHD simulations enhanced using optimization techniques. Results show that a fraction of the flyer remains at solid density at impact, that the fraction of material at solid density decreases with increasing magnetic pressure, and that the observed abrupt decrease in the quartz shock velocity is well correlated with the melt transition in the aluminum flyer.

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The Sandia Z accelerator has become a unique platform to study matter at extreme conditions. The large currents (20 MA, 200-300 ns rise time) and magnetic fields (several MG) produced by Z generate magnetic compression in the multi-Mbar regime, enabling quasi-isentropic compression experiments (ICE) to several Mbar stresses. Thus, the Z platform is useful in determining high stress material isentropes, performing phase transition studies (including rapid solidification), obtaining constitutive property information, and estimating material strength at high stress. Furthermore, the magnetic pressure can also accelerate macroscopic flyer plates to velocities in excess of 30 km/s. Thus, impact experiments can be performed to ultra-high pressures. Furthermore, the adiabatic release response of materials can be investigated through shock and release experiments, allowing hot, dense liquid states to be probed. The Z platform allows a large expanse of the equation of state surface to be explored enabling new and exciting material dynamics experiments. Specific examples from each of the areas mentioned above will be discussed.

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This report describes the complete revision of a deuterium equation of state (EOS) model published in 1972. It uses the same general approach as the 1972 EOS, i.e., the so-called 'chemical model,' but incorporates a number of theoretical advances that have taken place during the past thirty years. Three phases are included: a molecular solid, an atomic solid, and a fluid phase consisting of both molecular and atomic species. Ionization and the insulator-metal transition are also included. The most important improvements are in the liquid perturbation theory, the treatment of molecular vibrations and rotations, and the ionization equilibrium and mixture models. In addition, new experimental data and theoretical calculations are used to calibrate certain model parameters, notably the zero-Kelvin isotherms for the molecular and atomic solids, and the quantum corrections to the liquid phase. The report gives a general overview of the model, followed by detailed discussions of the most important theoretical issues and extensive comparisons with the many experimental data that have been obtained during the last thirty years. Questions about the validity of the chemical model are also considered. Implications for modeling the 'giant planets' are also discussed.

A new laser trigger system (LTS) has been installed on Z that benefits the experimenter with reduced temporal jitter on the x-ray output, the confidence to use command triggers for time sensitive diagnostics and the ability to shape the current pulse at the load. This paper presents work on the pulse shaping aspects of the new LTS. Pulse shaping is possible because the trigger system is based on 36 individual lasers, one per each pulsed power module, instead of a single laser for the entire machine. The firing time of each module can be individually controlled to create an overall waveform that is the linear superposition of all 36 modules. In addition, each module can be set to a long- or short-pulse mode for added flexibility. The current waveform has been stretched from ∼100 ns to ∼250 ns. A circuit model has been developed with BERTHA Code, which contains the independent timing feature of the new LTS to predict and design pulse shapes. The ability to pulse-shape directly benefits isentropic compression experiments (ICE) and equation of state measurements (EOS) for the shock physics programs at Sandia National Laboratories. With the new LTS, the maximum isentropic loading applied to Cu samples 750 um thick has been doubled to 3.2 Mb without generating a shockwave. Macroscopically thick sample of Al, 1.5 mm, have been isentropically compressed to 1.7 Mb. Also, shockless Ti flyer-plates have been launched to 21 km·s-1, remaining in the solid state until impact.

The high-pressure response of cryogenic liquid deuterium (LD{sub 2}) has been studied to pressures of {approx}400GPa and densities of {approx}1.5g/cm{sup 3}. Using intense magnetic pressure produced by the Sandia National Laboratories Z accelerator, macroscopic aluminum or titanium flyer plates, several mm in lateral dimensions and a few hundred microns in thickness, have been launched to velocities in excess of 22 km/s, producing constant pressure drive times of approximately 30 ns in plate impact, shock wave experiments. This flyer plate technique was used to perform shock wave experiments on LD{sub 2} to examine its high-pressure equation of state. Using an impedance matching method, Hugoniot measurements of LD{sub 2} were obtained in the pressure range of {approx}22-100GPa. Results of these experiments indicate a peak compression ratio of approximately 4.3 on the Hugoniot. In contrast, previously reported Hugoniot states inferred from laser-driven experiments indicate a peak compression ratio of approximately 5.5-6 in this same pressure range. The stiff Hugoniot response observed in the present impedance matching experiments was confirmed in simultaneous, independent measurements of the relative transit times of shock waves reverberating within the sample cell, between the front aluminum drive plate and the rear sapphire window. The relative timing was found to be sensitive to the density compression along the principal Hugoniot. Finally, mechanical reshock measurements of LD{sub 2} using sapphire, aluminum, and {alpha}-quartz anvils were made. These results also indicate a stiff response, in agreement with the Hugoniot and reverberating wave measurements. Using simple model-independent arguments based on wave propagation, the principal Hugoniot, reverberating wave, and sapphire anvil reshock measurements are shown to be internally self-consistent, making a strong case for a Hugoniot response with a maximum compression ratio of {approx}4.3-4.5. The trends observed in the present data are in very good agreement with several ab initio models and a recent chemical picture model for LD{sub 2}, but in disagreement with previously reported laser-driven shock results. Due to this disagreement, significant emphasis is placed on the discussion of uncertainties, and the potential systematic errors associated with each measurement.

The intense magnetic field generated in the 20 MA Z-machine is used to accelerate metallic flyer plates to high velocity (peak velocity {approx}20-30 km/s) for the purpose of generating strong shocks (peak pressure {approx}5-10 Mb) in equation of state experiments. We have used the Sandia developed, 2D magneto-hydrodynamic (MHD) simulation code ALEGRA to investigate the physics of accelerating flyer plates using multi-megabar magnetic drive pressures. Through detailed analysis of experimental data using ALEGRA, we developed a 2D, predictive MHD model for simulating material science experiments on Z. The ALEGRA MHD model accurately produces measured time dependent flyer velocities. Details of the ALEGRA model are presented. Simulation and experimental results are compared and contrasted for shots using standard and shaped current pulses whose peak drive pressure is {approx}2 Mb. Isentropic compression of Al to 1.7 Mb is achieved by shaping the current pulse.

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Using intense magnetic pressure, a method was developed to launch flyer plates to velocities in excess of 20 km s{sup -1}. This technique was used to perform plate-impact, shock wave experiments on cryogenic liquid deuterium (LD{sub 2}) to examine its high-pressure equation of state (EOS). Using an impedance matching method, Hugoniot measurements were obtained in the pressure range of 22--100 GPa. The results of these experiments disagree with the previously reported Hugoniot measurements of LD2 in the pressure range above {approx}40 GPa, but are in good agreement with first principles, ab initio models for hydrogen and its isotopes.

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