Publications

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This paper explores the role of electro-thermal instabilities on the dynamics of magnetically accelerated implosion systems. Electro-thermal instabilities result from non-uniform heating due to temperature dependence in the conductivity of a material. Comparatively little is known about these types of instabilities compared to the well known Magneto-Rayleigh-Taylor (MRT) instability. We present simulations that show electrothermal instabilities form immediately after the surface material of a conductor melts and can act as a significant seed to subsequent MRT instability growth. We also present the results of several experiments performed on Sandia National Laboratories Z accelerator to investigate signatures of electrothermal instability growth on well characterized initially solid aluminum and copper rods driven with a 20 MA, 100 ns risetime current pulse. These experiments show excellent agreement with electrothermal instability simulations and exhibit larger instability growth than can be explained by MRT theory alone. © 2012 American Institute of Physics.

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The double shock layered high-velocity flyer plate is one new capability being developed on Sandia's Z machine. With this technique, dynamic material data at high energy densities can be obtained at points in phase space which lie neither on principal Hugoniots nor on quasi-isentropic ramp curves. We discuss the double shock capability development experiments being performed on Z. © 2012 American Institute of Physics.

We describe a technique for measuring the pressure and density of a metallic solid, shocklessly compressed to multi-megabar pressure, through x-ray radiography of a magnetically driven, cylindrical liner implosion. Shockless compression of the liner produces material states that correspond approximately to the principal compression isentrope (quasi-isentrope). This technique is used to determine the principal quasi-isentrope of solid beryllium to a peak pressure of 2.4 Mbar from x-ray images of a high current (20 MA), fast (∼100 ns) liner implosion. © 2012 American Institute of Physics.

For the past decade, a large, interdisciplinary team at Sandia National Laboratories has been refining the Z Machine (20+ MA and 10+ MGauss) into a mature, robust, and precise platform for material dynamics experiments in the multi-Mbar pressure regime. In particular, significant effort has gone into effectively coupling condensed matter theory, magneto-hydrodynamic simulation, and electromagnetic modeling to produce a fully self-consistent simulation capability able to very accurately predict the performance of the Z machine and various experimental load configurations. This capability has been instrumental in the ability to develop experimental platforms to routinely perform magnetic ramp compression experiments to over 4 Mbar, and magnetically accelerate flyer plates to over 40 km/s, creating over 20 Mbar impact pressures. Furthermore, a strong tie has been developed between the condensed matter theory and the experimental program. This coupling has been proven time and again to be extremely fruitful, with the capability of both theory and experiment being challenged and advanced through this close interrelationship. This paper will provide an overview of the material dynamics platform and discuss several examples of the use of Z to perform extreme material dynamics studies with unprecedented accuracy in support of basic science, planetary astrophysics, inertial confinement fusion, and the emerging field of high energy density laboratory physics. © 2012 American Institute of Physics.

For the past decade, a large, interdisciplinary team at Sandia National Laboratories has been refining the Z Machine (20+ MA and 10+ MGauss) into a mature, robust, and precise platform for material dynamics experiments in the multi-Mbar pressure regime. In particular, significant effort has gone into effectively coupling condensed matter theory, magneto-hydrodynamic simulation, and electromagnetic modeling to produce a fully self-consistent simulation capability able to very accurately predict the performance of the Z machine and various experimental load configurations. This capability has been instrumental in the ability to develop experimental platforms to routinely perform magnetic ramp compression experiments to over 4 Mbar, and magnetically accelerate flyer plates to over 40 km/s, creating over 20 Mbar impact pressures. Furthermore, a strong tie has been developed between the condensed matter theory and the experimental program. This coupling has been proven time and again to be extremely fruitful, with the capability of both theory and experiment being challenged and advanced through this close interrelationship. This paper will provide an overview of the material dynamics platform and discuss several examples of the use of Z to perform extreme material dynamics studies with unprecedented accuracy in support of basic science, planetary astrophysics, inertial confinement fusion, and the emerging field of high energy density laboratory physics. © 2012 American Institute of Physics.

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Current pulse shaping techniques, originally developed for planar dynamic material experiments on the Z-machine [M. K. Matzen, Phys. Plasmas 12, 055503 (2005)], are adapted to the design of controlled cylindrical liner implosions. By driving these targets with a current pulse shape that prevents shock formation inside the liner, shock heating is avoided along with the corresponding decrease in electrical conductivity ahead of the magnetic diffusion wave penetrating the liner. This results in an imploding liner with a significant amount of its mass in the solid phase and at multi-megabar pressures. Pressures in the solid region of a shaped pulse driven beryllium liner fielded on the Z-machine are inferred to 5.5 Mbar, while simulations suggest implosion velocities greater than 50 kms-1. These solid liner experiments are diagnosed with multi-frame monochromatic x-ray backlighting which is used to infer the material density and pressure. This work has led to a new platform on the Z-machine that can be used to perform off-Hugoniot measurements at higher pressures than are accessible through magnetically driven planar geometries. © 2012 American Institute of Physics.

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Planar shock experiments were conducted on granular tungsten carbide (WC) and tantalum oxide (Ta{sub 2}O{sub 5}) using the Z machine and a 2-stage gas gun. Additional shock experiments were also conducted on a nearly fully dense form of Ta{sub 2}O{sub 5}. The experiments on WC yield some of the highest pressure results for granular materials obtained to date. Because of the high distention of Ta{sub 2}O{sub 5}, the pressures obtained were significantly lower, but the very high temperatures generated led to large contributions of thermal energy to the material response. These experiments demonstrate that the Z machine can be used to obtain accurate shock data on granular materials. The data on Ta{sub 2}O{sub 5} were utilized in making improvements to the P-{lambda} model for high pressures; the model is found to capture the results not only of the Z and gas gun experiments but also those from laser experiments on low density aerogels. The results are also used to illustrate an approach for generating an equation of state using only the limited data coming from nanoindentation. Although the EOS generated in this manner is rather simplistic, for this material it gives reasonably good results.

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