Walter Louis Atchison

Damage initiation and evolution, failure, and recollection processes under axisymmetric convergence were studied in the Russian-Damage experimental series, a joint effort between the Los Alamos National Laboratory and the All-Russian Institute of Experimental Physics. A helical explosive magnetic generator was used to drive a cylindrical liner shell to produce shock wave loading of a concentric cylindrical target shell. Shock wave amplitude was controlled by the liner-to-target spacing and by the magnetic field amplitude. Variation of the current pulse duration produced either a single impact, to study damage initiation through failure, or a double impact, to study failure with recollection. Both full and partial recollection of the main crack was obtained. By fielding high-precision diagnostics to measure the dynamic drive conditions and material response and by employing post-shot metallographic analysis, this project produced well-characterized experimental data across a range of damage and recollection levels for the chosen material, aluminum. We present selected experimental results to illustrate the methodology and utility of this experimental technique. (C) 2014 AIP Publishing LLC.

Summary form only given. Megagauss magnetic fields on metallic surfaces form plasmas that have higher temperatures than would be expected based upon simple diffusion models, as has been demonstrated by a series of aluminum "thick-wire," i.e., rod, experiments that have been conducted on the University of Nevada, Reno (UNR) Zebra generator (2 TW, 1 MA, 100 ns). One physical situation of current interest where such fields and plasmas may be encountered is Magnetized Target Fusion (MTF), where magnetically driven liners compress magnetized plasmas to fusion temperatures, e.g., the Russian MAGO program, the MagLIF program recently initiated at the Sandia National Laboratories and the FRCHX experiments being conducted at the Air Force Research Laboratory. Computations by Garanin and the UNR team have predicted and/or "matched" many of the previously reported Zebra experimental observations, including an observed magnetic field threshold for surface plasma formation. In this paper we report Eulerian computations that show improved agreement with the observations. It is quite common to use computations that "match the experiment" to interpret the experimental results, and we report an interpretation based upon the Eulerian results. However, we have also compared the Eulerian computations with the Lagrangian calculations of Garanin and Lagrangian calculations performed with the computer code Raven. The comparisons between the codes give rise to serious concerns about the differences between Eulerian and Lagrangian simulations and about the differences between van der Waals and Maxwell-construct equations-of-state.

Summary form only given. High performance, condensed matter, liner implosions powered by very high current drive provide an important capability for many physics experiments including shock physics, high magnetic field generation by flux compression, and quasi isentropic, P-dV, compression of materials ranging from solids to plasmas. Where large amounts of kinetic energy, high efficiency of conversion of electrical to kinetic energy, and/or high final implosion velocities are required, the traditional approach is to match the liner implosion time to the energy delivery time of the pulsed power driver. In this approach, the liner converges, arriving at the target while the power source continues to deliver current: late in, but not after, the end of the power pulse. This approach leads to driving the implosion with near-maximum current at small radius, maximizing the driving magnetic field, and maximizing the magnetic pressure through most of the implosion. This approach can lead to relatively high efficiency and minimum implosion time, but it also leads to high accelerations during most of the implosion. The principle obstacle to achieving high precision (accurately cylindrical) implosions is the development of magnetoRayleigh Taylor-like (MRT) instability at the magnetic field / liner interface. While it is the precision of the inner surface of the liner that is important in many applications, especially condensed matter experiments, growth of the MRT instability on the outer surface frequently results in distortions that penetrate through the full thickness of the liner, ruining the precision of the inner surface and making it unsuitable for many shock wave experiments. Instability growth increases with acceleration and is inhibited by material strength and by refining the surface precision, (to reduce the magnitude of initial perturbations). High precision implosions have been reported for cases in which the driving field was sufficiently low that the material at the liner / field interface is not melted, maintaining material strength throughout the liner. An approach to reaching these conditions will be discussed.

We consider an ALT-1,2-like [1,2] system to deliver up to 60-70 MA currents in the liner load and accelerate ~20 g/cm cylindrical liners to ~20 km/s (ALT-1,2: ~31 MA, ~13 g/cm, ~12 km/s). The system is intended for the ALT-3 experiment to test the efficiency of magnetic implosion of impacting liners and to verify the possibility of shock-wave measurements at up to 1 TPa pressures. We describe the physical configuration of the system and its diagnostic suite, which differ significantly from similar systems [3,4]. As compared with [4], changes are made to the physical configuration of individual system units and a number of system parameters: we increase load inductance by a factor of 1.5, use a We consider an ALT-1,2-like [1,2] system to deliver up to 60-70 MA currents in the liner load and accelerate ~20 g/cm cylindrical liners to ~20 km/s (ALT-1,2: ~31 MA, ~13 g/cm, ~12 km/s). The system is intended for the ALT-3 experiment to test the efficiency of magnetic implosion of impacting liners and to verify the possibility of shock-wave measurements at up to 1 TPa pressures. We describe the physical configuration of the system and its diagnostic suite, which differ significantly from similar systems [3,4]. As compared with [4], changes are made to the physical configuration of individual system units and a number of system parameters: we increase load inductance by a factor of 1.5, use a different transmission line and an AI liner (instead of envisaged two-layer liners) etc. Simulated characteristics of the system are presented.different transmission line and an AI liner (instead of envisaged two-layer liners) etc. Simulated characteristics of the system are presented.

ALT-3 (Advanced Liner Technology) is a collaboration between VNIIEF and LANL that aims to conduct high velocity material experiments and measure shock velocities at pressures near 1 TPa. The DEMG (Disk Explosive Magnetic Generator) is used to drive a >;60MA current that accelerates an aluminum liner to speeds in excess of 20 km/s. A simple circuit model is presented that models the DEMG to reasonable accuracy and is then coupled with 1-D magnetohydrodynamic simulations of both the liner and target. 2-D hydrodynamic simulations of the target are also presented along with implications for measurement accuracy and expected results.

Ranchero is a coaxial magnetic flux compression generator (FCG) initiated simultaneously on-axis, which is intended to produce currents approaching 100 MA. We continue with both applications and development, striving for both cost effectiveness and high performance. In this paper we discuss on-going work, provide details of the sophisticated detonation system, and show high current predictions from our active modeling effort. We intend further tests that will push the limits of Ranchero performance.

Summary form only given. Understanding the physical processes that can lead to the formation of plasma on the surface of metals subjected to megagauss magnetic fields and magnetic pressures of 0.1 Mbar and more is vital for both basic science and a wide variety of applications. "Thick" wire, i.e., rod, experiments on the University of Nevada, Reno (UNR) Zebra generator (2 TW, 1 MA, 100 ns) have provided an extensive data base on aluminum surface plasma formation. "Cold start" magnetohydrodynamic (MHD) computer models, one using a Lagrangian technique with an equation-of-state (EOS) that has VanderWaals loops and the second using an Eulerian technique with a Maxwell-construct EOS, have satisfactorily predicted many of the observations and trends in the observations as experimental parameters are varied. UNR Eulerian modeling has computationally predicted a magnetic field threshold for plasma formation and has led to a conclusion that the plasma formation in the Zebra experiments is predominantly a thermal process driven by Ohmic heating, although the modeling demonstrated significant dependence on the choice of equation-of-state (EOS) and resistivity models. In this paper, we examine the sensitivity of the computational results to various computational aspects such as physical model (e.g., with or without thermal conduction), computational approach (Eulerian or Lagrangian), computational grid size, time-step control, vacuum treatment, EOS (Maxwell construct or VanderWaals loops), and other computational issues. We also discuss the insight into experimental behavior that can be learned from the computations.

Numerical simulations of experiments in which plasma is formed on an aluminum surface by megagauss magnetic fields provide the first computational demonstration of a magnetic-field threshold that must be reached for aluminum plasma to begin to form. The computed times of plasma initiation agree reasonably well with the observations across the full range of rod diameters, leading to the conclusion that plasma formation is a thermal process. Computationally, plasma forms first in low-density material that is resistive enough to expand across the magnetic field and yet conductive enough that Ohmic heating exceeds expansion cooling.