Walter Louis Atchison

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.

Summary form only given. When, where, and how plasma forms on metal surfaces driven by intense current are important questions for both basic science and applications. The thermal ionization of the surface of thick metal, in response to a pulsed multi-megagauss magnetic field, is being investigated with detailed experiments 1-3 and numerical modeling 4-8 . Aluminum 6061 rods with initial radii (R 0 from 0.25-1.00 mm) larger than the magnetic skin depth are pulsed with the 1.0-MA, 100-ns Zebra generator. The surface is examined with time-resolved imaging, radiometry, spectroscopy, and laser shadowgraphy. The surface magnetic field (B s ) rises at 30-80 MG/us, with corresponding peak B s of 1.5-4 MG For these rise rates, thermal plasma is observed to form when B s reaches 2.2 MG. Optical emission from the plasma surface is initially non-uniform, but becomes quite highly uniform as T BB increases. While the current is rising linearly, the Al surfaces expand at 3-4 km/s, with no evidence, after surface plasma forms, of either re-pinching or outward acceleration. At peak current, T BB is 20 eV for R 0 =3D 0.50 mm rods, but only 0.7 eV for R 0 =3D 1.00 mm rods. Strong plasma fluting develops in the first case, while extremely smooth expansion occurs in the second (indicating resistive vapor). Moreover, after peak current, plasma (if formed) accelerates (to 10 km/s), while resistive vapor continues expanding at constant speed. The well-characterized experiment is providing a benchmark for radiation-MHD modeling. VNIIEF-UP and UNR-MHRDR modeling have achieved results that agree well with observations. Plasma is formed in low density material resistive enough to expand across the magnetic field, yet conductive enough that ohmic heating exceeds expansion cooling as the expanding material undergoes the liquid-vapor transition. An analytic calculation indicates ohmic heating should produce plasma, consistent with numerical and experimental observations.

A series of experiments to study the behavior of thick wires (0.5 mm to 2 mm in diameter) driven by currents of about 1 MA have recently been conducted on the Zebra facility at the University of Nevada, Reno. The objective of these experiments was to study plasma formation on the surface of conductors under the impact of megagauss magnetic fields. Laser shadowgraphy, filtered optical and extreme ultraviolet photodiodes, and extreme ultraviolet spectroscopy used in the experiments provided data on radial expansion of wires and plasma radiation. This paper focuses on numerical simulations of these experiments. Simulations with wires having a diameter of 1.6 mm and less demonstrated plasma formation with temperatures above 3 eV, which is in preliminary agreement with the experiment. For 2 mm diameter wires, although plasma can be observed in the simulations, it has substantially smaller optical thickness than in the simulations of the smaller-diameter wires, and the radiation fluxes prove to be much lower. This can shed light on the experimental results, where the radiation of the 2 mm wires was very weak. The simulated time dependences of the wire radii agree rather well with the experimental results obtained using laser diagnostics and light imaging. The experimental data of the photodiodes also agree well with the simulated time dependence of the detected radiation.

We are developing a new high explosive pulsed power (HEPP) system based on the 1.4 m long Ranchero generator which was developed in 1999 for driving solid density z-pinch loads. The new application requires approximately 40 MA to implode similar liners, but the liners cannot tolerate the 65 ¿s, 3 MA current pulse associated with delivering the initial magnetic flux to the 200 nH generator. To circumvent this problem, we have designed a system with an internal start switch and four explosively formed fuse (EFF) opening switches. The integral start switch is installed between the output glide plane and the armature. It functions in the same manner as a standard input crowbar switch when armature motion begins, but initially isolates the load. The circuit is completed during the flux loading phase using post hole convolutes. Each convolute attaches the inner (coaxial) output transmission line to the outside of the outer coax through a penetration of the outer coaxial line. The attachment is made with the conductor of an EFF at each location. The EFFs conduct 0.75 MA each, and are actuated just after the internal start switch connects to the load. EFFs operating at these parameters have been tested in the past. The post hole convolutes must withstand as much as 80 kV at peak dI/dt during the Ranchero load current pulse. We describe the design of this new HEPP system in detail, and give the experimental results available at conference time. In addition, we discuss the work we are doing to test the upper current limits of a single standard size Ranchero module. Calculations have suggested that the generator could function at up to ~120 MA, the rule of thumb we follow (1 MA/cm) suggests 90 MA, and simple flux compression calculations, along with the ~4 MA seed current available from our capacitor bank, suggests 118 MA is the currently available upper limit.

The paper discusses devices with a Disk Explosive Magnetic flux compression Generator (DEMG), which are similar to the ALT-1,2 experimental devices and are intended for testing the possibility of producing 1-3 TPa (10-30 Mbar) pressures and the possibility of measuring Hugoniots of different materials at such pressures. It is expected that two-layer, cylindrical liners, Al+Fe and/or Al+W, will be used, driven by 4-5 MG magnetic fields to ~ 20 km/s velocities. The paper presents and discusses simulated characteristics of these devices, in which currents, energies and powers delivered to the liner load can reach ~ 70 MA, ~ 40 MJ and ~ 20 TW and exceed those in the ALT-1,2 devices by a factor of ~2, ~ 4 and ~ 7, respectively.