T. Goodrich

A Zebra Experiment to Study Plasma Formation by Megagauss Fields

An alternative concept for fusion energy production is magnetized target fusion using metal liners to compress a mixture of magnetic flux and plasma fuel. In liner flux compression experiments, megagauss fields are produced at peak compression that heats the surfaces of aluminum walls of the liner cavity. Some radiation magnetohydrodynamic (MHD) modeling indicates that plasma formation should occur between 3 and 5 MG; however, such modeling depends on assumed material properties, which are a topic of ongoing research. Load hardware and diagnostics have been developed to study metal vapor and plasma formation on aluminum surfaces subjected to pulsed megagauss fields on the University of Nevada Zebra facility. The experiment is designed to study this interesting threshold for plasma formation. A current of 1 MA is pulsed along a stationary central rod to generate magnetic fields of 2-4 MG. The goal is to observe and diagnose the formation of metal vapor and plasma in the vicinity of the rod. The simple geometry enables easy access by diagnostics, which include magnetic sensors, filtered photodiode measurements, optical imaging, and laser schlieren, shadowgraphy, and interferometry. From these measurements, the magnetic field, the temperature of the surface metal plasma, the radiation field, and the growth of instabilities can be inferred. The diagnostics are time resolved to individually examine the distinct phases of heating, surface plasma formation predicted by radiation MHD modeling, and instability.

Development of an Experiment to Study Plasma Formation by Megagauss Fields

Load hardware and diagnostics have been developed to study metal vapor and plasma formed from aluminum surfaces by pulsed MG fields on Zebra. Radiation MHD modeling indicates plasma formation should occur between 3-5 MG, but such modeling depends on assumed material properties, which are a topic of ongoing research. The experiment is designed to learn about this interesting threshold for plasma formation. A current of 1 MA is pulsed along a stationary, central wire, to generate magnetic fields of 3-5 MG. The goal is to observe and diagnose the formation of metal vapor and plasma in the vicinity of the wire. The simple geometry enables easy access by diagnostics, which include magnetic sensors, filtered photodiode measurements, optical imaging, and laser schlieren, shadowgraphy, interferomerry and Faraday rotation. From these measurements the magnetic field, the density and temperature of the surface metal plasma, the radiation field, and the growth of instabilities will be inferred. Predictions of experimental data will be calculated from numerical simulations and compared with experimental results. The diagnostics are time resolved, so as to examine individually the distinct phases of compression, plasma formation, radiation-magnetohydrodynamic evolution, and instability. Diagnostics have being developed using a small HV pulser.

The Challenge of Wall-Plasma Interaction with Pulsed MG Fields Parallel to the Wall

Experiments suitable for a variety of pulsed power facilities are being developed to study plasma formation and stability on the surface of typical liner materials in the megagauss (MG) regime. Understanding the plasma properties near the surface is likely to be critical for the design of Magnetized Target Fusion experiments, where the plasma density in the region near the wall can play an important role in setting the transport from hot fuel to the cold boundary. From the perspective of diagnostic access and simplicity, the surface of a stationary conductor with large enough current to generate MG surface field offers advantages compared with studying the surface of a moving liner. This paper reports on recent experiments at UNR that have generated magnetic fields in the range of about 0.2 to 3 MG, which confirm the viability of future experiments planned at Atlas and/or Shiva Star. Diagnostics reported here involve electrical measurements, streak camera photography, and surface luminosity. Additional diagnostic measurements and numerical modeling will be reported in the future.

Plasma Formation and Evolution from an Aluminum Surface Driven by a MG Field

Summary form only given. Applying a magnetic field of several megagauss to a surface drives an interesting interplay of magnetic diffusion, hydrodynamics, and radiative energy transfer. This physics is important in wire-array Z-pinches, high current fuses, magnetically insulated transmission lines, ultrahigh magnetic field generators, magnetized target fusion, and astrophysics. To investigate such plasmas experimentally, 1 MA was driven through a 1 -mm-diameter cylindrical aluminum rod, using the UNR Zebra generator. The 70-ns current rise was sufficiently short that the current skin depth was a small fraction of the conductor radius. Diagnostics included optical imaging to a time-gated intensified CCD camera and a streak camera, magnetic field probes, photodiodes, photomultipliers, and laser shadowgraphy, schlieren, interferometry, and Faraday rotation. These yielded information on the threshold for plasma formation, the expansion of the aluminum, the temperature at the transition between optically thick and optically thin matter, and the growth of the unstable m=0 mode driven by the curvature of the magnetic field. Plasma formation due to ohmic heating was distinguished from plasma formation due to high electric fields or electrical contacts by comparing shots with wire loads vs. loads machined from a solid aluminum cylinder to have a 1-mm-diameter central length but large-diameter contacts. Time-gated images show markedly more uniform light from the machined load than from the wire load. The relatively simple experimental setup was chosen in the hope of providing a benchmark with which to test and improve radiation-magnetohydrodynamics modeling. Measurements have been compared with the results of RAVEN and MHRDR computer simulations, using various assumptions for equation of state, electrical conductivity, and radiation. The simulations yield observed quantities such as luminosity, laser shadowgraphs, and m=0 mode growth. They also yield many additional interesting details, such as the propagation of a compression wave from the surface to the axis and back, with a resultant rapid radial expansion of the surface after peak current.

Thermal aluminum plasma formation and evolution by pulsed megagauss field

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 experiments1-3 and numerical modeling4-8. Aluminum 6061 rods with initial radii (R0 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 (Bs) rises at 30-80 MG/us, with corresponding peak Bs of 1.5-4 MG For these rise rates, thermal plasma is observed to form when Bs reaches 2.2 MG. Optical emission from the plasma surface is initially non-uniform, but becomes quite highly uniform as TBB 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, TBB is 20 eV for R0 =3D 0.50 mm rods, but only 0.7 eV for R0 =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.

Thermal ionization of an aluminum surface by pulsed megagauss field

When, where, and how plasma forms on metal surfaces driven by intense current are important questions for both basic science and applications. The question of the conductivity of a metal surface under pulsed megagauss magnetic field has been posed since at least 1959, when Fowler et al.1 produced fields above 10 MG. The thermal ionization of the surface of thick metal, in response to a pulsed multi-megagauss magnetic field, is being investigated with well-characterized experiments2,3 and detailed 1-D and 2-D numerical modeling4–6. Aluminum rods with radii larger than the magnetic skin depth are pulsed with the 1.0-MA, 100-ns Zebra generator. A novel mechanical connection eliminates nonthermal precursor plasma, which in earlier experiments was produced by electric-field-driven electron avalanche and arcing electrical contacts. The surface was examined with time-resolved imaging, pyrometry, spectroscopy, and laser shadowgraphy. Thermal plasma forms when the surface magnetic field reaches 2.0 MG, in agreement with recent theoretical results4. Measurement of the surface temperature, expansion velocity, and ionization state, as a function of applied field, constrains the choice of models used in the radiation-magnetohydrodynamic simulations, which include the Eulerian MHRDR code and the Lagrangian RAVEN and UP codes. Numerical predictions can vary by orders of magnitude, but, for MHRDR modeling, the computed times of plasma formation agree well with observations if a standard SESAME Maxwell-construct EOS is used in conjunction with a VNIIEF resistivity model. An analytic calculation indicates ohmic heating should produce plasma, consistent with numerical and experimental observations.

Analysis of Plasma Formation in an Experiment with Pulsed Megagauss Field on 1.0-mm Diameter Aluminum Rods

Summary form only given. The physics of the interaction between large magnetic field and conducting media is important to wire-array z-pinches, high current fuses, magnetically insulated transmission lines, ultrahigh magnetic field generators, magnetized target fusion, and astrophysics. In an experiment on the 1 MA UNR Zebra Marx generator, megagauss magnetic field was pulsed on the surface of 1.0-mm-diameter aluminum rods. This rod diameter is large enough to confine current to a skin layer, so that the effects of magnetic diffusion are important, yet small enough to enable magnetic field in the range of a few megagauss; a regime where the formation of plasma on conducting surfaces is expected. Furthermore, to obtain experimental results with a one dimensional character to benchmark Radiation-MHD codes, loads were designed so that the growth of instability leaves the wire approximately axially uniform throughout the current rise. Rods with 1.0-mm-diameter fit this condition for the Zebra bank. An effort was made to distinguish plasma formation due to ohmic heating from plasma formation due to high electric fields or electrical contacts. Standard 1.0-mm-diameter wire loads were compared to loads machined from a solid aluminum cylinder to form a 1 -mm-diameter central length which transitioned smoothly to large-diameter contacts. Diagnostics included V-dot and B-dot probes, streak and time-gated intensified CCD cameras, photodiodes and photomultipliers, and laser shadowgraphy, schlieren, and interferometry. Filtered photodiodes measured radiation from the heated surface of the load. Assuming blackbody emission yields a surface temperature of order 10-eV near the time of peak current. Images from a time-gated intensified CCD camera and a streak camera give snapshots of complex surface phenomena, a time history of the expansion of the rod, and potentially uncover wave propagation speeds in the compressed aluminum. Images obtained support our expectation of a slowly expanding, highly axially symmetric surface during current rise, followed by fast expansion as the field strength diminishes. Laser diagnostics give evidence of plasma formation, as m=0 perturbation growth is observed after peak current. V-dot and B-dot data are being analyzed to obtain insight into the total energy deposition and the dynamic impedance of the load.

Potential Magnetized Target Fusion Targets for Atlas

Summary form only given. The near-term goal of magnetized target fusion research is to compress plasma inside a liner to thermonuclear temperatures. Two candidate plasma targets are the field-reversed configuration (FRC), and the stabilized hard-core Z pinch or MAGO configuration. Advantages of the FRC are high beta intrinsic to the configuration, a separatrix in the magnetic field that can isolate the fusion fuel from the walls, and experimentally demonstrated translation of the FRC from a formation coil into an imploding liner geometry. Advantages of the MAGO are experimentally demonstrated operation at high-density, self-organized stability for a wide range of beta values, wall confinement of fuel as needed for compression to beta greater than unity, and a coaxial geometry well suited for in-situ formation in an Atlas chamber. The major issues for the FRC are its poorly understood magnetohydrodynamic stability and the relatively complex hardware required for formation. For MAGO the major issues are mitigation of wall impurities generated by wall-plasma interactions intrinsic to the configuration, and generally less extensive diagnostics of experimental parameters connected with high-energy-density experimental conditions. This paper will present results of modeling aimed at comparison of these targets in a liner-compression context. An FRC compression experiment is being planned for the Air Force Research Laboratory Shiva Star facility. Higher energy compression experiments of either target are possible on Atlas. To make a sound judgment about the potential of magnetized target fusion, both plasma targets should be tested experimentally as soon as funding can be obtained

A Semi-Analytic Liner Implosion Model for Flux Compression on Atlas

Summary form only given. A flux compression experiment is being designed for the Atlas pulsed power facility. The purpose is to investigate generation of megagauss fields with liner technology in the geometry needed for compression of a stabilized diffuse z pinch.1 To survey possible parameters quickly and conveniently, a semi-analytic model has been developed that computes liner motion under the assumption that the liner remains cylindrically symmetric during the implosion and the metal of the liner is incompressible.2 Thus the liner thickness increases during implosion in a predictable way to conserve liner mass. Equations are derived for the time variation of liner position and circuit current including the effect of back pressure from the compressed flux. The model allows using realistic Atlas circuit parameters. The equations are integrated using the Matlab program and a standard Runge Kutta method. Recently the model has been extended to account for a shunt resistor and the resulting time-dependent current that would be generated inside the liner.3 The important advantage of a shunt resistor is that an auxiliary power supply is not needed to generate the seed flux which liner motion will compress. By tapping the power of Atlas to generate the seed flux, the incremental cost of a flux compression experiment is minimized. The selection of shunt material and dimensions must consider both the heating of the shunt and the amount of trapped flux, which along with the liner kinetic energy determines the final level of compressed magnetic field. Initial results suggest that readily available materials (a steel shunt and an aluminum liner) and properly chosen dimensions give a workable combination that generates magnetic field of several megagauss.