T. Awe

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.

Field Reversed Configuration target status for Magnetized Target Fusion

Summary form only given. The Los Alamos National Laboratory (LANL) collaboration with Air Force Research Laboratory (AFRL) collaboration is close to a physics demonstration of compressional heating in a Magneto Inertial Fusion (MIF) plasma target. These first Magnetized Target Fusion (MTF) experiments will use solid aluminum flux compressor shells. The experimental high density Field Reversed Configuration (FRC) can be made to translate fast enough so that FRC lifetime is not an issue. We show some initial translation data from the Los Alamos FRC experiment FRXL that characterize the translated target plasma. We have taken advantage of the LANL experience so that a near duplicate of FRXL has come up in several months. The solid liner MTF is only one of several magnetized, pulsed MIF fusion schemes that are being pursued. We outline the present status of MTF including target formation, translation to a trapping region, and compression results.

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.

Overview of FRC translatin experimental collaboration for magnetized target fusion

We present and overview the experimental high density Field Reversed Configurationi (FRC) approach for application to a physics demonstration of magnetized target fusion (MTF). This MT target plasma continues to be developed at the Los Alamos FRC experiment FRXL. The first translated FRXL FRC data will be shown, where the translation speeds exceed 15cm/usec, which yields a translation time substantially shorter than the FRC lifetimes. The conical theta coil is expected to generate toroidal magnetic field and helicity and increase stability and lifetime. The implications of the present data for MTF experiments will be discussed, along with the hardware, diagnostics, and pre-compression plasma formation and trapping experiments.

Experiments on field reversed configuration (FRC) formation and their compression using liners

Three types of experiments developing FRC formation, injection, and compression are described: field-compression, FRC formation-translation-capture, and FRC formation—translation—capture—compression. All involve the generation of primarily axial guide and mirror magnetic fields with ∼ 2 Tesla peak fields, using ∼5 ms rise time discharges into 9 pulsed magnet coils surrounding the liner implosion portion of the device. The field compression and FRC compression experiments use 12 MA, 4.5 MJ Shiva Star capacitor bank axial discharges to drive implosions of 30 cm tall, 10 cm diameter, 1 mm thick Al shells or liners. The FRC capture experiments are a pre-requisite for the destructive FRC compression experiments. All FRC experiments use 3 capacitor discharges into a segmented theta coil surrounding the FRC formation region to establish a bias field, accomplish pre-ionization of deuterium gas, and provide the reverse field main theta discharge (∼ 1 MA) which forms the FRC. This is aided by two cusp field discharges. The guide and mirror fields enable translation of the FRC and its capture in the liner interior region. Diagnostics include pulsed power (current and voltage), magnetic field, field exclusion, laser interferometry, imaging and spectroscopy, radiography, and both activation and time-of-flight neutron detection. Design features and operating parameters are guided by 2D-MHD simulations. Supported by DOE-OFES.

Experimental study of plasma formation on an aluminum surface pulsed with megagauss magnetic field

Plasma formation on the surface of thick metal, in response to a pulsed multi-megagauss magnetic field, has been experimentally investigated. Aluminum rods with initial diameters ranging from 0.5 to 2.0 mm are pulsed with the 1.0 MA, 100 ns Zebra generator. Surface magnetic field rates of change vary from 20 to 80 MG/µs, with corresponding peak fields of 1 to 6 MG. For all rod diameters selected, the magnetic field penetration depth is smaller than the thickness of the conductor, enabling study of the material response in the “liner” or “thick-wire” regime. A novel mechanical connection was developed that eliminates non-thermal precursor plasma, which was produced by electric-field-driven electron avalanche and arcing electrical contacts in earlier experiments. The dynamics of the pulsed aluminum rod and resultant surface plasma are examined with time-resolved imaging, pyrometry, spectroscopy, and laser shadowgraphy. Thermal plasma forms when the surface magnetic field reaches 2.0 MG, with no clear dependence on the rise time of the applied field. Plasma forms at lower current and reaches higher peak temperature when the initial rod diameter is reduced. Surface temperature and instability growth both decrease dramatically when the initial rod diameter exceeds 1.6 mm, suggesting a transition to a regime in which highly resistive vapor forms, instead of plasma. For rods with initial diameter 1.25 mm or below, which clearly demonstrate surface plasma formation before peak current, maximum surface temperatures are of order 10 eV, and radial expansion velocities are approximately 2—4 km/s during the current rise. Emission lines from multiply ionized aluminum atoms are characterized through time gated EUV spectroscopy. The measurement of the time-evolution of the surface temperature, aluminum expansion rate, and ionization state, as a function of applied field, significantly constrains the choice of models used in radiation-magnetohydrodynamic simulations.