Andrew Oxner

The dense Z-pinch program at the University of Nevada, Reno

A new program of research into the physics of dense z-pinches is being initiated around a high-repetition-rate two-terawatt generator (formerly Zebra/HDZP-II: 2MV, 1.2 MA, 100 ns, 200 kJ, 1.9 Ω final line impedance) transferred to the University of Nevada. Reno Physics Department from Los Alamos National Laboratory. Areas for study include the early-time evolution of a current-driven wire, the plasma turbulence around and between wires, the suppression or reduction of instabilities, the nature of x-ray bright spots, and the tailoring of the x-ray emission spectrum. Novel loads that introduce a stabilizing velocity shear or density profile will be examined, along with configurations that promise to increase the quantity, hardness, stability, and reproducibility of x-ray emission. A wide variety of diagnostics are being developed, so as to diagnose the plasma thoroughly and make detailed comparisons between experiment, computer simulation, and theory. These include x-ray, soft x-ray, and extreme ultraviolet...

Advanced x-ray and extreme ultraviolet diagnostics and first applications to x-pinch plasma experiments at the Nevada Terawatt Facility

A wide variety of x-ray and extreme ultraviolet (EUV) diagnostics are being developed to study z-pinch plasmas at the Nevada Terawatt Facility at the University of Nevada, Reno. Time-resolved x-ray/EUV imaging and spectroscopy, x-ray polarization spectroscopy, and backlighting will be employed to measure profiles of plasma temperature, density, flow, and charge state, and to investigate electron distribution functions and magnetic fields. The instruments are state-of-the-art applications of glass capillary converters (GCC), multilayer mirrors (MLM), and crystals. New devices include: a novel GCC-based two-dimensional imaging spectrometer, a six-channel crystal/MLM spectrometer (“polychromator”) with a transmission grating spectrometer, and two sets of x-ray/EUV polarimeters/spectrometers. An x-pinch backlighter is under development. X-ray polarimeter/spectrometer, a survey spectrometer, a multichannel time-gated x-ray pinhole camera, and filtered fast x-ray diodes have observed the structure of Ti and Fe ...

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.

Operation regimes of magnetically insulated transmission lines

Magnetically insulated transmission lines (MITLs) are commonly used for efficient power transport in the vacuum section of pulsed power devices. Plasma forming from metal surfaces limits the power transmitted to a load through MITLs. It eventually shunts the load, producing so-called MITL closure. Fundamental experiments are being performed on high intensity power transmission through coaxial cylindrical vacuum transmission lines. A current that rises to 1 MA in 100 ns is driven through the MITLs by a 2-MV, 2-/spl Omega/ pulse generator (Zebra). The condition of the MITL surfaces is carefully controlled and characterized before each shot. Differential B-dot probes measure the current before and after the MITL, to determine the time of gap closure. Optical imaging and laser diagnostics observe the plasma evolution in the gap with time and space resolution. The radial gap of the cylindrical vacuum transmission line has been systematically varied, and the time of MITL closure measured. They increase with the radial gap size in a discontinuous manner. Critical transitions (discontinuous jumps in closure time) appear to separate distinct MITL operation regimes. This is the first experiment and data set of this kind known to the authors. Electromagnetic-particle-in-cell and radiation-magnetohydrodynamic computer modeling assist the experiment, being used to refine the experimental design and to interpret the results.

X-ray temporal, spatial and spectral study of 0.9 MA X-pinch Ti, Fe, Mo, W and Pt radiation sources. Energetic electron beam and hard X-ray generation

The X-ray emission of Ti, Fe, Mo, W, and Pt X-pinches currently being studied at the Nevada Terawatt Facility (0.9-1.0 MA, 100 ns). New X-ray diagnostics for time-resolved spectroscopy and imaging has been developed and used in X-pinch experiments. Total X-ray/EUV yield was more than 10 kJ. The minimum X-ray pulse duration was 1.1 ns. For Ti, Mo and W it was observed that X-ray pulses occurred in two or three groups in the narrow time intervals after the start of the current. Most of the compact emitting region has been observed for a planar-loop Mo X-pinch. Strong jets were observed (Ti, Fe, Mo) directed toward the discharge axis, perpendicular to the wires. A structure of the X-pinch includes energetic electron beams directed toward the anode and along wires. A pulse anisotropic hard X-ray radiation was observed moving upwards along the axial axis with an energy of several hundred keV (Mo). The size of the source was smaller than 1 mm.

Powerful microfocus x-ray and hard x-ray 1 MA x-pinch plasma source for imaging, spectroscopy, and polarimetry

The x-ray emission of Ti, Fe, Mo, W and Pt x-pinches are currently bieng studied at the Nevada Terawatt Facility z- pinch machine (0.9-1.0 MA, 100 ns). New x-ray diagnostics for time-resolved spectroscopy and imaging has been developed and used in x-pinch experiments. The total x- ray/EUV yield was more than 10 kJ. The minimum x-ray pulse duration was 1.1 ns (Mo, W, Pt). For Ti, Mo and W pinches x-ray pulses occurred in two or three groups in the narrow time intervals after the start of the current. The most compact emitting region has been observed for a planar-loop Mo x-pinch (the number of hot spots ranging from 1-5 with a minimum size smaller than 30 micrometers at (lambda) <1.5-2 Angstoms). Strong jets were observed (Ti, Fe, Mo) directed toward the discharge axis, perpendicular to the wires. A structure of an x-pinch includes energetic electron beams directed toward the anode and along wires. The total beam energy increases from Ti to W. A pulse of hard x-ray radiation was observed moving upwards along the axial axis with an energy of several hundred keV(Mo). The size of this source was smaller than 1 mm. The measurements of temperature and density of x-pinch plasmas were based on theoretical modeling of K-shell Ti and L-shell Mo spectra (Te=1.5 keV for Ti, 0.8 keV for Mo, Ne up to 2- 3x1022 cm-3 with 1-10% of hot electrons).

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.

Two-terawatt Zebra Z-pinch at the Nevada terawatt facility

A high-repetition-rate, 2-TW Z-pinch (Zebra or HDZP-II from LANL: 2 MV, 1.2 MA, 100 ns, 200 kJ, 1.9 ohm) has been assembled to investigate the early-time evolution of a current-driven wire, the plasma turbulence around and between wires, the acceleration of a plasma current sheet by a magnetic field, and the suppression or reduction of plasma instabilities, and to generate radiation for applications. The heating, expansion, and dynamics of wires driven by current prepulses similar to those at SNL-Z is being examined in isolated wires and soon in SNL-Z wire arrays. 290 trillion watts of X-rays can now be generated by a few cubic millimeters of plasma. The source of this plasma is the Z-pinch. This plasma confinement device drives a giant current through a tiny load, compressing and heating it with extreme current-produced magnetic fields. The Z-pinch suffers from plasma instabilities that limit its performance. The ultimate performance limit of the Z-pinch is unknown: another order of magnitude increase in X-ray power levels may be possible. Such an improvement would open up new applications. Understanding the dense Z-pinch is vital to the search to ameliorate it. This article describes the activation of the 2-TW Zebra Z-pinch, the development of diagnostics, and an initial single-wire experiment.

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.

X-ray temporal, spatial and spectral study of 0.9 MA X-pinch Ti, Fe, Mo, W and Pt radiation sources

The X-ray emission of Ti, Fe, Mo, W, and Pt X-pinches currently being studied at the Nevada Terawatt Facility (0.9-1.0 MA, 100 ns). New X-ray diagnostics for time-resolved spectroscopy and imaging has been developed and used in X-pinch experiments. Total X-ray/EUV yield was more than 10 kJ. The minimum X-ray pulse duration was 1.1 ns. For Ti, Mo and W it was observed that X-ray pulses occurred in two or three groups in the narrow time intervals after the start of the current. Most of the compact emitting region has been observed for a planar-loop Mo X-pinch. Strong jets were observed (Ti, Fe, Mo) directed toward the discharge axis, perpendicular to the wires. A structure of the X-pinch includes energetic electron beams directed toward the anode and along wires. A pulse anisotropic hard X-ray radiation was observed moving upwards along the axial axis with an energy of several hundred keV (Mo). The size of the source was smaller than 1 mm.