Nicholas D. Ouart

Spectroscopy and implosion dynamics of low wire number nested arrays on the 1 MA COBRA generator

Low wire number nested array Z-pinch experiments have been carried out with wires made of aluminum, stainless steel (uniform), and combinations of these two materials (mixed) on the 1MA COBRA generator at Cornell University [J. D. Douglass, J. B. Greenly, D. A. Hammer et al., in Proceedings of the 15th IEEE International Pulsed Power Conference (IEEE, Piscataway, NJ, 2005)]. The outer array consisted of eight wires, whereas the inner array had four or eight wires. The 10μm Al wires were alloy 5056 and the 6.25μm stainless steel wires were alloy SS304. The diagnostic suite included fast-x-ray and extreme ultraviolet (EUV) detectors, a time-gated x-ray pinhole camera, x-ray spectrometers, and laser shadow imaging. The main focus was made on the spectroscopic study of plasma evolution after the main x-ray burst though the data from photoconducting detector (PCD) and EUV signals over the whole period of current, and in addition laser shadowgraphy images before the main x-ray burst were analyzed. Modeling of t...

Studies of Radiative and Implosion Characteristics From Brass Planar Wire Arrays

Experiments with single-planar wire arrays (SPWA) and double PWAs (DPWAs) with brass 310 wires were carried out on the 1-MA Zebra generator at the University of Nevada, Reno. Brass 310 (70% Cu and 30% Zn) PWAs have either 10 or 16 wires with diameters of 10.9 or 7.62 ¿m, respectively. The diagnostic suite included a bolometer, fast X-ray detectors, an axially resolved time-integrated spectrometer, a time-gated spectrometer, a time-gated pinhole camera, and a streak camera. A wire dynamic model was applied to study implosion characteristics, and non-LTE Cu and Zn kinetic models were used to model L-shell radiation from brass. The analysis of the time-gated spectra showed a correlation between the modeled electron temperature and the X-ray signal, and it agrees well with the maximum values from the time-integrated spatially resolved spectra. Modeling of time-gated and time-integrated spectra from brass PWAs indicates stronger opacity effects in L-shell lines for DPWAs.

Study of Electron Beams in Wire Arrays at 1-MA

We analyzed the characteristics of energetic electron beams produced by implosions of multiplanar wire arrays, nested cylindrical wire arrays (NCWAs), and compact cylindrical wire arrays (CCWAs), as well as X-pinches. In this paper, filtered soft and hard X-ray (HXR) diodes, spatially resolved time-integrated and time-gated spatially integrated LiF crystal spectrometers, and a Faraday cup were fielded to study the time evolution and spatial distribution of electron beams. Observed Faraday cup signals (electron cutoff energy, with EB from 42 to 250 keV) always coincide with HXR signals, and their pulse shape is similar to the shape of HXR pulses. The dependence of the total energy of the electron beam (Eb) on the wire material and the geometry of the wire-array load was studied. The electron-beam energies increase with the atomic number of the wire material. Aluminum tracer wires were found to decrease Eb in loads with Cu, Mo, or W wires. The intensity of cold K- and L-shell time-gated spectra correlate with corresponding amplitudes of HXR peaks and Faraday cup signals. The timing of correlation between the onset of energetic electron beams, HXR generation, and appearance of trailing mass for NCWAs and CCWAs is presented and discussed.

Z

In our experiments, we compared planar wire arrays, to which little energy could be coupled via the conventional magnetic‐to‐kinetic conversion mechanism, to cylindrical wire arrays of comparable dimensions and mass. The planar wire arrays were shown to radiate much higher peak power and more energy in sub‐keV and keV spectral ranges, than cylindrical wire arrays. We tested the theoretical conjecture that enhanced resistivity due to the small‐scale inhomogeneity of wire‐array plasmas has a major effect on dynamics, energy coupling to and radiation performance of wire‐array Z‐pinches.

-Pinch Generators

The study of implosion dynamics and spectroscopy of X‐pinches and wire arrays with Al wires alloyed or coated with other near‐Z or higher‐Z materials is discussed. In particular, X‐pinches from two combined Al 5056 and Mo wires and composed from four identical Al 5056 (5%Mg) wires and Cu clad Al (90% Al and 10%Cu) are studied. In addition, wire arrays with Alumel wires (95% Ni and 5% Al) and with Al 5056 wires (uncoated) and coated with 5% NaF are investigated. Spatially‐resolved and integrated x‐ray spectral data and time integrated and time‐gated pinhole x‐ray images accumulated in these X‐pinch and wire array experiments on the UNR 1MA Zebra generator are analyzed. Modeling of K‐shell radiation from Mg provides K‐shell plasma parameters for all Al 5056 wire experiments, whereas modeling of L‐shell radiation from Ni, Cu, and Mo provide parameters for L‐shell plasmas. The importance of using different materials or dopants for understanding of implosion dynamics of different wire materials is illustrated.

Radiation Yield and Dynamics of Planar Wire‐Array Plasma

Experiments with various wire loads from mid-atomic-number wires, which were performed on the university-scale 1-MA Zebra generator at the University of Nevada, Reno, during the last few years, are analyzed to assess the highest electron temperature reached. In particular, the results from experiments with planar wire arrays (PWAs) were considered. Load materials from mid-atomic-number such as stainless steel, Alumel, Cu, brass, Mo, and up to Ag were used to generate L-shell plasmas and to study plasma parameters. Though the full diagnostic set was utilized, the main focus was on X-ray spectroscopic data and on the non-local thermodynamic equilibrium kinetic modeling. As a result, the scaling of the maximum Te with the load material atomic number is presented for the first time in the range from Fe to Ag for L-shell plasmas from PWAs. The highest values of the electron temperature in L-shell plasmas, which are estimated from the modeling, were from both Ag PWAs and X-pinches. This work is important for the development of efficient X-ray radiators on university-scale Z-pinch generators.

Implosion dynamics and Spectroscopy of X-pinches and Wire arrays with doped Al wires on the UNR 1MA Z-pinch generator

X‐pinch experiments using conichrome alloy wires were implemented on 1MA COBRA facility at Cornell University. X‐ray spectral and imaging data were collected. In particular, the spatially‐resolved, time‐integrated x‐ray line K‐shell and L‐shell spectra of Cr, Co, and Ni ions from these alloy X‐pinches were recorded in one pulse using a mica crystal spectrometer. Non‐LTE kinetic models were applied to model these spectra. The results of this modeling are presented and the interpretation of K‐shell and L‐shell Cr, Co, and Ni as well as L‐shell Mo spectra from conichrome alloy X‐pinches are discussed.

Producing Kiloelectronvolt L-Shell Plasmas on Zebra at UNR

Summary form only given. Spatially-resolved and spatially-integrated X-ray spectral data and time-integrated and time-resolved pinhole X-ray images accumulated from Cu wire arrays and X-pinch experiments on the Zebra generator have been analyzed. In particular, Cu wire arrays were investigated with different number of wires and load sizes, and Cu X-pinches implemented with two and four wires with several diameters have been considered. Cu wire arrays were composed from both pure Cu wires and Cu alloys with 4% of Ni. Non-LTE kinetic models of Cu and Ni have been applied to account for the L-shell and K-shell radiation from the Cu and Ni ions. The advantage of using Cu alloys with a small percentage of Ni for diagnostics of Cu wire arrays will be illustrated. The resulting plasma parameters from modeling the Cu wire arrays and X-pinches will be discussed

Spectroscopic Modeling of X‐pinch Plasmas from Alloy Wires with Cr, Co, and Ni K‐ and L‐shell Radiators

Tungsten wire explosions are being intensively studied at Sandia National Laboratories. Any available x-ray spectral data accumulated in Z experiments with appropriate theoretical modeling can lead to better understanding of plasma evolution during a wire explosion. The present work focuses on the theoretical development of M-shell spectroscopy of W ions in the spectral range from 4 up to 8 Å. The majority of line emissions in this spectral region is composed of 31 → 41′, 51″ transitions. Atomic data were calculated using Cowan and MBPT codes for all isoelectronic sequences contributing into this spectral range. The non-LTE kinetic model was developed based on these atomic data. The sensitivity of this model to the number of included ions, configurations and levels, the electron density, ionization balance, and electron distribution function is discussed. The complete modeling of this spectrum allows a detailed diagnostic of a hotter plasma core in z-pinch experiments involving heavy ions.

Applications of the Non-LTE Models of Cu and Ni to Wire Arrays and X-Pinches from Cu Wires (Pure and Alloy with Ni) Produced on the 1-MA Pulsed Power Generator at UNR

An overview of the detailed spectroscopic analysis of more than fifty z-pinch shots at the Nevada Terawatt Facility is presented. Experimental x-ray spectra generated by Ti, Fe, Mo, W, and Pt z-pinches have been collected in wire implosions driven by a 0.7–1 MA current. Different configurations and masses of z-pinch loads are studied. In x-pinch experiments, pinhole images and damage to the anodes after shots evidence the presence of electron beams. Non-LTE collisional-radiative atomic kinetics models that include hot electrons have been developed and applied to interpret experimental x-ray spectra. In particular, diagnostics for K-shell spectra of Ti ions and L-shell spectra of Mo ions are presented.