M.C. Cooper

Radiation sources with planar wire arrays and planar foils for inertial confinement fusion and high energy density physics research

This article reports on the joint success of two independent lines of research, each of them being a multi-year international effort. One of these is the development of innovative sources, such as planar wire arrays (PWAs). PWAs turned out to be a prolific radiator, which act mainly as a resistor, even though the physical mechanism of efficient magnetic energy conversion into radiation still remains unclear. We review the results of our extensive studies of PWAs. We also report the new results of the experimental comparison PWAs with planar foil liners (another promising alternative to wire array loads at multi-mega-ampere generators). Pioneered at UNR, the PWA Z-pinch loads have later been tested at the Sandia National Laboratories (SNL) on the Saturn generator, on GIT-12 machine in Russia, and on the QiangGuang-1 generator in China, always successfully. Another of these is the drastic improvement in energy efficiency of pulsed-power systems, which started in early 1980s with Zuckers experiments at Nava...

Characterization of pure and mixed Ar, Kr and Xe gas jets generated by different nozzles and a study of X-ray radiation yields after interaction with a sub-ps laser pulse

Gas jets accelerated through a linear supersonic and a conical nozzle, comprising a monomer/cluster mix, were characterized at UNR using a Mach-Zehnder type interferometer and Rayleigh scattering. A comparison of the two nozzle types is presented, showing that the linear nozzle produces gas jets of an order of magnitude denser than the conical nozzle. The linear gas jets of Ar, Kr, and Xe as well as triple mixtures with different percentages of each of the aforementioned gases were characterized. The densest gas jets used Ar as the target gas, while the least dense jets came from Kr. Cluster radii of the pure gases were measured, and Xe gas jets were found to produce the largest gas clusters. A study of X-ray generation by gas jet-laser plasma was performed at the UNR Leopard laser (1.057 μm, 350 fs, ∼1019 W/cm2) on the linear nozzle. The gas jets were irradiated with a high-intensity sub-ps laser pulse. An absolute X-ray output of the laser-gas jet interactions measured by the calibrated PCDs is presente...

Double and Single Planar Wire Arrays on University-Scale Low-Impedance LTD Generator

Planar wire array (PWA) experiments were performed on Michigan Accelerator for Inductive Z-pinch Experiments, the University of Michigans low-impedance linear transformer driver (LTD)-driven generator (0.1 Ω, 0.5-1 MA, and 100-200 ns), for the first time. It was demonstrated that Al wire arrays [both double PWA (DPWA) and single PWA (SPWA)] can be successfully imploded at LTD generator even at the relatively low current of 0.3-0.5 MA. In particular, implosion characteristics and radiative properties of PWAs of different load configurations [for DPWA from Al and stainless steel wires with different wire diameters, interwire gaps, and interplanar gaps (IPGs) and for Al SPWA of different array widths and number of wires] were studied. The major difference from the DPWA experiments on high-impedance Zebra accelerator was in the current rise time that was influenced by the load inductance and was increased up to about 150 ns during the first campaign (and was even longer in the second campaign). The implosion dynamics of DPWAs strongly depends on the critical load parameter, the aspect ratio (the ratio of the array width to IPG) as for Al DPWAs on high-impedance Zebra, but some differences were observed, for low-aspect ratio loads in particular. Analysis of X-ray images and spectroscopy indicates that K-shell Al plasmas from Al PWAs reached the electron temperatures up to more than 450 eV and densities up to 2 × 1020 cm-3. Despite the low mass of the loads, opacity effects were observed in the most prominent K-shell Al lines almost in every shot.

Mixed double planar wire arrays on Michigan's Ltd generator

Summary form only given. Double Planar Wire Arrays (DPWA), which consist of two parallel rows of wires, have previously demonstrated high radiation efficiency, compact size, and usefulness for various applications in experiments on a University-scale high-impedance Z-pinch generator1. Recently, we successfully performed two experimental campaigns with PWAs on the University of Michigans low-impedance MAIZE (Linear Transformer Driver (LTD)-driven generator, 0.1Ω, 0.5-1 MA, 100-180 ns) in collaboration with the UM team. The details and the analysis of the results of the first experimental campaign can be found in Ref. [2]. The second experimental campaign was focused on studying the implosion and radiative characteristics of DPWAs using a diagnostic set similar to the first campaign, including: filtered X-ray diodes, X-ray spectrographs and pinhole cameras, and a new four-frame shadowgraphy system with 2-ns, 532 nm frequency doubled Nd:YAG laser. Here we present the results of four, mixed-DPWA shots with the load consisting of one plane with 6 Al wires of 10μm diameter and another plane of 6 stainless steel wires of 5.1 μm diameter. The rise-time of the current varies between 175 and 225 ns and shadowgraphy images cover the broad span of time from as early as 116 ns to as late as 304 ns. The shadowgraphy images show ablating and imploding mixed DPWAs that are very different from the images of uniform DPWAs. There is a clearly observed asymmetry of implosions of two wire array planes dependent on the material of each plane, (early time images in particular), captured also by X-ray pinhole images. WADM is used for the analysis of shadowgraphy images. X-ray spectra display both K-shell Al and L-shell Fe features analyzed with non-LTE modeling. Advantages of using mixed wire arrays are discussed.

Characterization and study of supersonic pure and mixed noble gas jets as a target for a sub-PS laser

Summary form only given. A gas jet containing a mixture of monomers and clusters was characterized and studied as an x-ray radiation source produced by a TW-class laser pulse. Gas jet parameters such as average density and cluster size were measured at the UNR Radiation Physics Laboratory using both optical interferometry and Rayleigh scattering techniques, respectively. Several noble gases were used in the gas jet: Ar, Kr, and Xe. Additionally, mixtures of two or three of those gases were also tested. By changing the gas jet backing pressure as well as the gas delay time between jet initiation and laser interaction with the jet, both the density and cluster size of the gas jets can be varied. Having control over the composition, density, and cluster size of the gas jets is important when considering them as targets for intense laser pulses. Our gas jets were irradiated with the 1057 nm short pulse (350 fs) UNR Leopard laser with an intensity of 1019 W/cm2 in the focus spot. Time resolved diagnostics included filtered Si-diode detectors (1.4-9 keV), filtered absolutely calibrated PCDs (>2.4 keV), and Faraday cups. An x-ray spectrometer and two three-channel x-ray pinhole cameras provided time integrated diagnostics on the gas jet plasma. Anisotropy of x-ray radiation with respect to laser beam polarization was observed in all spectral regions. The coefficient of conversion of laser energy into x-rays was measured with a maximum of 10-3. Most importantly, the mixtures of two or three gases each produced higher x-ray yields than the pure gases. Non-LTE modelling and a molecular dynamics (MD) code have been employed to determine plasma and cluster parameters. Electron temperatures and densities of the laser plasma of the mixed gases were higher than the pure gases.

Line emission from molybdenum high energy density plasma benchmarked with EBIT experiments

One of the first questions that should be considered to adequately describe radiation of high Z ions from High Energy Density (HED) plasmas and from even more extreme environments is: “How can we calculate and, most importantly, validate atomic properties and spectra of complex, high Z, highly charged ions in non-equilibrium as a function of plasma parameters and electron distribution function?” To contribute to answering this challenging question, we present a comprehensive experimental and theoretical study of the line emission from Mo HED plasmas benchmarked with LLNL EBIT data. Though there were some studies of Mo line radiation from pulsed power plasmas produced at UNR, Cornel University, and SNL before, there was no benchmarking with EBIT data. The analysis of X-ray spectra from high-Z HED plasma is always a very challenging topic because of contributions from numerous ionization stages and from multiple atomic processes in the plasma, such as, for example, dielectronic recombination, that is impossible to resolve in a rather narrow spectral range. We performed two types of Mo experiments at both the LLNL EBIT and at the Z-pinch generator at NTF/UNR to study line radiation in a spectral range between 3.5 and 5.5 Å. In particular, benchmarking experiments at the LLNL EBIT with Mo ions produced at electron beam energies from 2.75 keV up to 15 keV allowed us to break down these very complicated spectra into spectra with only few ionization stages and to select processes that influence them. The EBIT data were recorded using the EBIT Calorimeter Spectrometer and a crystal spectrometer with a Ge crystal. X-ray Mo spectra and pinhole images were collected from Z-pinch plasmas produced from various wire loads to provide different levels of opacity and electron beam effects. Non-LTE modeling and high-precision relativistic data were used to analyze L-shell Mo spectra from both experimental campaigns: an almost monoenergetic electron distribution function (EDF) for EBIT data and Maxwellian and non-Maxwellian EDFs for modeling of HED plasma spectra. The influence of different plasma processes including electron beams on Mo line radiation is summarized.

Modeling experiments of new compact hohlraum configuration with multiple parallel-driven x-ray sources with application of VisRad code

Summary form only given. A new compact Z-pinch x-ray hohlraum design with multiple parallel-driven x-ray sources was jointly proposed by the Sandia National Laboratories and the University of Nevada [1]. The first proof-of-principle experimental demonstration of the full configuration of this compact hohlraum with central reemission target and tailored shine shields (to provide a symmetric temperature distribution on the target) was achieved at the 1.7 MA UNR Zebra generator [2]. VisRad (PRISM Computational Sciences Co.), a 3-D view factor code, is used to simulate the multi-dimensional radiation environment within this new compact hohlraum configuration that incorporates multiple compact (mm-scale) planar wire array (PWA) x-ray sources that surround a reemission target in the center of the hohlraum cavity, allowing a reduction of hohlraum surface area and potentially providing a hotter x-ray environment. View factor modeling is a valuable design tool, allowing us to improve rapidly on experimental design and to demonstrate the feasibility of the concept for hohlraum and ICF studies on a 1-2 MA university-scale pulsed power platform. Double-PWA sources (DPWA) were modeled and used in experiments due to much better pulse shaping properties compared with single PWAs. Also, we are taking into account that the W DPWA is an anisotropic x-ray source and maximum radiation is emitted in the direction parallel to the wire planes. Different versions of compact hohlraum with two W DPWA sources and central cavity between them were analyzed using VisRad code. Simulations have predicted a reemission plastic target radiation temperature Trad ~ 39eV, showing good correlation to experimental data 37+3 eV The possibility of optimization of new compact configuration was demonstrated by changing relative volume of central cavity. Special emphasis is made on Trad uniformity at the reemission target surface by analysis of compact holraum configuration of 6 or more DPWA pinches proposed in Ref. [2] to reach better symmetry of hohlraum exposure. The scaling of this 6 DPWA sources hohlraum configuration using VisRad for higher current 20 MA generators (as Sandia National Laboratories Z facility) show that central target Trad ~ 85 eV is reachable. VisRad simulation has shown that x-ray power flux in new compact hohlraum might be ~1.3 times higher if W sources will be changed with Au sources.

Double and single planar wire arrays at high and low impedance university-scale generators

Single Planar Wire Arrays (SPWA) and Double Planar Wire Arrays (DPWA), which consist of one or two parallel rows of wires, respectively, have demonstrated high radiation efficiency (up to 30 kJ), compact size (1.5-3 mm), and usefulness for various applications in experiments on the high-impedance Zebra (1.9Ω, 1 MA, 100 ns). For example, DPWAs are very suitable for the new compact multi-source hohlraum concept, astrophysical applications, and as an excellent radiation source. Their implosion dynamics strongly depends on the critical load parameter, the aspect ratio Φ (width to inter-planar gap Δ) as well as on load wire material and mass. We have studied implosion dynamics and radiative properties of DPWAs at the enhanced Zebra current of 1.5-1.7 MA and have demonstrated the new regimes of implosions with asymmetric jets, no precursor formation, and very early radiation for larger sized (Δ=9 mm, Φ=0.54) and precursor formation and strong “cold” Ka emission for standard sized (Δ=6 mm, Φ=1.28) DPWAs.

Characteristics of the electron beam driven K-shell emission from brass wire array implosions on the zebra generator

Summary form only given. Brass wire array implosions on Zebra have produced characteristic K-shell emission. These Kα and Kβ photons are a result of high-energy electrons (>~10 keV) ionizing or exciting a 1s bound electron from ionization stages around the Ne-like charge states. The electron beam was measured using a Faraday cup. The K- and L-shell radiation was captured using time-gated and time-integrated spectrometers. The L-shell radiation comes from ionization stages around the Ne-like charge state that is mostly populated by a thermal electron energy distribution function. X-ray imaging was accomplished using time-integrated and time-gated pinhole cameras, with one camera using a Ross filter pair to image the K-shell emission. The diagnostic suite included various filtered x-ray diodes, bolometers, and laser shadowgraphy. A multi-zone non-LTE kinetics pinch model with radiation transport is used to obtain the plasma conditions by matching the observed x-ray emission. Monte-Carlo simulations are employed to infer information about the electron energy distribution function for the runaway electrons.

Analysis and comparison of x-ray image and x-ray burst features of high intensity laser beam jets interaction experiments on the leopard laser at UNR

Results of Ar gas-puff experiments performed on the high power Leopard laser at UNR are presented. The Leopard laser operated in two regimes: 350 fs, 40 TW pulse, or 0.8 ns, 25 GW pulse (pulse contrast from 10-5 to 10-7). Laser wavelength was 1.057 μm. The supersonic linear nozzle was compared with cylindrical tube sub-sonic nozzle. Flux density of laser radiation in focal spot was from 3×1016 W/cm2 (ns piulse) up to 2×1019 W/cm2 (fs pulse). The laser beam axis was positioned either along the jet plane or orthogonal to it at a distance of 1 mm from the nozzle output. Diagnostics included two sets of filtered Si-diodes (covered region from 1 to 55 keV), x-ray pinhole cameras, x-ray spectrometers, and Faraday cups. Specifically, x-ray images and structure of x-ray bursts are investigated and compared as a function of the orientation of the laser beam to the linear or cylindrical gas jet and laser pulse duration. The importance of analysis and comparison of x-ray image and x-ray burst features for a better understanding the mechanisms of the laser energy to x-ray conversion efficiency and future research directions are discussed.