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Small radius Bdot measurements of electrical current delivered by pulsed power devices are routinely compromised by electrode/convolute plasmas endemic to multiterawatt transmission lines. Inferences of delivered current unaffected by these issues have recently been obtained by numerically optimizing consistency between model predictions and local experimental velocimetry data, but these are only unique for shockless velocity profiles. Here, we describe a more general Bayesian method capable of inferring current despite the presence of shocks. Additionally, we describe uncertainty estimates and use of the technique on experimental data. This technique is the first to provide uncertainty estimates on the full current trace delivered to an inertial confinement fusion target. [ABSTRACT FROM AUTHOR]

We describe the details of a digital holographic microscopy diagnostic capable of quantifying both the topography and velocity of a km/s object with adjustable temporal sensitivity. This technique involves spatially multiplexing a double pulse reflected from a target with reference beams of precisely known temporal separation. [ABSTRACT FROM AUTHOR]

Electrothermal instability is responsible for degrading numerous applications of pulsed-power technology, yet the initial conditions from which it grows are not well understood. For the first time, metal surfaces have been tracked from characterization to self-emission. This reveals no clear correlation between non-uniform thermal emissions and surface metallurgical defects or crystallographic grains, while correlations are observed with surface topography for 5N metal but not 6061 metal. For 5N metal, surfaces with average roughness as small as 5 nm still admit thermal perturbations with δ T / T > 0.1. [ABSTRACT FROM AUTHOR]

Inhomogeneities in a current-carrying conductor promote non-uniform heating and expansion through the complex feedback between current density, electrical resistivity, Ohmic heating, temperature, and hydrodynamics. Three-dimensional-magnetohydrodynamic (3D-MHD) simulations suggest that μm-scale resistive inclusions or voids seed local overheating and through hydrodynamic explosion generate continuously growing craters which become several times larger than the initial perturbation. The ejected mass is the genesis of an electrothermally driven plasma filament which develops at lower current than plasmas on uniform surfaces adjacent to the defect. This result suggests that 1D or even 2D treatments are largely inadequate for detailed prediction of plasma formation. To test computational predictions, z-pinch experiments driven to 1 MA studied ultra-high-purity aluminum rods which were then machined to include pairs of quasi-hemispherical voids or "engineered defects (ED)" on the current-carrying surface. ED are the dominant current-density perturbation and reproducibly drive local overheating which can be compared with 3D-MHD simulation. Data from high-resolution-gated imagers of visible surface emissions confirm many simulation predictions, including the surface topography of local overheating, and the propensity for neighboring ED to prematurely source plasmas which then connect to form a plasma filament. Results also provide conditional support of theory which suggests heating similarity; that is, heating is independent of ED size for geometrically scaled ED. [ABSTRACT FROM AUTHOR]

Significant variations in plasma formation have been observed for thick aluminum rods with varying initial surface conditions when pulsed by an intense current in a z-pinch configuration. Rods were fabricated on a lathe to a diameter of 1.0 mm followed by a 300 μm wide and 6 mm long strip milled on the surface to remove a portion of the azimuthally extended machining marks left by the lathe. A subset of these rods was then electropolished, reducing the azimuthally extended machining marks and reducing the mean surface roughness. The fabrication process provided two types of rods with several surface finishes: a smooth surface (both the milled flat face and curved region), a rough surface (milled flat faced region), and an azimuthally extended rough surface (curved region), to study the effect of surface finish on plasma formation. Rods with azimuthally extended surface defects left by the machining process form plasma early compared to rods that have had the azimuthally extended features removed and/or reduce by electropolishing the surface or milling away a section of the surface. [ABSTRACT FROM AUTHOR]

Photonic Doppler Velocimetry (PDV) was used to investigate the exploding surface of coated metal driven by lineal current density increasing at 3 × 10 15 A/m/s. Aluminum-6061 rods were coated with three thicknesses (5 , 17 , and 41 μ m) of Parylene-N and driven to 850 kA in approximately 100 ns , with the metal thicker than the skin depth. According to PDV, the metal surface melts at 85 ± 13 T and expands with a constant acceleration. Then, at a surface magnetic field of 140 – 180 T , the metal acceleration begins to increase in time. Later, plasma forms on, and is correlated with PDV signal loss from, uncoated aluminum rods, 5 - μ m -coated rods, and 17 - μ m -coated rods, while no evidence suggests that plasma forms on 41 - μ m -coated metal. [ABSTRACT FROM AUTHOR]

Using the analogy between hydrodynamic and electrical current flow, we study how electrical current density j redistributes and amplifies due to two commonly encountered inhomogeneities in metals. First, we consider flow around a spherical resistive inclusion and find significant j amplification, independent of inclusion size. Hence, even μm-scale inclusions can affect performance in applications by creating localized regions of enhanced Joule heating. Next, we investigate j redistribution due to surface roughness, idealized as a sinusoidal perturbation with amplitude A and wavelength λ. Theory predicts that j amplification is determined by the ratio A / λ , so that even "smooth" surface finishes (i.e., small A) can generate significant amplification, if λ is correspondingly small. We compare theory with magnetohydrodynamic simulation to illustrate both the utility and limitations of the steady-state theory. [ABSTRACT FROM AUTHOR]

Experiments and modeling of x-ray radiography of millimeter diameter solid Al wires with laser-produced broadband x rays are reported. Experiments were performed using the 50-TW Leopard short-pulse laser in a laser and pulsed power chamber at the Nevada Terawatt Facility. To characterize broadband x rays and demonstrate a radiographic capability, bremsstrahlung, escaping electrons, and radiograph images of Al wires were simultaneously measured. The angularly resolved x-ray spectra are modeled by comparing measured bremsstrahlung signals in the range between 10 and ∼500 keV with hybrid particle-in-cell simulations. Transmission of Al wires from the radiograph images is further simulated with a Monte Carlo code. The measured transmission profiles of Al wires with three different diameters agree with calculations when a simulated x-ray spectrum composed of line emissions and bremsstrahlung is used with a source size of 600 ± 200 μm. Transmission calculations with only 22 keV Ag Kα or an exponential x-ray spectrum do not reproduce the measurement, suggesting that the accurate determination of an x-ray source spectrum, as well as the inclusion of the photon sensitivity of the detector, is critical in transmission calculations to infer the density of an object. The laser-based broadband x-ray radiography that was developed has been successfully implemented in a pulsed power chamber for future laser-pulsed-power coupled experiments. [ABSTRACT FROM AUTHOR]

We report the first observation of a significant change in plasma formation and evolution caused by a small change in the material composition (metal alloy). Thick copper rod alloys were varied in the initial diameter from 0.5 mm to 1.59 mm and pulsed to 1 mega-ampere (MA) peak current in 100 ns with a surface magnetic field rising nearly linearly at 45–80 MG/μs. The copper rods were diagnosed with surface radiometry, two-frame laser shadowgraphy, two-frame surface emission imaging, and time resolved load current measurements. Diagnostics determined time resolved surface temperature, surface expansion rates, time of surface plasma formation, and the growth rates of surface instabilities. Data indicate that copper alloy Cu-145 (99.5% Cu, 0%–0.7% Te, 0%–0.012% P) undergoes surface plasma formation when the surface magnetic field reaches 3.0 ± 0.1 MG whereas Cu-101 (>99.99% Cu) forms surface plasma at 3.5 ± 0.2 MG. The Cu-145 alloys also reach higher peak temperatures and form surface instabilities earlier in the current pulse. [ABSTRACT FROM AUTHOR]

Auto-magnetizing (AutoMag) liners [Slutz et al., Phys. Plasmas 24, 012704 (2017)] are designed to generate up to 100 T of axial magnetic field in the fuel for Magnetized Liner Inertial Fusion [Slutz et al., Phys. Plasmas 17, 056303 (2010)] without the need for external field coils. AutoMag liners (cylindrical tubes) are composed of discrete metallic helical conduction paths separated by electrically insulating material. Initially, helical current in the AutoMag liner produces internal axial magnetic field during a long (100 to 300 ns) current prepulse with an average current rise rate d I / d t = 5 k A / n s. After the cold fuel is magnetized, a rapidly rising current ( 200 k A / n s ) generates a calculated electric field of 64 M V / m between the helices. Such field is sufficient to force dielectric breakdown of the insulating material after which liner current is reoriented from helical to predominantly axial which ceases the AutoMag axial magnetic field production mechanism and the z-pinch liner implodes. Proof of concept experiments have been executed on the Mykonos linear transformer driver to measure the axial field produced by a variety of AutoMag liners and to evaluate what physical processes drive dielectric breakdown. A range of field strengths have been generated in various cm-scale liners in agreement with magnetic transient simulations including a measured field above 90 T at I = 350 kA. By varying the helical pitch angle, insulator material, and insulator geometry, favorable liner designs have been identified for which breakdown occurs under predictable and reproducible field conditions. [ABSTRACT FROM AUTHOR]

Ultrafast optical microscopy of metal z-pinch rods pulsed with megaampere current is contributing new data and critical insight into what provides the fundamental seed for the magneto-Rayleigh–Taylor (MRT) instability. A two-frame near infrared/visible intensified-charge-coupled device gated imager with 2-ns temporal resolution and 3- \mu \textm spatial resolution captured emissions from the nonuniformly Joule heated surfaces of ultrasmooth aluminum (Al) rods. Nonuniform surface emissions are consistently first observed from discrete, 10- \mu \textm scale, subelectronvolt spots. Aluminum 6061 alloy, with micrometer-scale nonmetallic resistive inclusions, forms several times more spots than 99.999% pure Al 5N; 5–10 ns later, azimuthally stretched elliptical spots and distinct strata (40–100 \mu \textm wide by 10 \mu \textm tall) are observed on Al 6061, but not on Al 5N. Such overheat strata, which are aligned parallel to the magnetic field, are highly effective seeds for MRT instability growth. These data give credence to the hypothesis that early nonuniform Joule heating, such as the electrothermal instability, may provide the dominant seed for MRT. [ABSTRACT FROM PUBLISHER]

A direct observation of the stratified electrothermal instability on the surface of thick metal is reported. Aluminum rods coated with 70 μm Parylene-N were driven to 1 MA in 100 ns, with the metal thicker than the skin depth. The dielectric coating suppressed plasma formation, enabling persistent observation of discrete azimuthally correlated stratified thermal perturbations perpendicular to the current whose wave numbers, k, grew exponentially with rate γ (k) = 0.06 ns-1 - (0.4 ns-1 µm2 rad-2) k2 in ~ 1 g / cm3, ~ 7000 K aluminum. [ABSTRACT FROM AUTHOR]