Nathan W. Moore

Opacity and gradients in aluminum wire array z-pinch implosions on the Z pulsed power facility

Aluminum wire array z pinches imploded on the Z generator are an extremely bright source of 1–2 keV radiation, with close to 400 kJ radiated at photon energies >1 keV and more than 50 kJ radiated in a single line (Al Ly-α). Opacity plays a critical role in the dynamics and K-shell radiation efficiency of these pinches. Where significant structure is present in the stagnated pinch this acts to reduce the effective opacity of the system as demonstrated by direct analysis of spectra. Analysis of time-integrated broadband spectra (0.8–25 keV) indicates electron temperatures ranging from a few 100 eV to a few keV are present, indicative of substantial temperature gradients.

A Renewed Capability for Gas Puff Science on Sandia's Z Machine

A comprehensive gas puff capability is being developed on the Z pulsed power generator. We describe the methodology employed for developing a gas puff load on Z, which combines characterization and modeling of the neutral gas mass flow from a supersonic nozzle, numerical modeling of the implosion of this mass profile, and experimental evaluation of these magnetic implosions on Z. We are beginning a multiyear science program to study gas puff z-pinch physics at high current, starting with an 8-cm diameter double-shell nozzle, which delivers a column of Ar gas that is imploded by the machines fast current pulse. The initial shots have been designed using numerical simulation with two radiation-magnetohydrodynamic codes. These calculations indicate that 1 mg/cm should provide optimal coupling to the driver and 1.6:1 middle:outer shell mass ratio will best balance the need for high implosion velocity against the need to mitigate the magnetic Rayleigh-Taylor instability. The models suggest 300-500-kJ Ar K-shell yield should be achievable on Z, and we report an initial commissioning shot at lower voltage in which 250 kJ was measured. Future experiments will pursue optimization of Ar and Kr K-shell X-ray sources, study fusion in deuterium gas puffs, and investigate the physics of gas puff implosions including energy coupling, instability growth, and radiation generation.

Modeling and Measurement of a Bistable Beam in a Microelectromechanical System

Design and fabrication of microelectromechanical systems (MEMS) can be costly, time consuming, and necessitating accurate models for their behavior. Current theoretical models of bistable beams in MEMS devices are limited to numerical or small deformation models and current measurement techniques are unable to fully characterize these devices as they only determine thresholds or have resolutions that are too coarse to adequately explore the force-deflection relationship of bistable mechanisms. Two analytical models are developed: a stepped Euler-Bernoulli beam and a large deformation model. To validate these models, a new technique for measuring in-plane mechanical properties of MEMS devices is introduced that measures normal and lateral forces against a probe tip, while electrostatic actuation and a force-feedback loop maintain the desired tip position. This allows true displacement-controlled measurements along two axes and facilitates automated positioning. Measurements validate the large deformation model and show that Euler-Bernoulli beam theory is inadequate for modeling the mechanisms bistable behavior. A parameter study in edge width using the large deformation model accounts for discrepancies between predicted and measured forces. The models utility is further demonstrated by an optimization study that redesigns the mechanism to be less sensitive to the edge width variation introduced in the manufacturing process.

The effect of gradients at stagnation on K-shell x-ray line emission in high-current Ar gas-puff implosions

Argon gas puffs have produced 330 kJ ± 9% of x-ray radiation above 3 keV photon energy in fast z-pinch implosions, with remarkably reproducible K-shell spectra and power pulses. This reproducibility in x-ray production is particularly significant in light of the variations in instability evolution observed between experiments. Soft x-ray power measurements and K-shell line ratios from a time-resolved spectrum at peak x-ray power suggest that plasma gradients in these high-mass pinches may limit the K-shell radiating mass, K-shell power, and K-shell yield from high-current gas puffs.

The Pull-Off Force and the Work of Adhesion: New Challenges at the Nanoscale

The pull-off force required to separate two surfaces has become a convenient metric for characterizing adhesion at the micro- and nanoscales using cantilever-based force sensors, such as an atomic force microscope (AFM), e.g., as a way to predict adhesion between materials used in MEMS/NEMS. Interfacial Force Microscopy (IFM) provides unique insight into this method, because its self-balancing force-feedback sensor avoids the snap-out instability of compliant sensors, and can estimate both the work of adhesion and the pull-off force that would be measured using a compliant cantilever. Here, IFM is used to illustrate the challenges of determining the work of adhesion from the pull-off force in a nanoscale geometry. Specifically, adhesion is evaluated between a conical, diamond indenter and three surfaces relevant to MEMS/NEMS adhesion problems: silicon, a model insulator and a compliant polymer surface.

Investigating the effect of adding an on-axis jet to Ar gas puff Z pinches on Z

Double-shell Ar gas puff implosions driven by 16.5 ± 0.5 MA on the Z generator at Sandia National Laboratories are very effective emitters of Ar K-shell radiation (photon energy >3 keV), producing yields of 330 ± 9% kJ [B. Jones et al., Phys. Plasmas 22, 020706 (2015)]. Previous simulations and experiments have reported dramatic increases in K-shell yields when adding an on-axis jet to double shell gas puffs for some configurations. We report on a series of experiments on Z testing Ar gas puff configurations with and without an on-axis jet guided by 3D magneto-hydrodynamic (MHD) simulations. Adding an on-axis jet was found to significantly improve the performance of some, but not all, configurations. The maximum observed K-shell yield of 375 ± 9% kJ was produced with a configuration that rapidly imploded onto an on-axis jet. A dramatic difference was observed in the plasma conditions at stagnation when a jet was used, producing a narrower stagnation column in experiments with a higher density but relative...

2-D RMHD Modeling Assessment of Current Flow, Plasma Conditions, and Doppler Effects in Recent Z Argon Experiments

By varying current-loss circuit parameters, the Mach2-tabular collisional radiative equilibrium 2-D radiation magnetohydrodynamic model was tuned to reproduce the radiative and electrical properties of three recent argon gas-puff experiments (same initial conditions) performed on the Z machine at Sandia National Laboratories. The model indicates that there were current losses occurring near or within the diode region of the Z machine during the stagnation phase of the implosion. The “good” simulation reproduces the experimental K-shell powers, K-shell yields, total powers, percentage of emission radiated in α lines, size of the K-shell emission region, and the average electron temperature near the time-of-peak K-shell power. The calculated atomic populations, ion temperatures, and radial velocities are used as input to a detailed multifrequency ray-trace radiation transport model that includes the Doppler effect. This model is employed to construct time-, space-, and energy-resolved synthetic spectra. The role the Doppler effect likely plays in the experiments is demonstrated by comparing synthetic spectra generated with and without this effect.

The effect of adding a center jet to Argon gas puff implosions at the Z facility

Argon gas puff implosions have been found to be an extremely efficient source of K-shell x-rays (3-4 keV) on the Z pulsed power facility at Sandia National Labs. producing over 300 kJ of K-shell emission in double-shell experiments. Experiments at Double Eagle [1] and Saturn [2] have previously found that adding a central jet of mass to their double-shell gas puff configurations increased the yield substantially.This presentation will describe recent experiments on Z that have investigated the effect of adding a central jet to doubleshell gas puffs on yield and plasma conditions. The central jet was added to two gas puff configurations: a 1:1.6 inner/outer shell mass ratio puff and a faster-imploding 1:1 mass ratio puff. Experimental results and comparisons to simulation will be described.

Molecular and Kinetic Models for High-Rate Thermal Degradation of Polyethylene

Thermal degradation of polyethylene is studied under the extremely high rate temperature ramps expected in laser-driven and X-ray ablation experiments-from 1010 to 1014 K/s in isochoric, condensed phases. The molecular evolution and macroscopic state variables are extracted as a function of density from reactive molecular dynamics simulations using the ReaxFF potential. The enthalpy, dissociation onset temperature, bond evolution, and observed cross-linking are shown to be rate dependent. These results are used to parametrize a kinetic rate model for the decomposition and coalescence of hydrocarbons as a function of temperature, temperature ramp rate, and density. The results are contrasted to first-order random-scission macrokinetic models often assumed for pyrolysis of linear polyethylene under ambient conditions.

New Capabilities for Hostile Environments on Z Grand Challenge LDRD - Final Status

The purpose of this project was to develop new physical simulation capabilities in order to support the science-based qualification of nonnuclear weapon components in hostile radiation environments. The project contributes directly to the goals of maintaining a safe, secure, and effective US nuclear stockpile, maintaining strategic deterrence at lower nuclear force levels, extending the life of the nuclear deterrent capability, and to be ready for technological surprise.