Christopher A. Grabowski

A high density field reversed configuration (FRC) target for magnetized target fusion: First internal profile measurements of a high density FRC

Magnetized target fusion (MTF) is a potentially low cost path to fusion, intermediate in plasma regime between magnetic and inertial fusion energy. It requires compression of a magnetized target plasma and consequent heating to fusion relevant conditions inside a converging flux conserver. To demonstrate the physics basis for MTF, a field reversed configuration (FRC) target plasma has been chosen that will ultimately be compressed within an imploding metal liner. The required FRC will need large density, and this regime is being explored by the FRX–L (FRC-Liner) experiment. All theta pinch formed FRCs have some shock heating during formation, but FRX–L depends further on large ohmic heating from magnetic flux annihilation to heat the high density (2–5×1022u200am−3), plasma to a temperature of Te+Ti≈500u200aeV. At the field null, anomalous resistivity is typically invoked to characterize the resistive like flux dissipation process. The first resistivity estimate for a high density collisional FRC is shown here. Th...

Performance of Dielectric Nanocomposites: Matrix-Free, Hairy Nanoparticle Assemblies and Amorphous Polymer–Nanoparticle Blends

Demands to increase the stored energy density of electrostatic capacitors have spurred the development of materials with enhanced dielectric breakdown, improved permittivity, and reduced dielectric loss. Polymer nanocomposites (PNCs), consisting of a blend of amorphous polymer and dielectric nanofillers, have been studied intensely to satisfy these goals; however, nanoparticle aggregates, field localization due to dielectric mismatch between particle and matrix, and the poorly understood role of interface compatibilization have challenged progress. To expand the understanding of the inter-relation between these factors and, thus, enable rational optimization of low and high contrast PNC dielectrics, we compare the dielectric performance of matrix-free hairy nanoparticle assemblies (aHNPs) to blended PNCs in the regime of low dielectric contrast to establish how morphology and interface impact energy storage and breakdown across different polymer matrices (polystyrene, PS, and poly(methyl methacrylate), PMMA) and nanoparticle loadings (0-50% (v/v) silica). The findings indicate that the route (aHNP versus blending) to well-dispersed morphology has, at most, a minor impact on breakdown strength trends with nanoparticle volume fraction; the only exception being at intermediate loadings of silica in PMMA (15% (v/v)). Conversely, aHNPs show substantial improvements in reducing dielectric loss and maintaining charge/discharge efficiency. For example, low-frequency dielectric loss (1 Hz-1 kHz) of PS and PMMA aHNP films was essentially unchanged up to a silica content of 50% (v/v), whereas traditional blends showed a monotonically increasing loss with silica loading. Similar benefits are seen via high-field polarization loop measurements where energy storage for ∼15% (v/v) silica loaded PMMA and PS aHNPs were 50% and 200% greater than respective comparable PNC blends. Overall, these findings on low dielectric contrast PNCs clearly point to the performance benefits of functionalizing the nanoparticle surface with high-molecular-weight polymers for polymer nanostructured dielectrics.

Dielectric performance of high permitivity nanocomposites: impact of polystyrene grafting on BaTiO3 and TiO2

Abstract Polymer nanocomposites are a promising concept to improve energy storage density of capacitors, but realizing their hypothetical gains has proved challenging. The introduction of high permittivity fillers often leads to reduction in breakdown strength due to field exclusion, which intensifies the applied electric field within the polymer matrix near nanoparticle interfaces. This has prompted research in developing new nanoparticle functionalization chemistries and processing concepts to maximize particle separation. Herein, we compare the dielectric performance of blended nanocomposites to matrix free assemblies of hairy (polymer grafted) nanoparticles (HNPs) that exhibit comparable overall morphology. The dielectric breakdown strength of polystyrene grafted BaTiO3 (PS@BaTiO3) systems was over 40% greater than a blended nanocomposite with similar loading (~25% v/v BaTiO3). Hairy nanoparticles with TiO2 cores followed similar trends in breakdown strength as a function of inorganic loading up to 40% v/v. Dielectric loss for PS@BaTiO3 HNPs was 2–5 times lower than analogous blended films for a wide frequency spectrum (1 Hz to 100 kHz). For content above 7% v/v, grafting the polymer chains to the BaTiO3 significantly improved energy storage efficiency. Overall this study indicates that polymer grafting improves capacitor performance relative to direct blending in likely two ways: (1) by mitigating interfacial transport to lower dielectric loss, irrespective of the dielectric contrast between matrix and nanoparticle, and (2) by restricting particle–particle hot-spots by establishing a finite minimum particle separation when the dielectric contrast between matrix and nanoparticle is large.

Electrical Control of Shape in Voxelated Liquid Crystalline Polymer Nanocomposites

Liquid crystal elastomers (LCEs) exhibit anisotropic mechanical, thermal, and optical properties. The director orientation within an LCE can be spatially localized into voxels [three-dimensional (3-D) volume elements] via photoalignment surfaces. Here, we prepare nanocomposites in which both the orientation of the LCE and single-walled carbon nanotube (SWNT) are locally and arbitrarily oriented in discrete voxels. The addition of SWNTs increases the stiffness of the LCE in the orientation direction, yielding a material with a 5:1 directional modulus contrast. The inclusion of SWNT modifies the thermomechanical response and, most notably, is shown to enable distinctive electromechanical deformation of the nanocomposite. Specifically, the incorporation of SWNTs sensitizes the LCE to a dc field, enabling uniaxial electrostriction along the orientation direction. We demonstrate that localized orientation of the LCE and SWNT allows complex 3-D shape transformations to be electrically triggered. Initial experiments indicate that the SWNT-polymer interfaces play a crucial role in enabling the electrostriction reported herein.

Results from compression of field reversed configuration using imploding solid liner

Summary form only given. The AFRL Shiva Star capacitor bank (1300 μF, up to 120 kV) used typically at 4 to 5 MJ stored energy, 10 to 15 MA current, 10 μs current rise time, has been used to drive metal shell (solid liner) implosions for compression of axial magnetic fields to multi-megagauss levels, suitable for compressing magnetized plasmas to Magneto-Inertial Fusion (MIF) conditions. MIF approaches use embedded magnetic field to reduce thermal conduction relative to inertial confinement fusion (ICF). MIF substantially reduces required implosion speed and convergence. Using a profiled thickness liner enables large electrode apertures and the injection of a field-reversed configuration (FRC) version of a magnetized plasma ring. Using a longer capture region than originally used, the FRC trapped flux lifetime was made comparable to implosion time and an integrated compression test was conducted. The FRC was compressed cylindrically by more than a factor of ten, with density up more than 100x, to >1018 cm-3 (a world FRC record), but temperatures were only in the range of 300-400 eV, compared to the intended several keV. Although compression to megabar pressures was inferred by the observed time and rate of liner rebound, we learned that heating rate during the first half of the compression was not high enough compared to the normal FRC decay rate. Principal diagnostics for this experiment were soft x-ray imaging, soft x-ray diodes, and neutron measurements. Measures that could double the trapped flux lifetime and pre-compression temperature of the FRC will be discussed.

The field-reversed configuration heating experiment on Shiva Star

Summary form only given. A collaborative research effort was launched in 2000 between the Air Force Research Laboratory and Los Alamos National Laboratory to investigate the formation of high density field-reversed configuration (FRC) plasmas for the purpose of then adiabatically compressing them to a high energy density (HED) state. The goal of the experimental system developed through this collaboration was to enable a low-cost approach to achieving thermonuclear fusion of the target plasma, which would then facilitate magneto-inertial fusion studies, laboratory astrophysical studies, and numerous other basic research studies connected with HED plasmas. This system was assembled at the Shiva Star facility at the Air Force Research Laboratory and referred to as the Field-Reversed Configuration Heating Experiment (FRCHX). The target FRC plasma is formed in the experiment through a reversed-field theta pinch that uses four to five capacitor banks. Once formed, the FRC is translated a short distance and then captured inside a magnetic well co-located and coaxial with an aluminum solid liner. Two additional capacitor banks establish guide fields and the end mirror fields for the magnetic well a few milliseconds before the FRC is formed. The Shiva Star High Energy Capacitor bank is then used to compress the FRC via the electromagnetic implosion of the surrounding solid liner, which is a 30-cm long, 10-cm diameter aluminum cylinder that is tapered at the top and bottom to reduce motion these locations and maintain electrical contact. Due to the overall circuit inductance the implosion requires -25 us for stagnation to occur, thus FRC formation process is started several microseconds after the liner implosion begins. This presentation will provide an overview of the latest FRCHX field coil and vacuum stand design, the pulsed power systems used, and the diagnostics employed on the experiment. The integration of the FRCHX systems into Shiva Star will be described, as well, and possible next steps will be presented.

Increasing performance of the FRCHX plasma injector system

Summary form only given. The Field-Reversed Configuration Heating Experiment (FRCHX) is an experiment developed in collaboration between the Air Force Research Laboratory (AFRL) and Los Alamos National Laboratory (LANL) to form compact (rs = 2.5~3.5 cm), high density (ne = 3~4×1016 cm-3) field-reversed configuration (FRC) plasmas intended for subsequent adiabatic compression to high energy density conditions. The FRC plasma is first formed via reversed-field theta pinch in a Deuterium background plasma. The theta coil which forms the FRC is conical to impart sufficient momentum to the plasma to translate it a short distance, where it is trapped by a magnetic well within a cylindrical flux-conserving aluminum solid liner. Once trapped, the FRC is diagnosed and/or compressed by the magnetically-driven implosion of the liner. The lifetime of the FRCs poloidal flux is an important parameter affecting plasma confinement during compression and ultimately its peak density, temperature, and neutron yield. Despite substantial improvements in lifetime recently achieved (14~16 μs now versus 7~9 μs previously1), a significantly longer lifetime (at least 2×) is still needed. We are therefore proposing to demonstrate substantially improved plasma target formation by merging two counterpropagating FRC plasmas within a central trapping/compression region. Trapped poloidal flux lifetimes 2 to 3 times longer with densities exceeding 1×1017 cm-3, temperatures (Te+Ti) exceeding 500 eV, and 4-5 T embedded magnetic fields are projected. These parameters surpass any achieved previously with uncompressed FRC plasmas. A discussion of the first FRCHX plasma formation system will be presented along with a description of the diagnostics used to diagnose the trapped FRC. The FRC parameters that were ultimately achieved with this system will also be provided. The presentation will conclude with an overview of the proposed FRC merging system and further details of the projected uncompressed FRC parameters anticipated with this new system.

In Situ TEM Characterization of Nanostructured Dielectrics

Polymer nanocomposites are being considered as potential materials for high energy capacitor due to the possibility of obtaining high dielectric breakdown strength characteristic of the organic matrix and large dielectric permittivity from inorganic filler [1,2]. However in most case the increase in the content of inorganic ceramic filler in polymer nanocomposite inevitably causes a decrease in the dielectric breakdown strength due to the agglomeration of fillers, the existence of defects, and inorganic-organic interfacial effects. Many studies have attempted to improve the compatibility of organic and inorganic, however in the large majority the dielectric breakdown strength is poor and the dielectric loss is high. Additionally, as nanostructured dielectrics are a relatively new class of materials for this application, reports of investigating fundamental mechanisms of dielectric breakdown for the dielectric nanocomposite on the nanoscale have been limited.