Research on forming, compressing, and accelerating milligram-range compact toroids using a meter diameter, two-stage, puffed gas, magnetic field embedded coaxial plasma gun is described. The compact toroids that are studied are similar to spheromaks, but they are threaded by an inner conductor. This research effort, named marauder (Magnetically Accelerated Ring to Achieve Ultra-high Directed Energy and Radiation), is not a magnetic confinement fusion program like most spheromak efforts. Rather, the ultimate goal of the present program is to compress toroids to high mass density and magnetic field intensity, and to accelerate the toroids to high speed. There are a variety of applications for compressed, accelerated toroids including fast opening switches, x-radiation production, radio frequency (rf) compression, as well as charge-neutral ion beam and inertial confinement fusion studies. Experiments performed to date to form and accelerate toroids have been diagnosed with magnetic probe arrays, laser interferometry, time and space resolved optical spectroscopy, and fast photography. Parts of the experiment have been designed by, and experimental results are interpreted with, the help of two-dimensional (2-D), time-dependent magnetohydrodynamic (MHD) numerical simulations. When not driven by a second discharge, the toroids relax to a Woltjer–Taylor equilibrium state that compares favorably to the results of 2-D equilibrium calculations and to 2-D time-dependent MHD simulations. Current, voltage, and magnetic probe data from toroids that are driven by an acceleration discharge are compared to 2-D MHD and to circuit solver/slug model predictions. Results suggest that compact toroids are formed in 7–15 μsec, and can be accelerated intact with material species the same as injected gas species and entrained mass ≥1/2 the injected mass.
Carbonic anhydrase VI (CA VI), encoded by type A transcripts of the gene Car6, is a secretory product of salivary glands and is found in the enamel pellicle. Because higher caries prevalence is associated with lower salivary concentrations of CA VI in humans, we tested whether CA VI protects enamel surfaces from caries induced by Streptococcus mutans, using Car6−/− mice, in which salivary CA VI expression is absent. We detected aberrant Car6 type A transcripts in Car6−/− mice, likely targets for nonsense-mediated mRNA decay. Expression of the intracellular stress-induced isoform of CA VI encoded by type B transcripts was restricted to parotid and submandibular glands of wild type mice. The salivary function of Car6−/− mice was normal as assessed by the histology and protein/glycoprotein profiles of glands, salivary flow rates and protein/glycoprotein compositions of saliva. Surprisingly, total smooth surface caries and sulcal caries in Car6−/− mice were more than 6-fold and 2-fold lower than in wild type mice after infection with S. mutans strain UA159. Recoveries of S. mutans and total microbiota from molars were also lower in Car6−/− mice. To explore possible mechanisms for increased caries susceptibility, we found no differences in S. mutans adherence to salivary pellicles, in vitro. Interestingly, higher levels of Lactobacillus murinus and an unidentified Streptococcus species were cultivated from the oral microbiota of Car6−/− mice. Collective results suggest salivary CA VI may promote caries by modulating the oral microbiota to favor S. mutans colonization and/or by the enzymatic production of acid within plaque.
The dynamics of imploding foil plasmas is considered using first-order theory to model the implosion and to investigate the effects of magnetohydrodynamic instabilities on the structure of the plasma sheath. The effects of the acceleration-produced magnetohydrodynamic (MHD) Rayleigh-Taylor instability and a wall-associated instability are studied for a variety of plasma implosion times for several pulsed power drivers. The basic physics of these instabilities is identified and models are developed to explain both linear and nonlinear behavior. These models are compared with the results of detailed two-dimensional magnetohydrodynamic simulations. Expressions for linear Rayleigh-Taylor growth are developed showing its dependence on driving current, plasma conductivity, and density gradient scale length. A nonlinear saturation model, based on magnetic field diffusion, is developed. The model for a wall instability involves the interaction of the plasma sheath with the electrode wall and the material ablated from the electrode. The growth of this instability is shown to be limited by field diffusion. Comparison with two-dimensional simulations has been excellent.
Recent developments in the kinetic theory of time correlation functions are applied to the problem of describing spectral line broadening in a fluid. The width and shift operator of Fano's formulation for dipole radiation is expressed in terms of the solution to a few-body problem, involving the shielded interaction of a single perturber with the radiating atom. No approximations are required in this reformulation, and previous theories of neutral and Stark broadening are shown to result from systematic small-parameter expansions to low order, A plasma-parameter expansion is suggested for the Starkbroadening case under usual experimental conditions, resulting in complete shielding of the atomperturber interaction.
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