851 research outputs found

    Ar-40 to Ar-39 dating of pseudotachylites from the Witwatersrand basin, South Africa, with implications for the formation of the Vredefort Dome

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    The formation of the Vredefort Dome, a structure in excess of 100 km in diameter and located in the approximate center of the Witwatersrand basin, is still the subject of lively geological controversy. It is widely accepted that its formation seems to have taken place in a single sudden event, herein referred to as the Vredefort event, accompanied by the release of gigantic amounts of energy. It is debated, however, whether this central event was an internal one, i.e., a cryptoexplosion triggered by volcanic or tectonic processes, or the impact of an extraterrestrial body. The results of this debate are presented

    A MIXED METHODS APPROACH TO UNDERSTANDING DOLLAR STORES AS FOOD RETAILERS

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    Introduction. Dollar stores are the fastest growing food retailer in the United States, prompting community restrictions. While dollar stores offer affordable food, there are concerns about product nutrition and community health impacts, particularly for historically underserved populations. This dissertation investigates the relationship between dollar store availability and household purchases of packaged foods high in sodium, saturated fat, and sugar (nutrients of concern), and explores the motivations behind purchasing food at dollar stores. Methods. This research uses mixed-methods to examine the impact of dollar store availability on consumer food purchasing habits. In the first study I analyze multi-level models using a national dataset of packaged food purchases to understand the association between county-level dollar store availability and household-level purchases of foods high in nutrients of concern. In the second study I investigate whether household dollar store use explains the relationships found in study one via mediation analysis. In the third study I conduct semi-structured interviews with dollar store shoppers with low incomes to learn what influences their decision to shop at dollar stores, how they want to see dollar stores improved, and their opinions on program and policy options. Results. Study one documented a positive association between dollar store availability and the purchase of foods high in each of the three nutrients of concern. The mediation analysis suggests that dollar store use may drive these associations for foods high in sugar and saturated fat but not sodium. Semi-structured interviews revealed that shoppers use dollar stores because of the convenience and low cost; purchase snacks, candy, and meal essentials; want dollar stores to offer fresh produce and higher quality protein; and support program and policy options that improve access to fruits and vegetables and increase purchasing power.Conclusion. These studies highlight the complex relationship between dollar store availability and consumer food purchasing behaviors. While dollar stores offer accessibility and affordability to underserved populations, they also contribute to the purchase of nutrients of concern. Understanding the structural factors and underlying motivations behind consumer choices at dollar stores is crucial for developing targeted interventions to promote healthier food options and to improve community health.Doctor of Philosoph

    Expanding the role of impurity spectroscopy for investigating the physics of high-Z dissipative divertors

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    New techniques that attempt to more fully exploit spectroscopic diagnostics in the divertor and pedestal region during highly dissipative scenarios are demonstrated using experimental results from recent low-Z seeding experiments on Alcator C-Mod, JET and ASDEX Upgrade. To exhaust power at high parallel heat flux, q ‖ > 1 GW/m 2 , while minimizing erosion, reactors with solid, high-Z plasma facing components (PFCs) are expected to use extrinsic impurity seeding. Due to transport and atomic physics processes which impact impurity ionization balance, so-called ‘non-coronal’ effects, we do not accurately know and have yet to demonstrate the maximum q ‖ which can be mitigated in a tokamak. Radiation enhancement for nitrogen is shown to arise primarily from changes in Li- and Be-like charge states on open field lines, but also through transport-driven enhancement of H- and He-like charge states in the pedestal region. Measurements are presented from nitrogen seeded H-mode and L-mode plasmas where emission from N 1+ through N 6+ are observed. Active charge exchange spectroscopy of partially ionized low-Z impuri- ties in the plasma edge is explored to measure N 5+ and N 6+ within the confined plasma, while passive UV and visible spectroscopy is used to measure N 1+ -N 4+ in the boundary. Examples from recent JET and Alcator C-Mod experiments which employ nitrogen seeding highlight how improving spectroscopic cov- erage can be used to gain empirical insight and provide more data to validate boundary simulations.EURATOM 63305

    Suppression of anomalous impurity transport in NBI-heated W7-X plasmas

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    Radial impurity density profiles in two Wendelstein 7-X (W7-X) experiments heated by neutral beam injection (NBI) are analyzed with respect to their impurity transport properties. Local impurity densities are derived from charge exchange reactions with the W7-X NBI system using simulated neutral densities cross-validated with beam emission spectroscopy. Impurity profiles of argon and carbon are found to show an evolving central accumulation inside half radius. The properties of the underlying impurity transport are assessed using the one-dimensional transport code pySTRAHL. Comparisons between simulation and experiment indicate transport dominated by anomalous diffusion outside half radius. The observed central impurity accumulation is found to match best simulations with purely (neo-)classical transport in the accumulation region. This data implies a suppression of the anomalous impurity transport channel to below 35% of the (neo-)classical one. Experimental data is found to be matched best when invoking a time evolving, inward propagating zone where anomalous impurity transport is suppressed. An additional central power deposition into a plasma with central impurity accumulation via electron cyclotron resonance heating is found to affect the (neo-)classical transport components in case of operation times below 200 ms only. For longer operation times, it is found to re-introduce an altered level of anomalous diffusion. The existence of an inward directed anomalous pinch as an alternative explanation for the central impurity accumulation cannot explain the observed profiles

    2D core ion temperature and impurity density measurements with Coherence Imaging Charge Exchange Recombination Spectroscopy (CICERS) at Wendelstein 7-X (invited)

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    Coherence Imaging Charge Exchange Recombination Spectroscopy (CICERS) is an imaging diagnostic installed in Wendelstein 7-X from which 2D maps of ion temperature (Ti) and impurity density (nZ) are obtained. The improved spatial resolution and coverage, as compared to standard Charge eXchange Recombination Spectroscopy (CXRS), with which these parameters can be assessed, come at the expense of spectral resolution, requiring the development of new strategies to isolate the active charge exchange contribution from passive and Bremsstrahlung radiation. In this work, a new approach based on the modeling of background radiation is presented and applied to the derivation of 2D Ti maps. These are compared to the Ti profiles derived from standard CXRS, which found excellent agreement up to the edge (ρ > 0.8). The CICERS view is implemented in the pyFIDAsim code, which is used to provide further insight into the spatial localization of the radiation as measured by the diagnostic. Moreover, an absolute intensity calibration is carried out, and, coupled with pyFIDAsim, the first 2D nC maps are obtained and validated against CXRS data

    Structural evolution of the 40 km wide Araguainha impact structure, central Brazil

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    Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)The 40 kill wide Araguainha structure in central Brazil is a shallowly eroded impact crater that presents unique insights into the final stages of complex crater formation. The dominant Structural features preserved at Araguainha relate directly to the centripetal movement of the target rocks during the collapse of the transient cavity. Slumping of the transient cavity walls resulted in inward-verging inclined folds and a km-scale anticline in the outer ring of the structure. The Folding stage was followed by radial and concentric faulting, With down-ward displacement of kilometer-scale blocks around the crater rim. The central uplift records evidence for kin-scale upward movement of crystalline basement rocks from the transient cavity floor, and lateral moment of sedimentary target rocks detached Front the cavity walls. Much of the structural grain in the central uplift relates to structural stacking of km-scale thrust sheets of sedimentary strata onto the core of crystalline basement rocks. Outward-plunging radial folds indicate tangential oblate shortening of the strata during the imbrication Of the thrust sheets. Each individual sheet records all early stage of folding and thickening due to non-coaxial strains, shortly before sheet imbrication. We attribute this folding and thickening phase to the kilometer-scale inward movement of the target strata from the transient cavity walls to the central uplift. The Outer parts of the central uplift record additional outward movement of the target rocks, possibly related to the collapse of the central uplift. An inner ring structure at 10-12 km from the crater center marks the extent of the deformation related to the outward movement of the target rocks.434701716Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Barringer Family FundClaude Leon Foundation of South AfricaFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)FAPESP [05/51530-3

    Impurity leakage mechanisms in the Wendelstein 7-X island divertor under friction-dominated conditions

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    The EMC3-Eirene code was used to study the main impurity leakage mechanism for the island divertor in the standard magnetic field configuration. It was found that under experimentally accessible plasma scenarios in the last experimental campaign, the majority of the island scrape-off layer was friction-force dominated. The impurity force balance was only thermal force dominated for upstream locations closed to the last closed flux surface, beyond the island X-point. No impurity neutral ionization was found in this location and hence the parallel impurity transport provides excellent impurity retention. It was found that impurities approach the confinement region nonetheless via perpendicular transport across the island O-point near the parallel flow stagnation region. This finding points out the specific role of the parallel flow stagnation region in providing lower parallel convective transport and long impurity residence times, which makes non-parallel transport channels more important or even the dominant driver of impurity leakage. In line with the relevance of the particle build-up in the flow stagnation region, different retention behavior as a function of density is seen for various species, which is shown to be due to ionization length changes as the plasma background density is increased
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