An Action Principle for Relativistic MHD

A covariant action principle for ideal relativistic magnetohydrodynamics (MHD) in terms of natural Eulerian field variables is given. This is done by generalizing the covariant Poisson bracket theory of Marsden et al., which uses a noncanonical bracket to effect constrained variations of an action functional. Various implications and extensions of this action principle are also discussed. Two significant by-products of this formalism are the introduction of a new divergence-free 4-vector variable for the magnetic field, and a new Lie-dragged form for the theory

Multi-GeV Electron Generation Using Texas Petawatt Laser

We present simulation results and experimental setup for multi-GeV electron generation by a laser plasma wake field accelerator (LWFA) driven by the Texas Petawatt (TPW) laser. Simulations show that, in plasma of density n(e) = 2 - 4 x cm(-3), the TPW laser pulse (1.1 PW, 170 fs) can self-guide over 5 Rayleigh ranges, while electrons self-injected into the LWFA can accelerate up to 7 GeV. Optical diagnostic methods employed to observe the laser beam self-guiding, electron trapping and plasma bubble formation and evolution are discussed. Electron beam diagnostics, including optical transition radiation (OTR) and electron gamma ray shower (EGS) generation, are discussed as well.Physic

A Ritual Geology

Robyn d’Avignon tells the history of West Africa’s centuries-old indigenous gold mining industries and its shared practices, prohibitions, and cosmological engagements

A Ritual Geology

Robyn d’Avignon tells the history of West Africa’s centuries-old indigenous gold mining industries and its shared practices, prohibitions, and cosmological engagements

The Unreasonable Effectiveness of the Air-Fluorescence Technique in Determining the EAS Shower Maximum

We review all existing air-fluorescence measurements of the elongation rate of extensive air showers (slope of mean EAS shower maximum (Xmax) vs log of shower energy E) above 1017 eV. We find remarkable agreement for all current and historic experiments over a 30 year period for the energy range from 1017 to 3x1018 eV. The mean elongation rate in this energy interval is near 80 gm/cm2/decade Above this energy, experiments in the Northern hemisphere are in good agreement with an average elongation rate of 48 +/- 10 gm/cm2/decade while Southern hemisphere experiments have a flatter elongation rate of 26 +/- 2 gm/cm2/decade We point out that, given the agreement at lower energies, possible systematic reasons for this difference are unlikely. Given this, the world elongation rate data alone may indicate a composition difference of UHECR in the Northern and Southern hemisphere and thus a diversity of UHECR sources in the Northern and Southern sky.Comment: Accepted by JET

Les enjeux de santé reproductive en Haïti « vus du bas » : le point de vue des acteurs de terrain à Port-au-Prince

Lors de la Conférence Internationale sur la Population et le Développement qui s’est tenue au Caire en 1994, la communauté internationale a adopté une nouvelle approche, jugée « révolutionnaire », celle de la santé sexuelle et reproductive. Avec ce nouveau paradigme, en Haïti, comme dans d’autres pays du Sud, les programmes en planification familiale ont, en théorie, élargi leurs horizons aux droits individuels en matière de reproduction, aux dynamiques de genre et à l’importance de la capacité décisionnelle des femmes. Plus de trente ans après la conférence du Caire, l’heure est au bilan. Cette recherche contribue à cet état des lieux en explorant, à travers des entretiens qualitatifs, les perceptions des acteurs impliqués dans la santé reproductive à Port-au-Prince : bailleurs de fonds, employés ministériels, ONG internationales et locales, et prestataires de soins. En identifiant les obstacles et les succès rencontrés sur le terrain, cette étude analyse les dynamiques actuelles de ces programmes dans le contexte haïtien. Les résultats montrent que, malgré l'adoption de l’approche en santé reproductive par la plupart des programmes en Haïti, ces derniers restent fragmentés et limités, souvent focalisés sur la planification familiale sans intégration d'autres aspects de la santé sexuelle. La dépendance aux financements internationaux et les priorités fluctuantes des bailleurs fragilisent le système de santé haïtien, tandis que les contraintes socioéconomiques et les lois restrictives, notamment en matière d'avortement, compliquent l'accès aux services de santé sexuelle et reproductive.At the International Conference on Population and Development held in Cairo in 1994, the international community adopted a new approach, deemed "revolutionary," that of sexual and reproductive health. With this new paradigm, in Haiti, as in other countries of the Global South, family planning programs have theoretically broadened their scope to include individual reproductive rights, gender dynamics, and the importance of women’s decision-making power. More than thirty years after the Cairo Conference, the time has come for an assessment. This research contributes to this assessment by exploring, through qualitative interviews, the perceptions of actors involved in reproductive health in Port-au-Prince: donors, ministry employees, international and local NGOs, and healthcare providers. By identifying the obstacles and successes encountered on the ground, this study analyzes the current dynamics of these programs within the Haitian context. The results indicate that, despite the adoption of the reproductive health approach, most programs in Haiti remain fragmented and limited, often focusing solely on family planning without integrating other aspects of sexual health. Dependence on international funding and the fluctuating priorities of donors weaken Haiti’s healthcare system, while socioeconomic constraints and restrictive laws, particularly regarding abortion, complicate access to sexual and reproductive health services

Modeling and Experimental Validation of the Performance of Phase Change Material Storage Tanks in Buildings

RÉSUMÉ Le stockage d’énergie thermique dans les bâtiments permet d’atténuer les pointes d’appel de puissance sur le réseau électrique et de synchroniser la demande énergétique à la disponibilité de ressources énergétiques renouvelables, telle l’énergie solaire. Les matériaux à changement de phase (MCP) peuvent être utilisés afin de permettre un tel stockage d’énergie thermique. Ceux-ci offrent une haute densité de stockage énergétique (principalement sous forme d’énergie latente) et un changement de phase à température quasiment constante. L’intégration de MCP dans un réservoir où circule un fluide caloporteur permet de créer un système de stockage actif. La charge et décharge énergétique du réservoir peuvent alors être contrôlées par le débit du fluide caloporteur envoyé dans le réservoir ou vers un contournement. Afin d’assurer une performance adéquate du réservoir de stockage à MCP dans un bâtiment, le comportement dynamique de celui-ci doit être prévisible. Or, le design du réservoir aura un impact crucial sur son fonctionnement : la température de changement de phase du matériau, la géométrie du réservoir et des capsules de MCP influenceront le comportement dynamique de celui-ci. Afin de permettre un design adéquat du réservoir, des outils permettant la simulation énergétique de tels systèmes sont nécessaires. Or, la modélisation du changement de phase, souvent basée sur une relation entre l’enthalpie et la température du matériau, présente certaines difficultés. Elle est, entre autre, limitée par les informations rendues disponibles par les manufacturiers, qui sont souvent incomplètes ou erronées. Les MCP ont aussi tendances à se comporter différemment lors de leur fusion et lors de leur solidification (i.e. présence d’hystérèse) et ils exhibent parfois des phénomènes de surfusion dont l’occurrence est plus stochastique que déterministe. De plus, peu de données expérimentales existent sur le comportement dynamique de tels réservoirs. Ainsi, les modèles numériques actuels sont limités à quelques géométries et rarement validés expérimentalement. Toutes ces problématiques sont soulevées dans cette thèse et des solutions sont abordées. La première partie (chapitre 4) offre une contribution à une méthode de caractérisation normalisée des matériaux à changement de phase, à travers une évaluation critique du traitement des données de la méthode « T-History ». Une proposition est faite quant à la variante qui devrait être adoptée pour déterminer la courbe enthalpie-température de MCP présentant une surfusion importante afin de fournir toutes les informations nécessaires pour la simulation numérique de leur comportement.----------ABSTRACT Thermal energy storage in buildings can attenuate peak power demand to the electric grid and synchronize the heating or cooling load to the availability of renewable energy, such as solar energy. Phase change materials (PCM) can be used to allow such storage of thermal energy. They offer high energy storage density (mainly through latent energy) and a quasi-constant phase change temperature. The integration of PCMs in a tank where a heat transfer fluid can circulate allows the creation of an active thermal storage system. Charging and discharging energy from the reservoir can be controlled by directing the heat transfer fluid into the tank or towards a by-pass. To ensure an adequate performance of the PCM storage tank in a building, its dynamic behaviour must be predictable. The PCM tank’s design will have a crucial impact on its operation: the material’s phase change temperature, the geometry of the tank and PCM capsules will influence the transient behaviour of the tank. To allow an adequate design of the tank, tools allowing the energy simulation of such systems are required. However, the modelling of phase change itself, often based upon a relation between the enthalpy and temperature of the material, presents some difficulties. It is, amongst other things, limited by the information made available by manufacturers, which are often incomplete or erroneous. PCMs have also the tendency to behave differently during their fusion and solidification processes (i.e. presence of hysteresis) and they exhibit at times a phenomenon of supercooling whose occurrence tends to be more stochastic than deterministic. Moreover, little experimental data exists on the transient behavior of such PCM storage tanks. The existing numerical models are limited to a few geometries and have rarely been validated experimentally. All these problems are explored in this thesis and solutions are addressed. The first section of this thesis (Chapter 4) offers a contribution towards a standardised characterisation method for phase change materials through a critical evaluation of the data processing in the T-History Method. A proposal is made concerning the data processing variant which should be adopted to determine the enthalpy-temperature curve of PCMs presenting an important degree of supercooling so that all the required information necessary for its numerical simulation is available. The second section (Chapter 5) concentrates on the detailed experimental testing of a real-scale horizontal storage tank containing rectangular PCM capsules

Subterranean Histories: Making `Artisanal' Miners on the West African Sahel.

Since the late 1990s, in the context of rising gold prices and pro-market legal reforms, dozens of multi-national corporations have opened gold mines across the West African Sahel. Increasingly, corporate security forces enter into violent conflicts with so-called “artisanal” miners who extract gold with handpicks and dynamite. Social scientists and journalists have slotted conflicts between these two categories of miners into narratives of Africa’s neoliberal resource “curse” and problems of governance. By focusing on longer histories of extraction and empire, I reframe this “clash” as one node in a far-reaching debate over the rights of agrarian residents, the state, and private capital to the Sahel’s mineral resources. Since French conquest of this region in the 1890s, private prospectors and geologists have systematically appropriated the gold discoveries of West African miners while simultaneously degrading their extractive practices as primitive, criminal, and wasteful. While the criminalization of “informal” economic practices is a central feature of the power dynamics that constitute “development” in much of Africa, scholars have largely overlooked these dynamics in the extractive sector. Scholarship on mining focuses almost exclusively on the exploitation of land, labor, and ecologies. By contrast, I argue that the co-option of African mineral knowledge, and not only nature, is central to the reproduction of mining capitalism in West Africa, and likely elsewhere. This project also details how the racial geographies of imperialism became incorporated into post-colonial regulations of technological practice. Rooted in a deep historical account of gold mining in eastern Senegal, this dissertation is based on two years of field research in Senegal, Guinea, and France.PhDAnthropology and HistoryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/133516/1/robdavig_1.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/133516/4/ROBDAVIG.pdfDescription of ROBDAVIG.pdf : Revised (Images removed due to copyright restrictions