Magnetic X-point collapse is investigated using a 2.5D fully relativistic particle-in-cell simulation, with varying strengths of guide-field as well as open and closed boundary conditions. In the zero guide-field case we discover a new signature of Hall-reconnection in the out-of-plane magnetic field, namely an octupolar pattern, as opposed to the wellstudied quadrupolar out-of-plane field of reconnection. The emergence of the octupolar components was found to be caused by ion currents and is a general feature of X-point collapse. The effect was shown to be independent of system size and ion mass and confined to a few ion inertial lengths from the reconnection current sheet. In a comparative study of tearing-mode reconnection, signatures of octupolar components are found only in the out-flow region. It is argued that space-craft observations of magnetic fields at reconnection sites may be used accordingly to identify the type of reconnection. Further, initial oscillatory reconnection is observed, prior to reconnection onset, generating electromagnetic waves at the upper-hybrid frequency, matching solar flare progenitor emission. When applying a guide-field, in both open and closed boundary conditions, thinner dissipation regions are obtained and the onset of reconnection is increasingly delayed. Investigations with open boundary conditions show that, for guide-fields close to the strength of the in-plane field, shear flows emerge, leading to the formation of electron flow vortices and magnetic islands. Asymmetries in the components of the generalised Ohm's law across the dissipation region are observed and inertial components are shown to play a role at the X-point. Extended in 3D geometry, it is shown that locations of magnetic islands and vortices are not constant along the height of the current-sheet. Vortices formed on opposite sides of the current-sheet travel in opposite directions along it, leading to a criss-cross vortex pattern. Similarly to oblique current sheets previously observed in 3D guide-field reconnection studies, vortex-tubes are inclined at the same angle as the magnetic field.

Abstract This thesis examines magnetic reconnection in the solar corona. Magnetic reconnection is the only mechanism which allows the magnetic topology of magnetized plasmas to be changed. Many of the dynamic processes in the Sun's atmosphere are believed to be driven by magnetic reconnection and studying the behaviour of such phenomena is a key step to understanding the reconnection mechanism. In Chapters 1 to 3, we discuss the physical and mathematical framework on which current magnetohydrodynamic reconnection models are based. The aim of the thesis is to investigate theoretical models of magnetic reconnection using variety of analytic and numerical techniques within the theoretical frame work of magnetohydrodynamics (MHD). In Chapter 4 we use a line-tied X-point collapse model for compressible plasmas to investigate the role of viscosity on the energy release mechanism. This model also provides the basis for the investigation of Chapter 5 which explores the impact of Hall currents in the transient X-point energy dissipation. Chapter 6 is concerned with how reconnection is modified in the presence of generalized Ohm's law which includes both Hall current and electron inertia contributions. In contrast to the closed X-point collapse geometry adopted for compressible plasmas previously, we find it more convenient to explore this problem using an open incompressible geometry in which plasma is continually entering and exiting the reconnection region. Specially, we find the scaling of the Hall-MHD system size analytically, rather than numerically as in the X-point problem of Chapter 5. Chapter 7 summarizes the results of investigations in Chapters 4, 5 and 6.

L’échange d’énergie par rayonnement est proportionnel à l’émissivité. Ce facteur dépend de la température et de la longueur d’onde mais aussi de l’état de surface, de la composition et de la phase. Sa mesure précise est donc toujours un défi à cause des influences connues ou inconnues. Par rapport à d’autres travaux, le but de ce travail est de l’étudier dans des conditions extrêmes, des températures cryogéniques aux hautes températures : Nous avons ainsi étudié à 80K l’absorption d’un diélectrique pour la protection thermique d’un satellite et celles de surfaces en or de différentes rugosités. A la température ambiante nous avons déterminé par une méthode en réflexion, l’émission de vitrages dans le but de la recherche d‘économie d’énergie. Nous avons aussi étudié la variation de l’émissivité pendant le changement de phase solidus-liquidus et α-β de second ordre. Autour de 100°C le soufre devient plus émissif lorsqu’il passe du solide au liquide mais par contre il n’y a pas de différence sur l’émissivité pour ses deux variétés allotropiques principales. A une beaucoup plus haute température, le silicium liquide se comporte comme un métal avec une émissivité très faible et un saut net a été constaté à son passage au point de fusion. Une variation marquée de l’émissivité pour les deux phases solides du SiC a été observée entre 8-11μm dans notre étude. Contrairement aux résultats de la littérature, les sommets d’émissivité diminuent progressivement avec l’augmentation de la température. Enfin trois points X, longueur d’onde où l’émissivité ne dépend pas de la température, ont été observés pour chaque phase. / Heat transferred by radiation is proportional to the emissivity. This coefficient depends not only on the wavelength and the temperature, but also on the surfaceroughness, the chemical composition and the phase. A precise measurement is always a challenge because of the known and unknown factors. Compared with others, this thesis aims at the studies in extreme conditions, from cryogenic temperature to very high temperature: The absorptivity of a dielectric applied to the thermal protector for the satellite and the emissivity of gold surfaces with different roughness have been measured at 80K. The emissivity of different windows has been determined by the reflection method at room temperature for the research of energy saving. We have also studied the variation of the emissivity during the solid to liquid and α-β phase transition. The sulfur becomes more emissive when it changes from solid to liquid around 100°C, but there is no difference on the emissivity between its two major allotropies. The liquid silicon behaves like a metal with a very low emissivity and an obvious bound has been measured when it crosses its melting point. A marked variation of the emissivity of SiC for its two solid-state phases has been observed between 8-11μm. Contrary to other results, the peak values of the emissivity attenuate with the increase of temperature. Finally, three X points at which the emissivity doesn’t depend on the temperature have been measured for each phase.

<div> <div> <p>Phase transition materials (PTM) have garnered immense interest in concurrent post-CMOS electronics, due to their unique properties such as - electrically driven abrupt resistance switching, hysteresis, and high selectivity. The phase transitions can be attributed to diverse material-specific phenomena, including- correlated electrons, filamentary ion diffusion, and dimerization. In this research, we explore the application space for these materials through extensive device-circuit co-design and propose new ideas harnessing their unique electrical properties. The abrupt transitions and high selectivity of PTMs enable steep (< 60 mV/decade) switching characteristics in Hyper-FET, a promising post-CMOS transistor. We explore device-circuit co-design methodology for Hyper-FET and identify the criterion for material down-selection. We evaluate the achievable voltage swing, energy-delay trade-off, and noise response for this novel device. In addition to the application in low power logic device, PTMs can actively facilitate non-volatile memory design. We propose a PTM augmented Spin Transfer Torque (STT) MRAM that utilizes selective phase transitions to boost the sense margin and stability of stored data, simultaneously. We show that such selective transitions can also be used to improve other MRAM designs with separate read/write paths, avoiding the possibility of read-write conflicts. Further, we analyze the application of PTMs as selectors in cross-point memories. We establish a general simulation framework for cross-point memory array with PTM based <i>selector</i>. We explore the biasing constraints, develop detailed design methodology, and deduce figures of merit for PTM selectors. We also develop a computationally efficient compact model to estimate the leakage through the sneak paths in a cross-point array. Subsequently, we present a new sense amplifier design utilizing PTM, which offers built-in tunable reference with low power and area demand. Finally, we show that the hysteretic characteristics of unipolar PTMs can be utilized to achieve highly efficient rectification. We validate the idea by demonstrating significant design improvements in a <i>Cockcroft-Walton Multiplier, </i>implemented with TS based rectifiers. We emphasize the need to explore other PTMs with high endurance, thermal stability, and faster switching to enable many more innovative applications in the future.</p></div></div>