Physics
Permanent URI for this communityhttp://hdl.handle.net/1903/2269
Browse
3 results
Search Results
Item Optimization of high-beta fusion devices against linear instabilities(2023) Gaur, Rahul; Dorland, William; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Magnetic confinement fusion is a technique in which a strong magnetic field is used tocontain a hot plasma, which enables nuclear fusion. In terms of overall energy efficiency, the two most promising magnetic confinement concepts are tokamaks (axisymmetric devices) and stel- larators (nonaxisymmetric devices). The power P produced by a magnetically confined nuclear fusion device is proportional to Vβ2B4, where V is the volume of the device, β is the plasma pressure - magnetic pressure ratio, and B is the magnetic field strength. Most tokamaks and stellarators currently in operation are low-β devices. In general, there are three ways to increase P , one may increase the operating β, the magnetic field or the volume of the device. The cost of these devices is proportional to V , making large enough devices expensive. Similarly, a large magnetic field (>10T) requires superconducting magnets that, even after the recent innovations in HTS (High-Temperature Superconductors), are expensive to manufacture. High-β devices are an attractive idea to efficiently produce fusion energy. However, a high-β generally also implies a large gradient in plasma pressure that can be a source of numerous instabilities. If fusion devices could be optimized against such instabilities, high-β operation would become an attractive approach compared to high field or large-volume reactors. Therefore, this thesis explores the optimization of high-β tokamak and stellarator equilibrium equilibria against linear instabilities. We will start by investigating the stability of high-β tokamaks and stellarator equilibria against the infinite-n ideal ballooning mode, an important pressure-driven MHD instability. We stabilize these equilibria against the ideal ballooning mode. To achieve this, we formulate a gradient-based adjoint technique and demonstrate its speed and effectiveness by stabilizing these equilibria. We also explain how this technique can be easily extended to low-n ideal-MHD modes in both tokamaks and stellarators. After demonstrating the adjoint technique for stabilizing against ideal MHD modes, wefirst analyze the kinetic stability of a sequence of axisymmetric equilibria. We study this by nu- merically solving the δf gyrokinetic model, a simplified version of the Vlasov-Maxwell model. Since these kinetic instabilities are driven by temperature and density gradients, we explore them by scanning multiple values of the plasma β, temperature and density gradients, and plasma boundary shapes, discovering interesting relationships between equilibrium-dependent quantities and growth rates of these instabilities. We then repeat the same process for two recently pub- lished stellarator equilibria with quasisymmetry — a favorable hidden symmetry in stellarators. With this study, we verify that our observations from high-β tokamaks can be generalized to quasisymmetric stellarators. From our microstability study, we find that electromagnetic effects are important for high-βdevices. Hence, using the numerical tools and knowledge derived from the previous chapters we build an optimization framework that searches for stable equilibria. Due to the similarity between axisymmetry and quasisymmetry, we then use the microstability optimizer to search for ideally and kinetically-stable, quasisymmetric, high-β stellarators.Item Cyclotron resonance gain in the presence of collisions(2017) Cole, Nightvid; Antonsen, Thomas M; Ott, Edward; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The conditions needed for the amplication of radiation by an ensemble of magnetized, relativistic electrons that are collisionally slowing down are investigated. The current study is aimed at extending the work of other researchers in developing solid-state sources of Terahertz radiation. The source type considered here is based on gyrotron-like dynamics of graphene electrons, or it can alternately be viewed as a solid state laser source that uses Landau levels as its band structure and is thus similar to a quantum cascade laser. Such sources are appealing because they oer the potential for a compact, tunable source of Terahertz radiation that could have commercial applications in scanning, communication, or energy transfer. An exploration is undertaken, using linear and nonlinear theories, of the conditions under which such sources might be viable, assuming realistic parameters. Classical physics is used, and the model involves electrons in monolayer graphene assumed to be pumped by a laser, follow classical laws of motion with the dissipation represented by a damping force term, and lose energy to the electromagnetic eld as well. The graphene is assumed to be in a homogeneous magnetic eld, and is sandwiched between two partially-transmissive mirrors so that the device acts as an oscillator. This thesis incorporates the results of two approaches to the study of the problem. In the rst approach, a linear model is derived semi-analytically, which is relevant to the conditions under which there is gain in the device and thus stable operation is possible, versus the regime in which there is no net gain. In the second approach, a numerical simulation is employed to explore the nonlinear regime and saturation behavior of the oscillator. The simulation and the linear model both assume the same original equations of motion for the eld and particles that interact self-consistently. The model used here is very simplied, but the aim here is to elucidate the basic principles and scaling behavior of such devices, not necessarily to calculate what the exact dynamics, outputs, and parameters of a fully commercially realized device will be.Item NANOSTRUCTURED MATERIALS FOR SOLAR HYDROGEN GENERATION(2010) Ma, Xiaofei; Zachariah, Michael R; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Hydrogen can be considered a nonpolluting and inexhaustible energy carrier for the future. However, hydrogen is not readily available for use as a fuel. It exists in bound form with other elements (e.g. water, hydrocarbons) and as such energy is required to abstract molecular hydrogen from various feedstocks. Solar energy due to its abundance and low cost is being considered as the energy source for environmentally safe hydrogen generation. This dissertation focuses on the development and characterization of nano-structured materials for solar thermochemical hydrogen generation, on the principle that concentrated solar radiation can be employed as the high-temperature energy source for driving an endothermic hydrogen generation process. The reaction mechanism and kinetics of different solar thermochemical processes using those nano-structured materials as reactants or catalysts were investigated. The experimental works in this dissertation can be divided into two main areas. The first area is to study the properties and reactivity of in-situ generated Zn nanocrystals (NCs) for solar thermochemical Zn/ZnO water splitting cycle for hydrogen production. The particle size-resolved kinetics of Zn NCs oxidation, evaporation, and hydrolysis were studied using a tandem ion-mobility method in which the first mobility characterization size selects the NCs, whereas the second mobility characterization measures changes in mass resulting from a chemical reaction of the NCs. The second part of the dissertation is concentrated on the investigation of in-situ generated nano-sized metal particles as catalysts in liquid hydrocarbon decomposition process for hydrogen generation. Catalytic decomposition of liquid fuels (n-octane, iso-octane, 1-octene, toluene and methylcyclohexane) was achieved in a continuous tubular aerosol reactor as a model for the solar initiated production of hydrogen, and easily separable CO free carbonaceous aerosol product. The effects of fuel molecular structure and catalyst concentration on the overall hydrogen yield were studied. Using the similar aerosol catalysis idea, ignition of liquid fuels catalyzed by in-situ generated metal nanoparticles was investigated. The morphological change of catalyst particles during fuel ignition process and the catalytic ignition mechanism are discussed.