Theses and Dissertations from UMD
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Item DIRECT FUSION DRIVE BASED ON CENTRIFUGAL MIRROR CONFINEMENT(2023) Carson, Jerry Lee; Sedwick, Raymond J; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A concept for direct fusion drive based on centrifugal mirror confinement of thermonuclear plasmas (DFD-CM) is described. In centrifugal mirrors, electric and magnetic fields are combined to confine the plasma within a rapidly rotating annulus of burning plasma fixed between two mirror magnets. High-energy fusion products leave the reactor core at a rate determined by the velocity of plasma rotation and the strength of the mirrors. Those departing through the aft jet-side mirror deposit their energy in a “warm plasma” which then expands through a magnetic nozzle to deliver jet power in the 100-1000 kW range. Fusion products departing through the forward, power-side mirror are converted to electricity to power the reactor. Moderate thrusts at attractive specific impulses (15000+ seconds) are possible. Findings are presented on centrifugal mirror reactor dynamics in propulsion applications, to include new insights into the relationship between mirror and centrifugal components of plasma confinement. Additionally, analysis is presented on reactor operability limits and characterization of viable configurations based on power density, technology constraints, and the ability to self-power. Physics of the warm plasma are discussed, to include estimates for fusion energy deposition. Finally, considerations for Alfvén’s “frozen-in” theorem relative to fusion plasmas and magnetic nozzle performance will be outlined.Analysis indicates the DFD-CM system can self-power, and would be relatively compact. For the 200 kW delivered jet power system, the volume of burning plasma in the CM fusion reactor is estimated to be on the order of 1 m3. Self-powering in propulsion applications requires DFD-CM reactor operation at M_θ>9. This in turn requires electric fields ranging from 40-90 MV/m, and mirror strengths up to 15T. The main losses in the propulsion system are due to heating and ionizing the propellant. These losses decrease with increasing specific impulse. This work has resulted in four contributions. To start, it is the first analysis of the end-to-end performance of direct fusion drive based on centrifugal mirror confinement of the burning plasma. It demonstrates that the concept is thermodynamically feasible with nominal cycle efficiencies of 50 percent based on fusion energy entering the propulsion system. The second contribution is characterization of CM fusion reactor performance and operability. A particular finding is that self-powering DFD-CM reactors in propulsion applications may need to operate at centrifugal Mach numbers greater than 9, as previously mentioned. The third contribution is the development and preliminary application of a set of engineering models of the reactor, warm plasma, and plasma acceleration and expansion. These models are considered moderate fidelity in that they account for first order effects, as well as salient second order effects. The fourth contribution is identifying the possibility that the burning plasma in the reactor and the warm plasma may be electrically coupled. The nature and implications of any coupling are uncertain, and the current research proceeds assuming that the coupling does not occur. However, the question indicates the need for further research.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.