Fossil resources provide the raw materials for manufacturing a majority of commodity chemicals and fuels, but the release of this buried carbon accelerates environmental crises related to rising levels of atmospheric CO2. Engineering direct and energy-efficient pathways to synthesize chemicals and fuels from sustainable reagents and using CO2-free renewable energy could mitigate these challenges. Promising strategies for developing such reaction processes utilize non-precious metal catalysts to address kinetic challenges and non-thermal plasma activation to circumvent thermodynamic constraints. Non-precious bimetallic catalysts were employed to selectively convert CO2 with H2 to the building block chemical CO, and in situ X-ray and infrared techniques revealed the properties of the catalytic components. Significant oxygen exchange between the ceria catalyst support material and gas-phase CO2 was quantified under reaction conditions, and NiFe bimetallic catalysts tuned the reaction selectivity while maintaining high activity. In order to eliminate H2 as a reagent, ethane (an underutilized shale gas fraction) was reacted with CO2 to produce alcohols. This reaction is not thermodynamically feasible under mild conditions, so non-thermal/non-equilibrium plasma activation was implemented in order to achieve a one-step, H2-independent process to synthesize alcohols and other oxygenates under ambient temperature and pressure. The ability to use non-thermal plasma to activate N2 at mild conditions introduces the possibility of moving beyond the carbon-based paradigm for chemicals and fuels. Non-thermal plasma has been used to synthesize ammonia under mild conditions, but the dearth of fundamental understanding of plasma-catalyst interactions handicaps the development of plasma catalytic N2 conversion processes. Therefore, an in situ FTIR reactor was employed to identify the surface reaction intermediates during plasma catalytic ammonia synthesis. These results provide the first direct evidence of catalytic surface reactions under plasma activation and reveal the presence of reaction pathways that are distinct from analogous thermocatalytic reactions. Finally, an energy-based analysis evaluates the environmental and economic outlook for plasma-activated nitrogen fixation processes.

DIII-D experiments demonstrate simultaneous stability measurements and control of resistive wall modes (RWMs) with toroidal mode numbers 𝓃= 1 and 𝓃= 2. RWMs with 𝓃 > 1 are sometimes observed on DIII-D following the successful feedback stabilization of the 𝓃 = 1 mode, motivating the development of multi-n control. A new optimal multi-mode feedback algorithm based on the VALEN physics code has been implemented on the DIII-D tokamak using a real-time GPU installed directly in the DIII-D plasma control system (PCS). In addition to stabilizing RWMs, the feedback can control the stable plasma error field response, enabling compensation of the typically unaddressed DIII-D 𝓃 = 2 error field component. Experiments recently demonstrated this algorithm’s ability to simultaneously control 𝓃 = 1and 𝓃= 2 perturbed fields for the first time in a tokamak, using reactor relevant external coils. Control was maintained for hundreds of wall-times above the 𝓃 = 1 no-wall pressure limit and approaching the 𝓃 = 1 and 𝓃 = 2 ideal-wall limit. Multi-mode feedback also improved the control of the ELM-driven 𝓃 = 1and 𝓃 = 2 fields which further validates the feedback performance. Furthermore, a rotating non-zero target was set for the feedback, allowing stability to be assessed by monitoring the rotating plasma response while maintaining control. This novel technique can be viewed as a closed-loop extension of active MHD spectroscopy, which has been used to validate stability models through comparisons of the plasma response to applied, open-loop perturbations. The closed-loop response measurements are consistent with open-loop MHD spectroscopy data over a range of 𝜷𝑛 approaching the 𝓃 = 1 ideal-wall limit, demonstrating the potential of this technique as a useful tool for measuring stability while maintaining control even as the marginal stability point is approached. These plasma response measurements were then fit to produce both VALEN and single-mode stability models. These models allow for important plasma stability information to be determined and have been shown to agree with experimentally observed RWM growth rates. This improved understanding and control of the 𝓃 = 1 and 𝓃 = 2 RWM will allow for more robust operation above the 𝓃 = 2 no-wall limit.

No description available.

In this work, we investigate numerically the evolution of perturbed Vlasov equilibria. according to the full nonlinear system with particular emphasis on analyzing the asymptotic states towards which the system evolves. The simulations are carried out with the numerical code that we have implemented on the Cray X-MP of the Pittsburgh Supercomputing Center and which is based on the splitting scheme algorithm. Maxwellian symmetric and one-sided bump-on-tail and two-stream type of equilibrium distributions are considered: the only distribution which seems to evolve towards a BGK equilibrium is the two-stream while the asymptotic states for the other distributions are better described by superpositions of possible BGK modes. Perturbations with wave-like dependence in space and both symmetric and non-symmetric dependence on velocity are considered. For weakly unstable modes, the problem of the discrepancy between different theoretical models about the scaling of the saturation amplitude with the growth rate is addressed for the first time with the splitting scheme algorithm. The results are in agreement with the ones obtained in the past with less accurate algorithms and do not exhibit spurious numerical effects present in those. Finally, collisions are included in the splitting scheme in the form of the Krook model and some simulations are performed whose results are in agreement with existing theoretical models. / Ph. D.

The work described in this thesis is an analysis of the Plasma Source Ion Implantation (PSII) process. A metal target is placed within a plasma, and pulsed to a high negative potential (10 - 50 kV). The electrons in the plasma close to the target are then repelled very rapidly, leaving an area of uniform positive charge. This causes an electric field to be set up between the plasma and the metal target. The ions close to the target are then accelerated towards the target by the electric field. The ions reach the target at high velocities, and implant deeply into the metal (-5 x 10-8 m), and form nitrides, which pin dislocations within the metal's atomic structure. The strength of the metal is therefore increased, and other properties such as the corrosion resistance of the metal are also improved. Metals that have undergone the PSII process have widely diverse applications. For example, in the motor industry, ion implanted metal punches last much longer than nitrided punches, while in the medical industry ion implanted metals are used for artificial limbs. A combination of a number of different analytic, numerical and simulation models are used to describe the PSII process, including the plasma behaviour and final nitrogen implantation profile in the metal target after the application of the voltage pulse. In all cases, a specific attempt has been made to realistically describe as closely as possible, the actual experimental arrangement at the University of Natal. For example: a waveform with a fast rise time, short plateau and exponential decay was used; the nitrogen plasma was more realistically described by a two species fluid to account for the measured N+, N; mix; and finally, the actual atomic composition for 304 stainless steel was used in the TAMIX particle simulation. This work thus models the whole PSII process, and could form the basis of future studies for the optimisation of the process. / Thesis (M.Sc.)-University of Natal, 1993.

Call number: LD2668 .T4 1962 H37

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

A numerical study is done of a plasma in contact with a cold solid surface that is emitting a neutral gas. Two numerical models have been developed to describe the dominant phenomena of surface plasma structures. The first model entails a steady-state, kinetic treatment of the transport equations in one space dimension and one velocity dimension, to determine self-consistently the distribution functions of the interacting species and the electrostatic potential near the solid surface. The dominant phenomena in this region are the ionization of the neutral gas and the acceleration of the resulting ions by the electrostatic field in a pre-sheath region. Other effects involved are a Debye sheath structure between the solid surface and pre-sheath, and collisional trapping and untrapping of electrons in an electrostatic potential well that is predicted in the pre-sheath region. Results are presented from a nondimensional model with a monatomic returning neutral species and for diatomic molecular hydrogen returning from the surface. For each set of physical parameters chosen, a one parameter family of solutions is obtained. The second numerical model involves a steady-state treatment of the transport equations in a (x,v∥,v⊥) phase space for the interacting species. Included in this model are ionization of the refluxing monatomic neutrals, a self-consistently determined electrostatic potential and a nonlinear Fokker-Planck treatment of ion-ion Coulomb collisions. Both the region near the surface dominated by kinetic effects and the region away from the surface in which Coulomb collisional effects are significant are treated. Results are presented which identify the correct physical solution for the region near the surface from the permitted family found with the kinetic model. Additionally, results are shown which span a temperature range from the high temperature kinetic regime where Coulomb collisional effects are negligible, to the low temperature, highly collisional fluid regime. At low temperatures the collisional model agrees well with standard fluid techniques.

Thesis (PhD (Electrical Engineering))--Cape Peninsula University of Technology, 2018. / The generation of electricity through the conventional conversion system such as thermal and hydroelectric plants may no longer be sufficient to meet the increasing demands and usage. One of the major reasons for shortage supply of electric power is due to the lack of fossil fuel and other conventional resources that are currently being used in Africa. In addition, the conversion process of the conventional system often causes pollution which contributes to global warming. Therefore, there is a need for this research to develop novel and alternative methods of generating electric power. Among these methods is the Magnetohydrodynamics (MHD) conversion system, which is a direct energy conversion system. In this system, plasma or ionised gas is directly converted into electric power with generating efficiency of about 62 %. The conversion process of the MHD system is based on the principle of Faraday’s Law of electromagnetism and fluid dynamics. The focus of the present study is to investigate alternative methods through which an MHD power generator can be coupled to the existing thermal plants in South Africa. In doing so, the thermal cycle efficiency of these conventional plants can be improved. Another goal of this study is to investigate the behaviour of an MHD generator prototype under exposure to plasma through simulation and experimentation in a laboratory setting.

by Lo Veng-cheong. / Bibliography : leaves 95-96 / Thesis (M.Phil.)--Chinese University of Hong Kong, 1982