Brent Foy (Committee Member), Gregory Kozlowski (Committee Member), Amit Sharma (Advisor)
Master of Science (MS)
The field of plasma confinement and path toward achieving thermonuclear fusion started with experimental devices like tokamak and has evolved into other more complex variants of magnetic plasma confinement such as stellarator and spherical-tokamaks. As the plasma confinement machines advance towards higher temperature and plasma density (thermonuclear fusion conditions) the role and nature of plasma-wall interaction such as possible edge plasma regimes, particle recycling at the walls and its consequence for erosion, migration and re-deposition of wall material and impurity generation, transport and radiation as well as issues of particle exhaust continues to be a dominant limiting factor due to the close proximity of the wall. The selection of optimal wall-material for the plasma-wall components is a complex process, and till date, it continues to be an important and challenging area in the field of study of plasma-wall interaction. Various metals, ceramics or graphites with desirable response to severe thermal loads and varying mechanical properties towards elastic deformation, plastic deformation, fatigue, and toughness have been proposed. Sputtering and wall-erosion which results in plasma contamination is an important determining factor for wall-material selection. Tungsten is considered as a possible candidate for plasma facing material because of its high thermal conductivity, low hydrogen retention, high atomic mass (high-Z), and, high melting point. In both, limiter and divertor configurations there is substantial recycling of particles on the wall due to continuous bombardment of wall material by both charged and neutral particles. Experimental studies have shown that the light particle species such as hydrogen and helium are able to penetrate into the tungsten wall and substantial trapping of helium in tungsten has been observed. Among other issues, blistering, fuzz formation, tritium retention, surface roughening, and intergranular embrittlement are major issues to be addressed. Considerable effort is invested towards developing a better understanding of the interactions of hydrogen, helium in tungsten matrix. In the present study we use classical molecular dynamics (MD) approach to study (a) hydrogen retention, (b) helium bubble formation in tungsten, and, (c) study the effect of the presence of helium bubbles in tungsten matrix on hydrogen retention. The hydrogen bombardment simulations span an energy range from 30 eV to 100 eV at three different substrate temperatures - 500K, 1200K and 2000K. The variation of hydrogen trapping on tungsten matrix surface orientation is examined by performing MD simulation for <100> and <111> surface orientations. The growth of helium clusters in tungsten matrix as a function of temperature and a varying number of helium atoms at the start of the simulation is performed to study solute saturation effects. The trapping of hydrogen in the presence of helium is studied through molecular dynamics study of the bombardment of hydrogen atoms on tungsten substrate with helium cluster distributed throughout the tungsten matrix. The results of this study show that the hydrogen trapping fraction grows almost linearly over the intermediate bombarding energy range with the exception of low incident energy for which higher hydrogen trapping is observed. The effect of substrate temperature for low energy hydrogen bombardment is found to be different from the high-energy incidence indicating a complex dynamics of atomic diffusion within the tungsten matrix. The surface orientation of the substrate also affects the trapping percentage of hydrogen. The formation and growth of helium cluster are found to be dependent on the temperature and the number of helium atoms per unit cell. In the presence of helium cluster, the trapping percentage of hydrogen is significantly affected, especially at low energy.
Department or Program
Department of Physics
Year Degree Awarded
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