Synergistic Effects of High Particle Fluxes and Transient Heat Loading on Material Performance in a Fusion Environment
Abstract
The work presented in this thesis focuses on the thermal and structural evolution of different materials when exposed to both high-flux ion irradiation and high intensity pulsed heat loading. Nuclear fusion devices create an intense radiation environment consisting of very energetic deuterium (D+) and helium (He+) ions. During operation, off-normal plasma events, such as edge-localized modes (ELMs), could cause intense heating of the plasma-facing component (PFC) surface, leading to melting and possible splashing into the fusion plasma. High-Z, refractory metals, such as tungsten (W), are therefore seen as favorable, due to their high melting point, high thermal conductivity, and low sputtering yield. However, potential splashing of the molten wall could contaminate the plasma and shut down the reactor. High-flux He+ wall loading could further exacerbate melting and splashing of the PFC surface, due to the growth of fiber form nanostructures, called fuzz, which possess a much lower mechanical and thermal strength than that of a pristine surface. Experiments performed throughout the dissertation attempt to qualify the effect of He+-induced surface structuring on the PFC thermal response during type-I ELMs. Elementary surface characterization revealed that He+ loading blurs clear melting and droplet emission thresholds observed on pristine surfaces during ELM-like heat loading, inducing thermal damage gradually through localized melting and conglomeration of fuzz tendrils. The reduced thermal conductivity of fuzz nanostructures led to increased levels of erosion due to fragmentation of molten material. Decreasing the imparted heat flux, at the sacrifice of higher frequencies, through ELM mitigation techniques showed the potential for an intermediate operating window that could heal fuzz nanostructures via annealing without the onset of splashing. Tests on transversally-oriented W microstructures (which will be used in ITER) revealed that radiation hardening along grain boundaries due to high-flux He+ loading may preferentially enhance brittle failure. Differences in penetration depth between experimental heat loading methods (millisecond laser vs. electron beam) affected heat deposition in and plasticity of the damaged surface. Simultaneous He+ particle loading and ELM-like heat loading inhibited fuzz formation due to repetitive shock-induced conglomeration. The addition of D+ ion irradiation appeared to further reduce evidence of early-stage fuzz formation, due to super-saturation of D in the near-surface layer. Significant structuring due to D+ particle loading may diminish the impact of ELM intensity on surface roughening and melting. Future studies need to expand upon the surface analysis presented throughout this dissertation and investigate the details of the subsurface to determine how intense thermal loading impacts gas trapping and migration. In addition, future PFC erosion research must utilize highly sensitive, in situ measurement techniques to obtain reliable information on material lifetime and performance.
Degree
Ph.D.
Advisors
Hassanein, Purdue University.
Subject Area
Nuclear engineering|Plasma physics|Materials science
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