Performance of Tungsten-Tantalum Alloys as Plasma Facing Components in Relevant Fusion Conditions
Currently nuclear fusion remains a major target for future energy production. However, numerous key issues still remain unresolved for both inertial confinement and magnetic confinement design concepts. Material selection for Plasma Facing Components (PFCs) is a major concern that needs further investigation and innovative solutions. Tungsten (W) has become a leading candidate for use in fusion devices because of several key thermal and physical properties like high melting point, high thermal conductivity, and low erosion rate. However, continued research on tungsten has revealed several major concerns when tungsten is exposed to relevant fusion conditions, including embrittlement, melting, and extreme morphology evolution leading to a nanostructure called ‘fuzz’. These issues have prompted the need for innovative solutions to design more robust PFC materials. The work in this thesis will investigate alloying tungsten with tantalum in order to elucidate the possible enhancements in tungsten PFC performance as a function of tantalum concentration. The scope of the experimental work discussed in this thesis covers three major areas. First, tungsten (W) and tungsten-tantalum alloys (W-Ta) were exposed low energy helium ion (He+) at various temperatures. The results of this experiment showed a significant difference in accumulated surface damage as a function of both temperature and Ta concentration. Scanning electron microscopy (SEM) and X-Ray Diffraction (XRD) data indicated that there may be a correlation between the observed morphology differences and the induced crystal structure change caused by the presence of Ta. These results were supported via X-Ray Photoelectron Spectroscopy (XPS) and Optical Reflectivity (OR). The second major area discussed is the exposure of pure W and W-Ta alloys to mixed and sequential He+/D+ ion beam irradiations. In these experiments the effect of dual ion irradiation is investigated by subjecting W and W-Ta samples to four different D+:He + ratios (100% He+, 60% D+ + 40% He+, 90% D+ + 10% He+ and 100% D+). SEM results revealed that increasing the D+ concentration leads to suppression of He + induced surface damage. Additional, sequential ion exposures were conducted to decouple the interaction between the He+ and D + during the dual ion irradiations. For the sequential experiments, W and W-Ta sample were first exposed to low energy He+ ions. This was then followed by exposures to low energy D+ ions at 1223 K. SEM results revealed similar response in the surface due to the dual ion beam irradiations. There was significant degradation and reintegration of the fuzz surface in response to low energy D+ irradiation at 1223 K. This experiment was repeated for W and W-Ta samples but the D+ exposure was conducted at 523 K. In this case, post irradiation SEM revealed that the D+ had no effect on the He+ induced morphology. This result indicated that the morphology suppression mechanism is based on a temperature dependent W-D interaction mechanism, like D desorption. This result is significant to the fusion community in that it suggests there may be an operation parameter space for future fusion devices which actively suppresses He+ induced surface damage on the PFCs. The final area discussed is the exposure of pure W and W-Ta samples to both single and dual ion beam irradiation along with simultaneous pulsed heat loading to simulate ELM-like transient events expected in future fusion devices. SEM data from this chapter revealed three main conclusions. First, there was a very apparent difference in the severity of the laser induced damage when comparing the W to the W-5Ta samples. This trend was consistent regardless of loading conditions. Further investigation suggested that the weaker W-5Ta samples fail more readily under the intense thermal stresses that are induced by the transient heat loading. Second, for the heat fluxes investigated in these experiments, there was essentially no significant difference in the resulting surface damage between the laser only exposures and the sequential ion irradiation followed by laser exposures. Finally, the presence of ion irradiation (single and dual) with simultaneous pulsed heat loading did show significant differences in the damage morphology. Specially, deeper trenches and pore formation were present and suggest a possible increase in the surface erosion. This increase seems smaller in the dual ion irradiation case. Additional erosion studies using a Mo witness plate revealed an increase in erosion of W when exposed to transient heat loading with simultaneous ion bombardment. This is important, and it details a critical decrease in PFC performance when synergistic loading effects are taking into account. In addition to the results presented, the work provided in this thesis creates a framework for the comprehensive analysis of alternative PFC materials to complex fusion conditions.
Sizyuk, Purdue University.
Nuclear engineering|Materials science
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