Renewal: Advanced ultrafine dispersion-strengthened tungsten alloys as burning plasma-facing component materials

Of the various materials options at the plasma-material interface in future burning-plasma magnetic fusion devices (i.e., graphite, liquid metals, etc.) refractory metals (molybdenum, tungsten etc.) are attractive for use during steady-state, high-temperature (700-1000 C) operation with heat flux ranging between 10-20 MW/m2. However, both molybdenum and tungsten have serious performance issues regarding radiation tolerance (brittle fracture, hardening, swelling, transmutation, etc. even at relatively high temperature) and hydrogen retention/permeation that must be addressed to make them a viable option for steady-state burning plasma operation in fusion reactors (e.g. DEMO). As a plasma-facing material, additional tungsten limitations include: irradiation-driven nanostructuring on tungsten surfaces (e.g. morphology evolution), blistering and embrittlement of tungsten due to high exposure from hydrogen and helium irradiation, and extremely low tolerances for tungsten impurity emission into the core plasma. However, compared to low Z materials such as carbon and beryllium, tungsten's high melting point, high thermal conductivity, low tritium retention, and low plasma-induced sputter threshold has rendered it one of the best candidate armor material options for the extreme fusion environments encountered in future plasma-burning fusion reactors.This proposal is focused in discovery and fundamental understanding of novel self-healing and adaptive materials for the PMI (plasma-material interface) envisioned for future plasma-burning extreme environments in thermonuclear fusion reactors that can provide enhanced radiation-tolerance or resistance. This proposal stems from the PI's DOE Early Career Award work on harnessing nanotechnology and mesoscale materials design in refractory metals to address gaps in PMI research. These gaps are, in part, linked to the lack of understanding of multi-scale interactions at the plasma-material interface and the limited discovery and development of novel material interfaces that can be designed to adapt to extreme fusion reactor conditions. This proposal supports the DOE FES Burning Plasma Science: Long Pulse—Materials & Fusion Nuclear Science mission by three general objectives: 1) materials development and thermo-mechanical testing to ultrafine dispersion-strengthened tungsten (UDSW), 2) elucidate the hydrogen isotope retention mechanisms in UDSW 3) establishing an understanding of dispersoid microstructure effects on near-surface helium behavior in UDSW.The overall goal of this program is to lay the foundational understanding of advanced materials for the plasma-material interface that enable plasma-burning fusion reactor operation. Development of materials unique to these extreme conditions take extraordinary effort and the goal is to study knowledge-gap hypothesis-driven questions in the context of PMI armor materials that motivate a more extensive development phase. The work in this program combines fundamental and discovery research that establishes yet unknown process-composition-property-function relations of novel tungsten-based plasma-facing armor material interfaces. The proposal provides a unique perspective at the interface of materials science and PMI. In particular 'function' in this context addresses the environment of long-pulsed plasma-burning fusion conditions. PMI studies proposed focus on significant PMI knowledge gaps identified by the PMI fusion community in the recent DOE Department of Energy Fusion Energy Sciences Workshop on Plasma Materials Interactions 2015. These gaps were articulated in research themes that included the need for establishing understanding of the reconstituted surfaces in PMI under reactor-relevant long-pulse prototypical environments. This gap is addressed in this proposal by carefully examining the incubation regimes of irradiation-driven defects and correlating to effects on PMI properties and their evolution over very large fluence (i.e. prototypical long-pulse) regimes. In particular this proposal investigates the characteristic behavior of surface composition and morphology of ultrafine dispersion-strengthened tungsten-based materials developed with spark plasma sintering and the properties intrinsic in UDSW materials to survive these harsh environments.