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, surface erosion even at relatively high temperatures) 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 materials for the extreme fusion environments (e.g., 0.1-1.0 dpa, > 5 MW/m2) encountered, for example, in the ITER burning plasma.
Given most of the attention in PMI has focused on conventional solid PFCs, there is a critical need for both fundamental investigation and translational development of transformative PFC technologies to meet the demand of reactor-relevant material interfaces facing the plasma. This program builds on the work from Prof. Allain’s DOE Early Career Grant (DE-SC0004032, Harnessing nanotechnology for fusion plasma-material interface research in an in-situ particle-surface interaction facility), which investigated both processing and high heat-flux performance of extreme-refined and multi-modal grained tungsten as a robust class of PMI materials. In this program we leverage this knowledge to investigate novel self-healing and adaptive materials for the PMI (plasma-material interface) that can provide enhanced radiation-tolerance or resistance. The context is focus on the plasma-material interface and the systems studied range from extreme-refined doped refractory metals (for optimum stabilization) to complex nanocomposites including the use of mesoporous metal systems as scaffolds for low-melting point metals (e.g. Li, Sn). We focus on both surface/interface and thermo-mechanical properties under simulated PMI extreme conditions using state-of-the art in-situ characterization.
The program focuses on tungsten-based materials and alternates that are introduced as either a bulk plasma-facing material of few mm thickness materials (i.e. envisioned to be harnessed or coupled to a thicker substrate tile) or a coating with properties that can have a positive effect on the edge plasma (e.g. reduction of erosion, reduction of heat flux deposition, lower Zeff, among others). Coupling of the refractory alloy to low-Z ultrathin films (e.g. lithium, boron, etc.) is explored along with low atomic concentration doping at grain boundaries. In addition, non-traditional materials such as porous and hierarchical structures at the nano and mesoscale as scaffolds for liquid metals are also explored. The materials design approach adopted here will also investigate and establish a new class of materials systems for the PMI. These materials could have either self-healing properties where “self-healing” is defined as materials that freely can repair themselves after damage without any external influence from their environment; or adaptive materials properties. Where “adaptive” is defined as intrinsic properties coupled to an external influence (e.g. radiation stimulant), whereby during exposure to a defined extreme environment the material performance is maintained or improved. This paradigm to PMI materials design is transformative in that the design of the plasma-material interface is closely coupled with its behavior under well-defined magnetic fusion plasma edge conditions and studied with robust in-situ performance characterization to decipher the dynamic material properties as they evolve during plasma exposure and thus help define the processing schemes to design the same. The overall program will be initiated exploring these complex refractory metal-based systems from both a process-structure-property point of view and also for their plasma-material interaction (PMI) functionality. PMI properties studied will include: multi-component thermal, physical and chemical sputtering, surface charge dynamics, ion reflection, ion-driven surface segregation and diffusion, ion-induced mixing, and surface topography.
