Plasma-facing materials for fusion energy: Historical review, challenges and future directions

The promise of fusion energy is real. Major countries worldwide aim to achieve commercial grid-connected fusion power generation by around 2050, with magnetic confinement fusion—represented by tokamaks—leading the way among various fusion approaches. However, identifying suitable plasma-facing materials (PFMs) for the fusion core may be the greatest challenge on the road to commercial fusion, serving as a bottleneck that constrains the safety, steady-state operation, lifetime, and cost of fusion devices. PFMs are challenged by heat fluxes up to 20 MW·m⁻², plasma exposure with temperatures of hundreds of millions of K and neutron irradiation of 14.1 MeV. This places extremely stringent demands on the comprehensive properties of PFMs, including thermomechanical properties, corrosion resistance, radiation resistance. No single material has been able to simultaneously meet all the requirements for ideal PFMs. In this paper, an overview of the development history of PFMs in tokamaks is provided. Stainless steel was used as PFM in early fusion devices and was abandoned due to their low melting points and the contamination of the plasma by high-Z impurities generated by sputtering. Then, the PFM community shifted to low-Z materials such as carbon and beryllium that are compatible with plasma. These low-Z PFMs were phased out mainly due to their high erosion and high tritium retention later. The PFMs community revisited the high-Z materials of high melting points and high sputtering threshold, leading to tungsten as the reference material. The implementation of PFMs has undergone a transition from high-Z to low-Z and back to high-Z materials, reflecting the dialectical principle of negation of negation in epistemology. This evolution represents progress in comprehensive understanding of material properties, plasma control, and fusion device design of the fusion community, laying a solid foundation and instilling confidence in the realization of fusion energy’s promise. This paper also introduces potential candidate materials for PFMs in future fusion devices primarily including refractory metals and ultra-high temperature ceramics and discusses their advantages and disadvantages as PFMs. Challenges and perspectives on the future development of PFMs are presented at the final section.