Behavior of tungsten under irradiation and plasma interaction

Tungsten, the primary material under consideration as the divertor material in magnetic-confinement nuclear fusion reactors, has been known for the last decade to form ‘fuzz’—a layer of microscopic, high-void-fraction features on the surface—after only a few hours of exposure to helium plasma. Fuzz has also been observed in molybdenum, tantalum, and several other metals. Helium bubbles in tungsten and other metals are also known to change the hardness of the surface, accumulate at grain boundaries and dislocations, and increase hydrogen isotope retention. This article reviews helium- and hydrogen-induced surface evolution, including fuzz formation, in tungsten and other plasma-facing materials, as well as modeling and experimental efforts that have been undertaken to understand the mechanisms of fuzz formation, helium and hydrogen transport in plasma-facing materials, and relevant atomic-scale and electronic effects relevant to plasma-facing materials.

https://iopscience.iop.org/article/10.1088/2053-1591/aa8c22/pdf

Helium, hydrogen, and fuzz in plasma-facing materials

This review of the potential of robust plasma facing components (PFCs) with liquid surfaces for applications in future D/T fusion device summarizes the critical issues for liquid surfaces and research being done worldwide in confinement facilities, and supporting R&D in plasma surface interactions. In the paper are a set of questions and related criteria by which we will judge the progress and readiness of liquid surface PFCs. Part-II (separate paper) will cover R&D on the technology-oriented aspects of liquid surfaces including the liquid surfaces as integrated first walls in tritium breeding blankets, tritium retention and recovery, and safety.

Liquid surfaces for fusion plasma facing components—A critical review. Part I: Physics and PSI

The development of tritium breeder, neutron multiplier and flow channel insert materials for the breeding blanket of the DEMO reactor is reviewed. Present emphasis is on the ITER test blanket module (TBM); lithium metatitanate (Li2TiO3) and lithium orthosilicate (Li4SiO4) pebbles have been developed by leading TBM parties. Beryllium pebbles have been selected as the neutron multiplier. Good progress has been made in their fabrication; however, verification of the design by experiments is in the planning stage. Irradiation data are also limited, but the decrease in thermal conductivity of beryllium due to irradiation followed by swelling is a concern. Tests at ITER are regarded as a major milestone. For the DEMO reactor, improvement of the breeder has been attempted to obtain a higher lithium content, and Be12Ti and other beryllide intermetallic compounds that have superior chemical stability have been studied. LiPb eutectic has been considered as a DEMO blanket in the liquid breeder option and is used as a coolant to achieve a higher outlet temperature; a SiC flow channel insert is used to prevent magnetohydrodynamic pressure drop and corrosion. A significant technical gap between ITER TBM and DEMO is recognized, and the world fusion community is working on ITER TBM and DEMO blanket development in parallel.

https://iopscience.iop.org/article/10.1088/1741-4326/aa7e4e/pdf

Functional materials for breeding blankets—status and developments

A review of the LIBS analysis for the plasma-facing components diagnostics

Group 5 elements (niobium and tantalum) and group 6 elements (molybdenum and tungsten) were exposed to helium plasma, and the resulting surface structures were observed by electron microscopy. Group 5 elements showed hole structures, where the size of the holes ranged from several tens of nm to a few hundred nm in diameter, while group 6 elements showed fiber-like structures. As a first step in understanding such differences, the difference in helium agglomeration energies and changes in the stress tensor as a function of the number of He atoms at interstitial sites were investigated for each element using density functional theory. The calculations revealed that helium atoms prefer to agglomerate in both of these groups. However, helium in group 6 elements can agglomerate more easily than group 5 elements due to higher binding energy. These results indicate a possible correlation between the shape of helium plasma induced surface nanostructures and the atomic level properties due to helium agglomeration.

Comparison between helium plasma induced surface structures in group 5 (Nb, Ta) and group 6 elements (Mo, W)

Systematic study of He induced nano-fiber formation of W and other period 6 transition metals

Energy deposition and melt deformation on the ITER first wall due to disruptions and vertical displacement events

Helium plasma irradiation to various metals leads to peculiar morphology changes as forming fiberform nanostructures called fuzz when a certain set of conditions is satisfied. In this Tutorial, we overview experimental observations about the fuzz growth process, conditions, growth rate, etc. on tungsten (W), and fuzz growth observed on other various metals. The fuzz growth mechanism, open questions about fuzz growth, and current understandings on these issues are discussed based on simulation and theoretical works. Several applications (gas sensors and photocatalytic application) with fuzz are shown. We hope that this Tutorial will help new researchers in this field to perform experiments and simulations to reveal new perspectives of fuzz.

Growth of fiberform nanostructures on metal surfaces by helium plasma irradiation

Design studies of innovatively small fusion reactor based on biomass-fusion hybrid concept: GNOME

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