S6-3
Surface damage of tungsten by heat and particle loadings
タングステンの表面損傷
Y. Ueda上田良夫
Graduate School of Engineering, University of Osaka, Yamadaoka 2-1 , Suita, Osaka 565-0871, Jpan
大阪大学工学研究科 〒565-0871 大阪府吹田市山田丘2-1

In this presentation, present knowledge and issues of tungsten surface damage caused by transient heatpulses and plasma (hydrogen isotopes and helium) loadings are reviewed. Since tungsten materials arebrittle, surface damage could also cause large scale cracks penetrating throughout tungsten blocks.
Surface damage could cause enhanced erosion and dust formation, which need to be limited below
acceptable levels in terms of W material lifetime, safety and core plasma performance.

Tungsten is foreseen as a plasma facing material for
ITER and DEMO because of its low sputtering yield,
high melting point and high thermal conductivity. But thesurface damage of tungsten caused by transient heatloadings unmitigated disruption/VDE with high stored
energy plasmas as well as by unmitigated ELMs are ofgreat concerns for enhanced erosion and impact of coreplasma performance. In addition, hydrogen isotopes andhelium mixed plasmas would also affect surface
morphology changes such as blisters and He inducedstructure.
In terms of transient heat loadings by major
disruptions (MD), vertical displacement events (VDE)and edge localized modes (ELM) could be applied to thedivertor surfaces (mainly MD and ELM) and the baffle
surfaces (VDE). Although there still remain large
uncertainties in estimation of deposition energies, surface
melting of tungsten might occur without mitigation. Meltlayer motion driven plasma pressure and JxB forcescould cause surface unevenness which enhances furthermelting and ejection of droplets, which could be serious
concerns for core plasma contamination and mobilizing
tungsten dusts. For DEMO reactors, significant meltingof tungsten must be avoided due to these reasons. In
order to avoid melting of tungsten, appropriate measuresfor disruption and ELM mitigation, which will be themost important subjects for ITER using full W divertor
from day one.
Even with success of mitigation of transient heatloadings, thermal fatigue by high cycle transients causedby mitigated ELM’s is another concern. High cycle heatpulses with pulse energy density even much below amelting threshold (~1MJ/m2 for ~0.5 ms pulse length)
might bring about surface cracking, local melting anddroplet and grain ejection. We still do not know how
these processes have impacts on stable operations of
DEMO reactors. Surface protection and/or surface
repair/refurbishment might be necessary.
Particle loadings (hydrogen isotopes and helium), in
general, cause thin damaged layers containing trappedand solute atoms whose mechanical and thermal
properties are degraded by solution of these atoms,nanometric bubbles (by both hydrogen and helium),blistering (mainly by hydrogen in fusion environments at
low temperatures), large holes (by helium at elevatedtemperatures), and nanostructure (W fuzz, by helium atmedium temperatures). Hydrogen blistering could not be
an issue in a fusion environment due to mainly
engineering surfaces of W monoblocks and plasma
impurities (Be and He). W fuzz could bring aboutenhanced release of tungsten through unipolar arcing, butmore studies on the formation, erosion, anddisappearance of W fuzz by pulsed heat are necessary tocorrectly evaluate this effect on core plasma performance[1].
Combined effects of heat and particles could enhancesurface damage and erosion by pulsed heat only in somecases. But some other experimental results showed
alleviation of surface damage (e.g. W fuzz and He holes)
by ELM-like pulsed heat. Therefore, there still remainsome uncertainties of surface morphology changes bycombined loadings. Although transient heat loadings
should be reduced to some acceptable level, effects ofoccasional modest transients on surface damage
alleviation should also be examined [1].

Reference[1] Y. Ueda et al., “Research status and issues oftungsten plasma facing materials for ITER and beyond”,
Fusion Engineering and Design, Vol. 89, October 2014,
Pages 901–906.