The influence of plasma-surface interaction on the performance of tungsten at the ITER divertor vertical targets

Abstract

The tungsten (W) material in the high heat flux regions of the ITER divertor will be exposed to high fluxes of low-energy particles (e.g. H, D, T, He, Ne and/or N). Combined with long-pulse operations, this implies fluences well in excess of the highest values reached in today's tokamak experiments. Shaping of the individual monoblock top surface and tilting of the vertical targets for leading-edge protection lead to an increased surface heat flux, and thus increased surface temperature and a reduced margin to remain below the temperature at which recrystallization and grain growth begin. Significant morphology changes are known to occur on W after exposure to high fluences of low-energy particles, be it H or He. An analysis of the formation conditions of these morphology changes is made in relation to the conditions expected at the vertical targets during different phases of operations. It is concluded that both H and He-related effects can occur in ITER. In particular, the case of He-induced nanostructure (also known as 'fuzz') is reviewed. Fuzz formation appears possible over a limited region of the outer vertical target, the inner target being generally a net Be deposition area. A simple analysis of the fuzz growth rate including the effect of edge-localized modes (ELMs) and the reduced thermal conductivity of fuzz shows that the fuzz thickness is likely to be limited by the occurrence of annealing during ELM-induced thermal excursions. Not only the morphology, but the material mechanical and thermal properties can be modified by plasma exposure. A review of the existing literature is made, but the existing data are insufficient to conclude quantitatively on the importance and extent of these effects for ITER. As a consequence of the high surface temperatures in ITER, W recrystallization is an important effect to consider, since it leads to a decrease in material strength. An approach is proposed here to develop an operational budget for the W material, i.e. the time the divertor material can be operated at a given temperature before a significant fraction of the material is recrystallized. In general, while it is clear that significant surface damage can occur during ITER operations, the tolerable level of damage in terms of plasma operations currently remains unknown.

Surface modification of W–V alloy exposed to high heat flux helium neutral beams

“…Blistering and lids exfoliation are shown on the surface of W-5V after exposed to repeated He pulses at a low peak surface temperature of 900 °C, while porous structure and coral-structure are dominant at a high peak surface temperature of 1800 °C. He bubble growth and rupture are the main factors of He-induced surface modification [20,21]. The effect of surface temperature and He fluence on He bubble evolution and surface modification is discussed.…”

Impact of ELM control techniques on tungsten sputtering in the DIII-D divertor and extrapolations to ITER

“…ITER will operate with a partially detached divertor with relatively cold electron temperatures (< 5 eV) at the strike point to mitigate W sputtering [8]. The electron temperature may increase moving radially away from the divertor targets, but this region is expected to be coated with beryllium deposits, shielding the W surface from erosion [9].…”

Motion of W and He atoms during formation of W fuzz

“…Perhaps the most well studied system is the tungsten-helium system, due to the relevance of tungsten as a plasma-facing material for proposed fusion confinement devices [2]. A recent review documents in detail the changes that can occur during exposure of W to various plasma conditions, including He [3]. Some of the most consequential changes that can occur during He plasma exposure are a change in the surface thermal conductivity making it more difficult to handle the extreme heat flux associated with controlled fusion confinement devices, embrittlement of the surface due to the presence of He atoms making transient heat loads more likely to result in surface cracking and morphology changes to the surface.…”