Integrated model predictions on the impact of substrate damage on gas dynamics during ITER burning-plasma operations

Abstract

Divertor design and choice of plasma-facing materials (PFM) will be essential to the success of next-generation fusion reactors as they operate under more powerful scenarios. Understanding and controlling interactions between the plasma and PFM is essential to making these choices. Within these plasma–material interactions and especially in tungsten (W), the interplay between the most abundant plasma species (hydrogen isotopes and helium, He) with the wall material alters fuel retention. However, this interplay is yet to be sufficiently understood to confidently project fuel retention levels to future fusion devices. The paper presents a series of integrated simulations of fusion plasmas and their interaction with tungsten. Specifically, this study assesses the impact of He plasma pre-exposure on hydrogenic species retention during 100 s of burning plasma operations (BPO) in ITER. Multiple pre-exposure scenarios are considered, including sub-surface damage resulting from exposures in the linear device PISCES and from early ITER He-operation. The predictions from these consecutive He-BPO exposures show that fuel content and spatial distribution in the material are largely determined by the He-induced damage, as manifest in: (i) changes in surface temperature expected during BPO have little effect on fuel retention in the presence of He-induced damage; (ii) gas content stabilizes quickly in substrates pre-exposed in PISCES, at levels set by the concentration of pre-existing vacancies, while it continues to increase in substrates initially pristine or pre-exposed to ITER He plasmas; (iii) the presence of He and He–V clusters in the near-surface region locally increases hydrogenic retention, but decreases its permeation; this results in hydrogenic species that remain closer to the surface in pre-damaged substrates, while the bulk content is higher for initially pristine cases. In summary, the interaction and binding of D and T with the pre-existing He–V clusters modifies retention and permeation of hydrogen species during ITER BPO.

ERO modeling and analysis of tungsten erosion and migration from a toroidally symmetric source in the DIII-D divertor

“…Results presented here also highlight the necessity of consistently modeling plasma conditions, electric fields and resulting E × B drifts inside and outside the sheath to accurately predict impurity transport and subsequent material erosion. Moreover, this work underlines the importance of a comprehensive sheath model in integrated modeling of impurity erosion, transport and redeposition [47] to account for the radial transport of low-Z impurities near divertor surface due to radial E × B drift in the sheath, and to accurately predict tungsten erosion, e.g. in ITER divertor.…”

Impact of Soret effect on hydrogen and helium retention in PFC tungsten under ELM-like conditions

“…Figure 4(b) shows the integrated deuterium content over the thickness of the tungsten slab as a function of time in a similar way as helium in figure 4(a). The same general remarks can be made about the deuterium behavior as for helium, with the difference that the deuterium content is higher than the helium content mainly because of the differences in their respective implantation profiles: for helium implantation, the peak of the implantation profile is located at a depth of 1.6 nm below the surface, while the respective peak is at 2.2 nm below the surface for deuterium [23]. The times to reach 99% of the periodic steady state are estimated to be ∼10 300 s without accounting for the Soret effect and ∼4400 s with the Soret effect accounted for, which are longer than those in the helium case because of the higher overall content for deuterium.…”

“…We found that, for all species examined, their total content over the depth of the tungsten slab simulated, after reaching their steady periodic state, was reduced (on average) by a factor of ∼3. While the actual value of the total content is strongly dependent on gas implantation and neutron irradiation conditions, especially on the location of the peak implantation profile [23], which would vary from one location to another in a divertor wall, the ratio of the species content between the cases with and without accounting for the Soret effect would be similar, under the assumption that transport of hydrogen isotopes is not heavily limited by self-clustering of helium and hydrogen binding to the resulting helium clusters. However, atomistic calculations have shown a strong propensity for helium self-clustering and overpressurized helium bubble formation, and a relatively strong trapping interaction of hydrogen with the helium clusters and bubbles.…”

“…for each species obtained for pristine tungsten through binary collision approximation [22] simulations corresponding to ITER burning plasma operation at peak heat flux in the divertor as described in [23]. In this study, the incident species flux is taken to be constant, as the main focus of this work is to examine the impact of the Soret effect.…”

Modeling tungsten response under helium plasma irradiation: a review

“…Recently, Lasa et al [140,141] reported an integrated model coupling the background plasma transport, the nearsurface sheath effects, the erosion and transport of wall materials across the scrape-off layer (SOF), sputtering and implantation of ions impacting on the material, and the dynamics of the subsurface gas atoms. The workflow of the model is shown in figure 13.…”