(323d) Impact of Soret Effect on Hydrogen and Helium Retention in Plasma-Facing Tungsten Under ELM-like Conditions

Plasma-facing components (PFCs) in nuclear fusion reactors are expected to be exposed to extremely high particle fluxes and heat loads that modify the PFC materials microstructure. These fluxes will create strong gradients of temperature and concentration of diverse species, including He and H atoms and small mobile helium clusters. In addition to these impurity species, intrinsic defects, such as self-interstitial atoms (SIAs), also are generated in the PFC near-surface region. The aforementioned thermal and concentration gradients will strongly affect the migration of these defects and impurity species in the PFC material.

To fundamentally understand such effects, we have used nonequilibrium molecular-dynamics (NEMD) simulations to analyze the transport of He, mobile helium clusters, H, and SIAs in the presence of a thermal gradient in tungsten, the chosen PFC material for the divertor of ITER (International Thermonuclear Experimental Reactor). We have found that all the species examined tend to migrate toward the hot regions of the tungsten sample [1]. The resulting species concentration profiles are exponential distributions in the direction of the imposed temperature gradient, rising toward the hot regions of the sample, in agreement with irreversible thermodynamics analysis [1]. For all the species examined, intrinsic point defects and impurities, we have found that the resulting species flux is directed opposite to the heat flux, indicating that species transport is governed by a Soret effect, namely, thermal-gradient-driven diffusion, characterized by a negative heat of transport that drives species transport uphill, i.e., from the cooler to the hot regions of the tungsten sample [1,2].

The findings of our thermal and species transport analysis have been implemented in our cluster-dynamics code, Xolotl, which has been used to compute temperature and species profiles over spatiotemporal scales representative of PFC tungsten under typical reactor operating conditions, including extreme heat loads at the plasma-facing surface characteristic of plasma instabilities that induce edge localized modes (ELMs) [2]. We demonstrate that the steady-state species profiles, when properly accounting for the Soret effect, vary significantly from those where temperature-gradient-driven transport is not accounted for and discuss the implications of such a Soret effect on the response to plasma exposure of plasma-facing tungsten. Although our cluster-dynamics simulations do not yet include self-clustering of helium or hydrogen blister formation, our simulation results show that the Soret effect substantially reduces helium and hydrogenic species retention inside PFC tungsten [2].

[1] E. Martinez, N. Mathew, D. Perez, S. Blondel, D. Dasgupta, B. D. Wirth, and D. Maroudas, J. Appl. Phys. 130, 215904 (2021).

[2] D. Dasgupta, S. Blondel, E. Martinez, D. Maroudas, and B. D. Wirth, Nucl. Fusion, under review (2023).