(335e) Soret Diffusion of Helium and Intrinsic Point Defects in Tungsten

Plasma-facing materials (PFMs) in nuclear fusion reactors must withstand extreme conditions, with high heat and particle fluxes that modify their microstructure. These fluxes create strong temperature gradients and highly non-equilibrium concentrations of diverse species in the PFM. In addition to the helium (He) ash and hydrogenic species, neutrons generated in the fusion reaction will create intrinsic point defects, namely, vacancies and self-interstitial atoms (SIAs), and their clusters. Additional vacancies and SIAs are generated by trap mutation reactions undergone by helium clusters in the PFM bulk and in regions near the plasma-exposed surfaces. These defects, as well as He atoms and small mobile helium clusters, will then migrate in the presence of concentration and temperature gradients.

In this work, we use non-equilibrium molecular-dynamics (NEMD) simulations to study the transport of He and SIAs in the presence of a thermal gradient in tungsten PFM and discover evidence for Soret diffusion of He and SIAs in tungsten. We find that, in all cases, the intrinsic point defects and impurity atoms tend to migrate toward the hot regions of the material and calculate their concentration profiles in the direction of the temperature gradient. We also analyze thermal and species transport in tungsten within the framework of irreversible thermodynamics, which has been implemented in the cluster-dynamics code Xolotl. The resulting concentration profiles from the NEMD simulations are in agreement with the predictions of irreversible thermodynamics. We compute a negative heat of transport for each species analyzed, which indicates that the respective driven species fluxes are directed opposite to the heat flux. We demonstrate that when Soret diffusion or thermomigration, i.e., drift species transport driven by the thermal gradient, is considered, the resulting steady-state profiles differ significantly from those when species transport is decoupled from heat transport.