Vacancy and Helium-Cluster Thermomigration in Tungsten

Abstract

Strong thermal gradients in tungsten plasma-facing components can bias the migration of irradiation-induced defects and thereby alter their redistribution, interaction, and retention. Here, we extend an atomistic jump-statistics framework previously developed for hydrogen to quantify the thermomigration of a single vacancy and interstitial He$_1$--He$_4$ clusters in body-centered-cubic tungsten. Non-equilibrium molecular-dynamics simulations performed over several thermal gradients are used to determine the directional jump ratio, while geometrically derived projected jump lengths convert the fitted directional-bias coefficient into the heat-of-transport coefficient $Q^*=αkT^2$, where $k$ is the Boltzmann constant, $T$ the temperature, and $α$ a constant. All investigated defects exhibit negative values of $α$, corresponding to net migration toward the hot region. He$_4$ has the largest response, $α=-2.2457\times10^{-2}$ K$^{-1}$, followed closely by the vacancy, whereas He$_3$ exhibits the weakest response despite its intermediate cluster size. Independent constant-temperature simulations over 1000--1600 K show that He$_3$ is also the least mobile cluster and has the largest effective migration energy, $E_m^{\mathrm{eff}}=0.271$ eV. In contrast, He$_4$ is more mobile than He$_3$ and has the smallest effective migration energy, 0.087 eV. Agreement between the He$_3$ concentration profile predicted from the jump statistics and that obtained from a long molecular-dynamics trajectory provides an independent validation of the method. These results show that thermomigration cannot be inferred from cluster size, diffusivity, or migration energy alone and provide defect-specific heat-of-transport parameters for higher-length-scale models of tungsten under fusion-relevant thermal gradients.