Maroudas Dimitrios, Blondel Sophie, Hu Lin, Hammond Karl D, Wirth Brian D
Department of Chemical Engineering, University of Massachusetts, Amherst, MA 01003-9303, USA.
J Phys Condens Matter. 2016 Feb 17;28(6):064004. doi: 10.1088/0953-8984/28/6/064004. Epub 2016 Jan 21.
We report a hierarchical multi-scale modeling study of implanted helium segregation on surfaces of tungsten, considered as a plasma facing component in nuclear fusion reactors. We employ a hierarchy of atomic-scale simulations based on a reliable interatomic interaction potential, including molecular-statics simulations to understand the origin of helium surface segregation, targeted molecular-dynamics (MD) simulations of near-surface cluster reactions, and large-scale MD simulations of implanted helium evolution in plasma-exposed tungsten. We find that small, mobile He n (1⩽ n ⩽ 7) clusters in the near-surface region are attracted to the surface due to an elastic interaction force that provides the thermodynamic driving force for surface segregation. This elastic interaction force induces drift fluxes of these mobile He n clusters, which increase substantially as the migrating clusters approach the surface, facilitating helium segregation on the surface. Moreover, the clusters' drift toward the surface enables cluster reactions, most importantly trap mutation, in the near-surface region at rates much higher than in the bulk material. These near-surface cluster dynamics have significant effects on the surface morphology, near-surface defect structures, and the amount of helium retained in the material upon plasma exposure. We integrate the findings of such atomic-scale simulations into a properly parameterized and validated spatially dependent, continuum-scale reaction-diffusion cluster dynamics model, capable of predicting implanted helium evolution, surface segregation, and its near-surface effects in tungsten. This cluster-dynamics model sets the stage for development of fully atomistically informed coarse-grained models for computationally efficient simulation predictions of helium surface segregation, as well as helium retention and surface morphological evolution, toward optimal design of plasma facing components.
我们报告了一项关于钨表面植入氦元素偏析的分层多尺度建模研究，钨被视为核聚变反应堆中的等离子体面对部件。我们基于可靠的原子间相互作用势进行了一系列原子尺度模拟，包括用于理解氦表面偏析起源的分子静态模拟、近表面团簇反应的靶向分子动力学（MD）模拟以及等离子体暴露钨中植入氦元素演化的大规模MD模拟。我们发现，近表面区域中较小的、可移动的He_n（1⩽ n ⩽ 7）团簇由于弹性相互作用力而被吸引到表面，该力为表面偏析提供了热力学驱动力。这种弹性相互作用力诱导了这些可移动He_n团簇的漂移通量，随着迁移团簇接近表面，漂移通量显著增加，促进了氦在表面的偏析。此外，团簇向表面的漂移使得近表面区域能够以比体材料中高得多的速率发生团簇反应，最重要的是陷阱突变。这些近表面团簇动力学对表面形态、近表面缺陷结构以及等离子体暴露后材料中保留的氦量有显著影响。我们将此类原子尺度模拟的结果整合到一个经过适当参数化和验证的空间相关连续尺度反应扩散团簇动力学模型中，该模型能够预测钨中植入氦元素的演化、表面偏析及其近表面效应。这种团簇动力学模型为开发完全基于原子信息的粗粒度模型奠定了基础，以便对氦表面偏析、氦保留和表面形态演化进行计算高效的模拟预测，从而实现等离子体面对部件的优化设计。
