Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Plasma Physics
Volume 66, Number 13
Monday–Friday, November 8–12, 2021; Pittsburgh, PA
Session TM09: Mini-Conference: Modeling of Material Evolution During Low-Energy H and He Plasma ExposureOn Demand
|
Hide Abstracts |
Chair: Jean Paul Allain, University of Illinois at Urbana-Champaign Room: Rooms 403-405 |
Thursday, November 11, 2021 9:30AM - 10:00AM |
TM09.00001: Continuum-scale Modeling of Surface Morphological Response of Plasma-Facing Tungsten Dwaipayan Dasgupta, Chao-Shou Chen, Brian D Wirth, Dimitrios Maroudas Under fusion reactor operating conditions, implantation of helium ions is responsible for the formation of a fragile fiber-like crystalline nanostructure on the plasma-facing component (PFC) tungsten surface, known as “fuzz”, with detrimental effects on the reactor performance. Numerous atomistic simulation studies have provided insights into this complex surface nanostructure formation. However, such atomic-scale studies cannot access the spatiotemporal scales of micrometers and hours relevant to fuzz formation. |
Thursday, November 11, 2021 10:00AM - 10:20AM |
TM09.00002: Mechanical Properties of Plasma-Exposed Tungsten Asanka T Weerasinghe, Brian D Wirth, Dimitrios Maroudas Based on molecular-dynamics simulations using properly parametrized interatomic potentials and recently developed machine learning potentials, we examine systematically the effects of plasma exposure related defects on the mechanical properties of plasma-facing component (PFC) tungsten. Our simulations reveal that empty voids are centers of dilatation resulting in the development of tensile stress in the tungsten matrix, whereas He-filled voids (nanobubbles) introduce compressive stress in PFC tungsten. We find that the elastic moduli of PFC tungsten, namely, the bulk, Young, and shear moduli, soften substantially as a function of He content in the tungsten matrix, in addition to the softening caused in the matrix with increasing temperature. Moreover, we establish exponential scaling relations for the elastic moduli of PFC tungsten as a function of its porosity and He content. We also find that He bubble growth significantly affects both the bulk modulus and the Poisson ratio of PFC tungsten, while its effect on the Young and shear moduli of the plasma-exposed material is weak. Beyond the elastic regime, our simulations reveal that the presence of voids reduces the ultimate tensile strength (UTS) of tungsten, which is a monotonically decreasing function of porosity. Introducing He into the voids to form He nanobubbles causes further systematic decrease of the tungsten UTS with increasing He content. The significant impact of He bubble formation and growth on the fracture mechanics and structural response of PFC tungsten also is characterized in detail. |
Thursday, November 11, 2021 10:20AM - 10:40AM |
TM09.00003: Pressure-based Helium Bubble Bursting Modeling Wahyu Setyawan, Giridhar Nandipati, Dwaipayan Dasgupta, Dimitrios Maroudas, Karl D Hammond, Sophie Blondel, Brian D Wirth Molecular dynamics is employed to simulate the growth of a helium bubble in tungsten sub-surfaces at until the bubble bursts. Simulations are performed at 933 K for W{100} and W{110} surface orientations. During the growth, a bubble moves towards the surface after punching a loop when the loop can glide towards the surface. For a given initial depth and surface orientation, a bubble must reach a critical size and gas density to burst. The critical size increases with depth. A bubble bursts more easily in W{100} than W{110}, presumably due to the more number of loop glide directions towards the surface in W{100}. Additionally, targeted simulations are performed at higher temperatures up to 2500 K. An equation-of-state is employed to model bubble pressure as a function of gas density and temperature. Subsequently, the data are used to develop a pressure-based bubble bursting model for kinetic Monte Carlo and cluster dynamics simulations. |
Thursday, November 11, 2021 10:40AM - 11:00AM |
TM09.00004: Improving the Bubble Bursting Model in Cluster Dynamics Simulations of Low-Energy Helium Plasma-Exposed Tungsten Sophie Blondel, Dwaipayan Dasgupta, Yogendra S Panchal, Brian D Wirth In this presentation, we describe a modified bubble bursting model which is implemented in Xolotl, a spatially-dependent reaction-diffusion cluster dynamics code to simulate the divertor surface response to fusion-relevant plasma exposure. In the base model, the helium release from over-pressurized bubbles is accounted as a stochastic event, which is dependent on the size of a given helium bubble and its distance from the free surface (Xolotl does not explicitly compute the pressure within a bubble). The predicted helium retention behavior agrees with observations from high flux molecular dynamics (MD) simulations, although the size and depth distributions of the bursting bubbles vary significantly. We investigate the fidelity of the modified model in which the partially random bursting events are replaced with a set of equivalent reactions with rates formulated to depend on size and distance in the same way as in the base model. This new implementation provides a performance gain because, unlike the base model, the time step of the solver does not need to be refined after each disruptive bursting event. We also propose improvements to the bursting model based on more extensive comparisons against MD data to provide a proxy for the bubble pressure in Xolotl. |
Thursday, November 11, 2021 11:00AM - 11:30AM |
TM09.00005: Simulations of Hydrogen and Helium Near Grain Boundaries in Plasma-Facing Tungsten Brandon S Laufer, Karl D Hammond We conduct molecular dynamics simulations of hydrogen and helium transport in plasma-facing tungsten in the presence of grain boundaries. These simulations serve as benchmarks for simulations based on continuum mechanics or kinetic Monte Carlo and also reveal and explore mechanisms of gas transport and surface evolution in plasma-facing materials. Helium and hydrogen are both strongly attracted to and immobilized on the grain boundaries, and grain boundaries serve as barriers that generally prevent hydrogen or helium from crossing them for times up to hundreds of nanoseconds. As it does in single-crystal systems, helium forms bubbles beneath the surface, but the attractive forces near the grain boundaries draw helium to them, sweeping out a helium-depleted region several nanometers in each direction from the plane of the grain boundary. Hydrogen, which does not form bubbles in the bulk without defects present, also collects along the grain boundary, and the regions on either side of the grain boundary are also depleted of hydrogen. We see little evidence, even at half a microsecond (fluences on the order of 1020 m−2 s−1), of any phenomena that would lead to hydrogen blister formation, though we cannot rule out the possibility that it will occur at much later times. |
Thursday, November 11, 2021 11:30AM - 11:50AM |
TM09.00006: The Mobility of Small, High-pressure Helium Bubbles in Tungsten via Frenkel Pair Nucleation and Annihilation Zachary J Bergstrom, Enrique Martinez Saez, Danny Perez Fusion reactor environments invariably lead to the formation of helium bubbles for which the nucleation, growth, and diffusion strongly impact plasma-facing component performance. This research characterizes a diffusion mechanism of small, over-pressurized bubbles in tungsten, which proceeds via successive Frenkel pair nucleation and annihilation, as a function of helium content, vacancy cluster size, and number of self-interstitials. Molecular dynamics was used to directly simulate the diffusion of bubbles at high-temperature with helium-per-vacancy ratios of 4.5-7. It is found that bubbles are most mobile when the Frenkel pair nucleation/annihilation rates are nearly equal and when the bubbles nucleate and annihilate a single self-interstitial. Peak diffusivity can be as high at 10-11 m2s-1, and decreases with size and temperature. The kinetics and energetics of bubble Frenkel pair nucleation/annihilation are described and a model for bubble diffusivity is developed which produces diffusion coefficients in good agreement with those calculated from molecular dynamics. These results provide valuable insight into the mobility of helium bubbles in plasma-facing components and input for higher-order modeling of the near-surface region of materials during plasma-surface interactions (LA-UR-20-30368). |
Thursday, November 11, 2021 11:50AM - 12:10PM |
TM09.00007: Effects of Displacement Cascades on Helium and Hydrogen Near Grain Boundaries in Tungsten Brandon S Laufer, Kenneth A Distefano, Megan C Herrington, Karl D Hammond We conduct molecular dynamics (MD) simulations of displacement cascades consistent with secondary neutron collisions to study the effect of neutron damage on hydrogen and helium in plasma-facing tungsten. We focus on 10–30 keV cascades—energies below the threshold at which electron–phonon coupling is particularly important—that result from primary knock-on atom (PKA) trajectories that interact with helium or hydrogen atoms. In the case of helium, the size and shape of the helium bubbles near the area of peak damage can change substantially, changing the local stress and occasionally resulting in the bubble bursting and venting of the bubble's helium content back to the plasma. For hydrogen, we focused on bicrystals in which the grain boundary is parallel to the surface—this results in a layer of hydrogen forming along the grain boundary. We then introduced PKA trajectories within 30 degrees of the grain boundary plane. The overall distribution of hydrogen is not significantly different before and after the cascade: though some PKA trajectories imparted enough energy to desorb hydrogen atoms from the surface, very few of them significantly altered the distribution of hydrogen beneath the surface, particularly the distribution on the grain boundary. Our results suggest that helium bubbles are likely to deform, coalesce, or burst in the presence of neutrons, and that hydrogen near grain boundaries—prior to the formation of blisters—may be relatively unaffected by the energy and damage from secondary neutron damage cascades. |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700