Bulletin of the American Physical Society
60th Annual Meeting of the APS Division of Plasma Physics
Volume 63, Number 11
Monday–Friday, November 5–9, 2018; Portland, Oregon
Session PM9: Mini-Conference on Plasma–Material Interactions in Fusion Devices: ITER and Beyond. IV. Hydrogen Retention/Release and Neutron Issues for PMI |
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Chair: Karl Hammond, University of Missouri Room: OCC C123 |
Wednesday, November 7, 2018 2:00PM - 2:20PM |
PM9.00001: Scale-bridging from the Atoms Up; Employing Machine Learning to Improve the Accuracy and Scalability of Molecular Dynamics Mitchell Wood, Mary Alice Cusentino, Aidan P Thompson Simulation methods such as Molecular Dynamics(MD) are powerful tools for examining behaviors of materials that originate on the atomic scale. On modern supercomputers it is possible to simulate many millions of atoms on time scales exceeding one microsecond. Fitting an interatomic potential(IAP) is a critical multiscaling link between electronic structure codes, such as density functional theory and MD that determines the accuracy of these simulation efforts. Our approach to constructing this multiscaling link employs machine learning using the Spectral Neighborhood Analysis Potential(SNAP) to learn the features of a DFT database. This work is focused on the tungsten-beryllium system to better understand the effects of beryllium transport to the tungsten diverter in ITER-like conditions, where accurate predictions of implantation depth profiles and surface behavior will be important.Experimentally, a beryllium-tungsten intermetallic forms under fusion relevant conditions, molecular dynamics simulations of both energetic beryllium implantation as well as beryllium deposition on a tungsten surface were performed to investigate this surface chemistry using this newly developed SNAP potential. |
Wednesday, November 7, 2018 2:20PM - 2:40PM |
PM9.00002: Prediction of Near Surface Gas Bubble Evolution in the ITER Divertor with Cluster Dynamics Sophie Blondel, David E Bernholdt, John Canik, Mark R Cianciosa, Davide Curreli, Jon T Drobny, Wael Elwasif, David L Green, Ane Lasa, Philip C Roth, Tim Younkin, Brian Wirth Plasma surface interactions in fusion tokamak reactors involve an inherently multiscale set of phenomena, for which current models are inadequate to predict the divertor response to and feedback on the plasma. In this presentation, we describe the latest code developments of Xolotl, a spatially-dependent reaction diffusion cluster dynamics code. Xolotl is part of a code-coupling effort to model both plasma and material simultaneously, including SOLPS for simulations of the edge plasma in steady-state conditions; the effect of the sheath at shallow magnetic angles, evaluated by hPIC; GITR calculations of migration and redeposition of impurities eroded from the surface; and the response of the wall surface to these plasma conditions modeled by coupling F-TRIDYN and Xolotl. The latter has been extended from helium only to mixed hydrogen-helium plasma, increasing the computational complexity due to the large number of clusters to model and requiring optimization. A simplified helium bubble bursting model is included to take into account the gas release happening when a bubble is near the surface in order to predict more realistic surface evolution and sub-surface composition under ITER conditions. Results from a range of locations in the divertor will be presented. |
Wednesday, November 7, 2018 2:40PM - 3:00PM |
PM9.00003: H Binding Energetics at Near-Surface He-V clusters and Bubbles in Tungsten Zachary Bergstrom, Li Yang, Brian Wirth We describe results of density functional theory calculations to investigate the interaction and binding energies of H with a wide range of He-V clusters as a function of depth below W surfaces of varying orientation. The results show that the behavior of H below W can be divided into three distinct regions, namely a strong influence region, a transition region and as the depth below the surface increases, the H cluster configurations with lowest energy are similar to those found in bulk W. Additionally, we have evaluated the H trapping energetics at planar He-W interfaces to assess the influence of helium density and surface orientation on de-trapping energies. Our results provide fundamental insight into the energetics of H and He that control the surface morphology and tritium retention behavior in W exposed to low-energy plasma conditions. |
Wednesday, November 7, 2018 3:00PM - 3:20PM |
PM9.00004: Evaluating the Impact of Helium Bubble Layers on Hydrogen Diffusion David Martin, Mary Alice Cusentino, Brian Wirth We describe a qualitative and quantitative assessment of hydrogen permeation in thin tungsten slabs with a (100) surface morphology using molecular dynamics. The simulations have been performed with and without low-energy 100 eV helium implantation at a flux of 4x1025 He m-2s-1 to a fluence of ~4x1019He m-2, and have involved either 60 eV H implantation or introduction of a concentrated hydrogen layer at ~10 nm below the surface of the W slab. The simulations reveal that H diffusion is reduced by the presence of the He nano-bubble layer, and that H is observed to segregate to, and trap at, the interface of the high pressure helium bubbles with the tungsten matrix. These results provide atomistic insight into the mechanisms controlled the reduced permeation and retention of hydrogen during mixed He-H plasma exposure. |
Wednesday, November 7, 2018 3:20PM - 3:40PM |
PM9.00005: Theoretical study of hydrogen desorption from tungsten in divertor conditions Jerome Guterl, Sergei Krasheninnikov, Igor Bykov, Stefan A Bringuier, Philip B Snyder Using a recently developed W-H interatomic potential [1], H thermal desorption from W surface is simulated for various material temperature T=1000K-1700K and low H surface coverage. It is observed that both atomic and molecular H desorption occurs at similar rates, and are of second order with respect to H surface coverage. Both desorption processes follow a H precursor state [2] induced by H-H short-range interactions on W surface. The second order nature of atomic desorption results from such precursor state. H desorption induced by incoming H ions may be comparable to thermal desorption when W surface is saturated with H. Ion-induced H desorption is characterized by simulating H particles impinging onto a fully covered W surface. At low incident energy <1eV, impinging H either deposit onto W surface, or induce molecular desorption. In contrast, impinging H at higher incident energy ~10eV induce atomic desorption of one or two separate H atoms. [1] Wang, Journal of Physics: Condensed Matter (2017) [2] Cassuto, A. Surface Science (1981) |
Wednesday, November 7, 2018 3:40PM - 4:00PM |
PM9.00006: Stress induced hydrogen self-trapping in tungsten Roman Smirnov, Sergei Krasheninnikov It has been observed experimentally that hydrogen concentrations as high as ~1 at.% can be retained in surface layer of plasma exposed tungsten samples. However, mechanism of retention of such high amounts of hydrogen is not understood. We present molecular dynamics simulations that demonstrate formation of self-trapped hydrogen platelets in tungsten induced by stresses, in particular, those produced by dislocations. We also show that hydrogen self-trapping in tungsten can occur spontaneously at high hydrogen concentrations ~10 at.%. The platelets are capable to retain large quantities of hydrogen, exceeding trapping capacity of other non-cavity defects in tungsten by orders of magnitude. The formation mechanism and properties of the hydrogen platelets formed in tungsten are described. The hydrogen de-trapping energy from the platelets is also obtained. Based on the simulation findings, we present a model of hydrogen retention via the dislocation-induced self-trapping, which describes retained quantities and dynamics of hydrogen in plasma exposed tungsten. |
Wednesday, November 7, 2018 4:00PM - 4:20PM |
PM9.00007: Simulation of the Effect of Hydrogen Repulsion on Clustering and Retention in Tungsten Brandon Laufer, Karl D Hammond Prior studies have shown that helium plasma exposure simulations with molecular dynamics can be sped up using direct implantation of helium for energies well below the sputtering threshold, which eliminates the time necessary to simulate atoms that reflect off the surface. In this study, we attempt to use the same approximation for hydrogen. However, because hydrogen atoms in tungsten are generally repelled from each other, and hydrogen is therefore not self-clustering, we also conducted several simulations in which hydrogen directly bombards the surface for comparison. The bombardment and implantation simulations significantly differed, even at short times. These differences include deeper penetration of hydrogen into tungsten in the bombardment case. We also observed that hydrogen trapped beneath the surface will, if it diffuses to the surface, generally have enough energy to desorb without forming an adsorbed layer, but that hydrogen directly impacting on the surface can be captured and adsorb on the surface. These observations suggest not only that hydrogen behavior in tungsten is very different than that of helium, but that hydrogen may require much more time and effort to achieve realistic, tractable models of hydrogen retention and diffusion in plasma-facing materials. |
Wednesday, November 7, 2018 4:20PM - 4:40PM |
PM9.00008: Effect of Trapping/Detrapping of SIA Clusters by Impurities on Damage Accumulation in Neutron Irradiated Tungsten: Comparison of 14 MeV-Neutron and High-Flux Isotope Reactor (HFIR) PKA Spectra Giridhar Nandipati, Wahyu Setyawan, Kenneth J Roche, Richard J Kurtz, Brian Wirth Using KSOME, simulation of radiation damage accumulation in neutron-irradiated tungsten at 1025 K was investigated to understand the influence of SIA cluster trapping and detrapping at impurities on the damage as a function of dose, dose rate, SIA detrapping activation barrier, impurity concentration and PKA spectrum. Impurities trap vacancies permanently while SIA clusters can detrap by overcoming an activation barrier. Since impurities can lower the rotation barrier of 1D diffusing SIA clusters, two cases of the migration dimensionality of detrapped SIA clusters were studied, one where they diffuse in a random 1D direction or alternately, where they retain their original 1D direction if larger than size 5. In the former case, SIA clusters effectively perform 3D diffusion. However, interestingly, the damage was lower for 1D diffusion. A detailed comparison of damage accumulation between two different PKA spectra, with respect to differences in defect production rates and impurity concentration will be presented. Finally, a comparison with experimental results, where applicable, is discussed. |
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