Bulletin of the American Physical Society
55th Annual Meeting of the APS Division of Plasma Physics
Volume 58, Number 16
Monday–Friday, November 11–15, 2013; Denver, Colorado
Session NM9: Mini-Conference: Plasma Material Interactions Involving Helium (SciDAC and Beyond) II |
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Chair: Sergei Krashenninikov University of California, San Diego, and Brian Wirth, University of Tennessee, Knoxville Room: Governor's Square 16 |
Wednesday, November 13, 2013 9:30AM - 9:50AM |
NM9.00001: Multiscale Coupling of Monte Carlo Binary-Collision-Approximation Codes with Particle-in-Cells for Plasma-Material Interaction Davide Curreli, Kyle Lindquist, David N. Ruzic Techniques based on Monte Carlo Binary Collision Approximation (BCA) are widely used for the evaluation of particle interactions with matter, but rarely coupled with a consistent kinetic plasma solver like a Particle-in-Cell. The TRIM code [Eckstein; Biersack and Haggmark, 1980] and its version including dynamic-composition TRIDYN [Moller and Eckstein, 1984] are two popular implementations of BCA, where single-particle projectiles interact with a target of amorphous material according to the classical Carbon-Krypton interaction potential. The effect of surface roughness can be included as well, thanks to the Fractal-TRIM method [Ruzic and Chiu, 1989]. In the present study we couple BCA codes with Particles-in-Cells. The Lagrangian treatment of particle motion usually implemented in PiC codes suggests a natural coupling of PiC's with BCA's, even if a number of caveats has to be taken into account, related to the discrete nature of computational particles, to the difference between the two approaches and most important to the multiple spatial and temporal scales involved. The break down of BCA at low energies (unless the projectiles are channeling through an oriented crystal layer [Hobler and Betz, 2001]) has been supplemented by Yamamura's semi-empirical relations. [Preview Abstract] |
Wednesday, November 13, 2013 9:50AM - 10:10AM |
NM9.00002: Object Kinetic Monte Carlo Simulations of Annealing of Cascade Damage in Tungsten Giridhar Nandipati, Wahyu Setyawan, Howard Heinisch, Richard Kurtz, Kenneth Roche, Brian Roche Results are presented for a series of annealing simulations of displacement cascades in tungsten (W) using the kinetic Monte Carlo (KMC) code kSOME (kinetic simulation of microstructure evolution), which is our newly developed lattice-based Object KMC simulation code. In principle, kSOME can deal with migration, emission, transformation and recombination of all types of intrinsic point defects and their complexes. In addition, the interaction of these point defects with sinks such as dislocations, grain boundaries and free surfaces is also treated. We have studied the long-time annealing of displacement cascades in W obtained from molecular dynamics (MD) simulations. A database of displacement cascades in W was obtained using MD simulations for temperatures of 800-1300K and primary knock-on atom (PKA) energies in the range of 2 to 40 keV. The input data for the KMC simulations, such as activation energies for migration and dissociation of defects, and their capture radii were obtained from atomic-level calculations. The evolution of radiation damage was investigated as a function of time, temperature, dose and dose-rate. The results for W are compared with those for similar simulations of cascades in $\alpha$-Fe. [Preview Abstract] |
Wednesday, November 13, 2013 10:10AM - 10:30AM |
NM9.00003: Development of Drift-Diffusion Models of Mobile Helium Cluster Dynamics Near Surfaces of Plasma-Exposed Tungsten Dimitrios Maroudas, Lin Hu We report on the development of continuum drift-diffusion models for the dynamics of mobile helium (He) clusters in near-surface regions of materials exposed to He plasmas in fusion reactors. In addition to Fickian diffusion, the models account for drift fluxes driven by surface/interface segregation forces; such thermodynamic driving forces become significant near surfaces/interfaces and areal defects such as grain boundaries (GBs). The continuum models for the evolution of the mobile cluster concentrations are linked hierarchically with molecular-dynamics simulations of mobile cluster migration and are parameterized based on atomistic computations over the range of mobile cluster size for various surface/GB orientations; these computations yield the energy profiles of structurally relaxed He-cluster configurations in the direction normal to the surface/GB and optimal cluster migration/reaction pathways. The atomistic predictions of diffusivities and segregation potentials provide the required constitutive information for the closure of the drift-diffusion transport equations. The transport modeling also is extended to cases of combined drift effects near multiple sinks, such as in regions where GBs intersect with the surface. [Preview Abstract] |
Wednesday, November 13, 2013 10:30AM - 10:50AM |
NM9.00004: Unit mechanisms of interstitial He cluster diffusion in W: molecular and accelerated molecular dynamics simulations Blas Uberuaga, Danny Perez Understanding unit mechanisms associated with He diffusion in W is critical for building physical models that are used for predicting performance of fusion energy systems. We use a combination of accelerated molecular dynamics (AMD) and conventional molecular dynamics (MD) to identify both the mechanisms and rates of migration for interstitial He clusters as a function of cluster size. AMD simulations reveal complex but definite mechanisms for diffusion of these clusters while MD shows that the migration mechanisms identified via AMD simulations dominate over a wide temperature range. Where there are deviations from a true Arrhenius behavior, we show that these can be explained by a super-basin effect in which a single pathway is still the bottleneck for leaving a complex arrangement of states that is sampled differently as a function of temperature. We discuss the implications for higher-level models of He evolution in W. [Preview Abstract] |
Wednesday, November 13, 2013 10:50AM - 11:10AM |
NM9.00005: Benchmarking Results for High-Performance Simulations of Plasma Facing Components with Xolotl Jay Jay Billings Predicting the operational lifetime and performance of Plasma Facing Components (PFCs) for magnetically-confined fusion devices is critical to the deployment of future commercial fusion reactors beyond ITER and DEMO. The difficulty of this task can be partially eased by developing high-performance computing tools to simulate the PFCs in a tokamak environment. We present on-going work on a new, high-performance simulator for PFCs, named Xolotl, that considers the material evolution in the divertor with cluster dynamics. We will describe a set of benchmark problems that we have developed for this problem in tungsten and iron and compare Xolotl's results. We will also briefly describe the governing reaction-diffusion equations solved by Xolotl, its dependence on the dilute limit for clusters and the reactions it considers. [Preview Abstract] |
Wednesday, November 13, 2013 11:10AM - 11:30AM |
NM9.00006: Discussion on Modeling Approaches |
Wednesday, November 13, 2013 11:30AM - 11:50AM |
NM9.00007: Modeling of Hydrogen Retention in Co-deposited Layers with Trap Distribution R.D. Smirnov, S.I. Krasheninnikov, M.J. Baldwin, R.P. Doerner It has become increasingly evident in recent years that plasma-wall interactions causing particle and energy exchange play crucial role in control of edge plasma and life-cycle of plasma-facing components in fusion devices. Under such conditions concurrent sputtering and re-deposition of wall material lead to formation of material layers with co-deposited hydrogen, which are largely responsible for retention of hydrogen fuel in fusion vessel. However, conventional approach to modeling of hydrogen retention with discrete traps appears insufficient to describe experimentally observed time evolution of hydrogen outgassing from co-deposits. In this work we present a model of retention with continuous spectrum of trap energies and apply it to a series of thermo-desorption experiments on beryllium co-deposits performed on PISCES device. We demonstrate that the model reproduces main features of the co-deposit outgassing experiments, including time evolution of the desorption flux in different thermal regimes. The implications of the obtained results on our understanding of the retention mechanisms are discussed. [Preview Abstract] |
Wednesday, November 13, 2013 11:50AM - 12:10PM |
NM9.00008: Evolution of Plasma-Exposed Tungsten Surfaces Due to Helium Diffusion and Bubble Growth Karl Hammond, Lin Hu, Dimitrios Maroudas, Brian Wirth Helium from linear plasma devices and tokamak plasmas causes the formation of microscopic features, termed ``fuzz'' or ``coral,'' on the surface of plasma-exposed materials after only a few hours of plasma exposure. The details of such surface modifications are only beginning to be understood. This study examines the initial and intermediate stages of fuzz formation by large-length-scale molecular dynamics (MD) simulations of helium-implanted tungsten over time scales of up to microseconds using single-crystalline and polycrystalline supercell models of tungsten. The large-scale MD simulations employ state-of-the-art many-body interatomic potentials and implantation depth distributions for the insertion of helium atoms into the tungsten matrix constructed based on MD simulations of helium-atom impingement onto tungsten surfaces under prescribed thermal and implantation conditions. The large-scale MD simulations reveal surface features formed via the sequence of helium implantation, diffusion of helium atoms and their aggregation to form bubbles, growth of bubbles and consequent production of tungsten self-interstitial atoms, organization of those atoms into prismatic loops, glide of those loops to the surface, and bubble rupture. [Preview Abstract] |
Wednesday, November 13, 2013 12:10PM - 12:30PM |
NM9.00009: Fractal-TRIM and TRYDYN simulations of D-Be and Ar-W, and extrapolation to He-W Kyle Lindquist, Davide Curreli, David Ruzic Simulations using the codes TRIDYN and Fractal-TRIM have been done for irradiation of deuterium on beryllium and argon on tungsten, at energies ranging from the sputtering threshold (few tens of eV's) up to hundreds of eV's. The sputtering yield has been calculated for these cases and for different concentrations of gas implanted into the metal target. The simulations have been compared with recent experimental measurements of sputtering yield for similar combinations of projectile/targets. The sputtering yield measured by Doerner [Doerner, Nishijima, Schwarz-Selinger, 2012] of D on Be, 6 times less than a pure beryllium target, is compatible with a deuterated beryllium target at concentrations of Be[60\%]D[40\%]. Sputtering yields for Ar on W have been compared with Nishijima [Nishijima, Baldwin, Doerner, 2011] experimental results and simulations for smooth surfaces. Efforts have also been made to incorporate rough and fuzzy tungsten surfaces. Similar calculations of helium irradiation on a flat and rough tungsten surface have been done, and preliminarily compared to the Yamamura semi-empirical relations. [Preview Abstract] |
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