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 NM9: Mini-Conference on Plasma–Material Interactions in Fusion Devices: ITER and Beyond. III. Helium and Impurity Effects on PMI |
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Chair: Ane Lasa Esquisabel, University of Tennessee Room: OCC C123 |
Wednesday, November 7, 2018 9:30AM - 9:55AM |
NM9.00001: Using 3He-containing plasma to track the motion of helium atoms in tungsten Russell Doerner, Sergei Krasheninnikov, Matthew Baldwin, Daisuke Nishijima, Marlene Patino Experiments using 3He-containing plasma have been performed in PISCES-A to allow quantification of the resultant He content in W samples using D ion Nuclear Reaction Analysis (NRA). Measurements at temperatures where fuzz forms on the W surface reveal that only a small amount (~25%) of the He in the samples after the exposure is located in the tendrils themselves, with the majority of the He contained in the W below the fuzz layer. Measurements (again at high temperature) with time varying 3He content in the plasma showed the dynamic behavior of the He bubbles, as 3He injected only early in the discharge was removed from the W by subsequent 4He plasma exposure. The 3He content in the W base scales with the 3He fluence when 3He is contained in the plasma at the end of the discharge, demonstrating that fuzz is not completely opaque for helium ions impinging on the sample. The continuous bubble rupturing process is also thought to be a material migration source at high temperature. 3He was also observed to be removed from W during subsequent 4He plasma at lower temperature (~500°C), but no material migration was detected. These measurements will help guide MD simulations of He behavior in W and provide data for bench-marking such codes. |
Wednesday, November 7, 2018 9:55AM - 10:15AM |
NM9.00002: Flux Effects on Helium-Induced Surface Evolution in Tungsten Karl D Hammond, Ian V Naeger, Wathsala Widanagamaachchi, Li-Ta Lo, Dimitrios Maroudas, Brian D. Wirth We investigate the effects of flux on helium behavior in plasma-facing tungsten using molecular dynamics. The simulations span several orders of magnitude and achieve simulation times of up to 2.5 μs, revealing concerted bubble-bursting events that are responsible for significant and very sudden changes in surface morphology. The helium depth distribution is very strongly flux-dependent, as helium becomes trapped near the surface at high flux because of the near-surface bubbles that form. Helium retention is also much lower at low flux than at high flux. Surface features correlate with bubble locations at low fluence, but at high fluence, bubbles merge together, venting to the plasma at one or more locations and leaving large interconnected cavities below the surface. Such ruptured bubbles may serve as pathways deeper into the material, allowing helium to bypass the near-surface layer of bubbles and form deeper and potentially much larger bubbles that can produce more sizeable surface features. |
Wednesday, November 7, 2018 10:15AM - 10:35AM |
NM9.00003: Effects of helium plasmas on tungsten surfaces characterized by helium ion microscopy, ion channeling, and in-situ spectroscopic ellipsometry Robert D Kolasinski, Dwaipayan Dasgupta, Josh A Whaley, Aaron Engel, Frances I Allen, Dean Alan Buchenauer, Dimitrios Maroudas, Brian Wirth We have used helium ion microscopy and in-situ spectroscopic ellipsometry to characterize how tungsten surfaces are modified by low-energy helium plasmas. The RF plasmas were generated in a small linear plasma device equipped with a Lisitano coil (exposure conditions: Γion = 8.5×1020 m-2 s-1; F = 7.9×1023 - 3.6×1025 m-2, Tsample = 450 - 930 °C.) Helium ion microscopy was used to image the initial surface morphology changes and nano-tendril growth. We then correlated the microscopy data with the surface optical properties measured by an in-situ, real-time spectroscopic ellipsometry system. We observed a continuous decrease in both the extinction coefficient and index of refraction as a function of fluence between wavelengths of 280-1000 nm. In addition to these results, we present preliminary low and medium energy surface channeling experiments. We considered the W(110) model system, using ion beams to assess the crystallinity of the near-surface. |
Wednesday, November 7, 2018 10:35AM - 11:00AM |
NM9.00004: Using Accelerated Molecular Dynamics Simulations to Understand Helium Bubble Evolution in Tungsten Blas Pedro Uberuaga, Xiang-Yang Liu, Luis Sandoval, Danny Perez, Arthur F. Voter Designing materials to withstand the extreme conditions of fusion requires understanding, at the atomic scale, material evolution at those same conditions. Accelerated molecular dynamics (AMD) methods, which extend the timescale of conventional molecular dynamics while retaining the fidelity of the interatomic interactions, is a key tool in developing this understanding. We have used these methods to examine the behavior of helium bubbles in tungsten with the goal of informing higher level models of basic mechanisms over timescales relevant for fusion conditions. This talk will summarize both past results as well as more recent work, highlighting and contrasting the growth mode of bubbles in bulk tungsten versus at a grain boundary, examining the effect of helium arrival rate on the development and evolution of bubble networks, and determining the rate of migration of embryonic bubbles. Together, these results complement other simulation studies and provide a more comprehensive picture of the dynamics associated with helium bubbles in tungsten. |
Wednesday, November 7, 2018 11:00AM - 11:25AM |
NM9.00005: Interaction of boron and nitrogen-rich plasmas with tungsten wall Yevgeny Raitses, Shurik Yatom, Vladislav Vekselman, Alexander Khrabry, Igor D Kaganovich There is an interest in the conditioning of tungsten divertor by controlled injection of boron and boron nitride powder during the plasma discharges [1]. However, it remains unclear how plasma consisting of boron, nitrogen and deuterium species interact with the tungsten wall. In this work, we study plasma material interaction using optical emission spectroscopy (OES) and high speed imaging for boron-rich ablating targets immersed in the nitrogen and nitrogen-hydrogen plasmas generated by two different methods: 1) the arc discharge between the tungsten electrodes [2] and 2) laser vaporization. In particular, our arc experiments revealed that boron ablation generates charged boron droplets. OES measurements demonstrated a strong effect of addition of the hydrogen to the nitrogen atmosphere on observed boron, nitrogen and tungsten species. For laser vaporization, we also detected molecular species including BN, B2N, N2. These results appear to be consistent with our thermodynamic simulations. [1] A. Borotolon et al., PSI Conference, Princeton, NJ, USA, June 2018; [2] Y. W. Yeh et al., Sci. Rep.-UK 7 (2017). |
Wednesday, November 7, 2018 11:25AM - 11:45AM |
NM9.00006: Quantification on the Effect of Uncertainty On Impurity Migration In PISCES-A Simulated With GITR Tim Younkin, Khachick Sargsyan, Tiernan Casey, Habib Nasri Najm, Russ Doerner, Daisuke Nishijima, David L Green, John Canik, Ane Lasa, Philip C Roth, Davide Curreli, Jon T Drobny, Parker Forehand, Brian Wirth The extreme heat, charged particle, and neutron flux / fluence to plasma facing materials in magnetically confined fusion devices has motivated research to understand, predict, and mitigate the associated detrimental effects. Of relevance to the ITER divertor is the helium interaction with the tungsten divertor, the resulting erosion and migration of impurities. The linear plasma device PISCES A [1] has performed dedicated experiments for ~10^22 m-2s-1 flux, 250 eV He exposed tungsten targets to assess the net and gross erosion of tungsten and volumetric transport. We present results of the erosion / migration / re-deposition of W during the experiment from the GITR (Global Impurity Transport) code coupled to materials response models. Specifically, the modeled and experimental W I emission spectroscopy data for the 429.4 nm wavelength and net erosion through target and collector mass difference measurements are compared. Bayesian inference is applied to the background plasma profile measurements (Te,ne) and data fitting in order to produce an uncertain input model for GITR. This allows for a bounding in the predictions of the experimental observables based on the uncertainty in the input data and models. [1] R.P. Doerner Nucl. Fusion 52 (2012) |
Wednesday, November 7, 2018 11:45AM - 12:05PM |
NM9.00007: Applied Statistical Model of Surface Morphology for Plasma-Material Interactions Jon T Drobny, Davide Curreli, Ane Lasa, Sophie Blondel, John Canik, David L Green, Philip C Roth, Tim Younkin, Brian Wirth Multi-scale simulations of fusion devices must include detailed models of plasma-material interactions including surface morphology and composition to successfully capture the complete physics of machine operation. Binary Collision Approximation (BCA) codes succeed at modeling atomic scale ion-matter interactions, including reflection, sputtering, and implantation. F-TRIDYN is a BCA code designed as an interface between plasma and material codes for the PSI2-SciDAC collaboration. In this collaboration, F-TRIDYN is coupled to hPIC to provide ion energy-angle distributions and to Xolotl and GITR for subsurface dynamics and impurity transport respectively. F-TRIDYN includes an explicit model of surface morphology. However, explicit surface morphology is computationally expensive. For this reason, a fast, statistical model of surface morphology has been developed. This talk will cover the implementation, computational performance, and applications of the statistical surface model in F-TRIDYN. |
Wednesday, November 7, 2018 12:05PM - 12:30PM |
NM9.00008: Modeling of Surface Morphological Evolution of Plasma-Facing Tungsten Dimitrios Maroudas, Dwaipayan Dasgupta, Robert D Kolasinski, Brian D Wirth We have developed an atomistically-informed, continuous-domain model to describe the surface morphological evolution of plasma-exposed tungsten (W). Based on this model, we have conducted self-consistent numerical simulations of the dynamics of the helium plasma-irradiated W surface morphology and carried out systematic comparisons of the simulation results with experimental measurements. The surface morphology predicted by our simulations is in good qualitative agreement with the experimental observations for the early stage of nanotendril formation on the W surface, a precursor to fuzz-like surface growth. Additionally, quantitative comparisons between the experimental data and the simulation results show that our model predicts satisfactorily the growth rate of the nanotendrils and provides reasonable estimates for the nanotendril width and arrangement on the W surface. Furthermore, we have investigated the dependence of the nanotendril width and arrangement on the W surface on the He fluence and surface temperature. |
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