Bulletin of the American Physical Society
57th Annual Meeting of the APS Division of Plasma Physics
Volume 60, Number 19
Monday–Friday, November 16–20, 2015; Savannah, Georgia
Session PM11: Mini-Conference: Measuring and Modeling of Plasma Material Interactions III |
Hide Abstracts |
Chair: David Ruzic, University of Illinois at Urbana-Champaign, John Canik, Oak Ridge National Laboratory Room: 103/104 |
Wednesday, November 18, 2015 2:00PM - 2:20PM |
PM11.00001: Why ions enter the sheath entrance at supersonic speed? Xianzhu Tang, Zehua Guo In a boundary plasma of a fusion device, the sheath Knudsen number, which is defined as the ratio of the plasma mean-free-path and the plasma Debye length, is much greater than unity, so one anticipates a collisionless sheath, even though the overall boundary plasma in the scrape-off layer is collisional. This is supposed to be the regime for which the Bohm criteria for the ion entry flow at the sheath entrance, $v\ge c_s$ with $c_s$ the sound speed, is usually satisfied at the equal sign. But numerical simulations using first-principles particle-in-cell codes tend to report a supersonic flow. Here we revisit the two-scale and transition layer analysis of the sheath-presheath transition, in tandem with the conventional Bohm criteria analysis, to understand why and how the supersonic sheath entry flow is established at the sheath entrance, which is a few Debye length away from the wall, and its impact on plasma particle and power load at the wall. Works upported by DOE OFES. [Preview Abstract] |
Wednesday, November 18, 2015 2:20PM - 2:40PM |
PM11.00002: Improving the sheath model used in plasma fluid transport codes John Canik, Xianzhu Tang Fluid plasma transport codes such as SOLPS treat the sheath at the interface of plasma and material as a boundary conditions on the fluid equations, enforcing standard conditions on the flow speed, electric potential, and the transmission of power to the wall for given plasma parameters. Recent computational studies using the VPIC particle-in-cell code have revealed several inaccuracies in the conventional values used for these, with higher flow speed, lower potential, and higher sheath transmission coefficients found in the first-principle simulations. The impact of the updated values for these boundary conditions on the plasma solution is investigated using SOLPS. For example, kinetic simulation show a substantially higher ion sheath heat transmission coefficient ($\sim$ 5.5-6) than are typically assumed ($\sim$ 2), and modestly lower electron coefficients. SOLPS simulations show that updating these values can significantly alter the plasma state from an initial equilibrium, increasing the electron and decreasing the ion temperatures. Further, the upstream density required to achieve low temperature, detached divertor conditions is increased by $\sim$ 10{\%}. The impact of further improvements to the SOLPS sheath model, including the flow speed and electric potential, will be presented. [Preview Abstract] |
Wednesday, November 18, 2015 2:40PM - 3:00PM |
PM11.00003: Two-point Analysis of SOLPS Modeling for a Slot Divertor P.C. Stangeby, J.M. Canik Two-Point Modeling, 2PM, is used to verify and quantify SOLPS code analysis of a narrow slot divertor configuration for a Fusion Nuclear Test Facility based on the Advanced Divertor concept, FNSF-AT [1]. The conventional 2PM includes free parameters, the power and momentum loss factors, f$_{pwr-loss}$, f$_{mom-loss}$. Kotov and Reiter showed that the 2PM is exact in the framework of the equations solved by the B2 code when f$_{pwr-loss}$ and f$_{mom-loss}$ are computed from the B2 code output [2]. An approximation of this procedure is applied here to the SOLPS analysis for FNSF-AT. As the ``upstream'' density at the outside midplane separatrix, n$_u$, is increased from 4.8e19 to 8.4e19 m$^{-3}$, the value of T$_{e-OSP}$ at the outer strike point drops from 70 to 2 eV while n$_{e\_OSP}$ increases from 2E20 to 2E21 m$^{-3}$. Over this range of conditions the 2PM values of T$_{e-OSP}$ were found to be within a factor of 2 of the SOLPS values. The relative roles of power and momentum loss are discussed.\par \vskip6pt \noindent [1] Garofalo et al, Nucl. Fusion 54 (2014) 073015.\par \noindent [2] Plasma Phys. Control. Fusion 51 (2009) 115002. [Preview Abstract] |
Wednesday, November 18, 2015 3:00PM - 3:20PM |
PM11.00004: A Molecular Dynamics Study of Sub-Surface Mixed Hydrogen-Helium Bubbles in Tungsten Zachary Bergstrom, Mary Alice Cusentino, Brian Wirth Fusion reactor materials experience high ion fluxes and operating temperatures which pose significant problems to plasma facing components. Among these issues is the formation of sub-surface helium and hydrogen bubbles in the divertor, of which tungsten is the prime candidate. Bubbles below the surface can grow by loop punching producing significant surface roughening and deformation. Molecular dynamics (MD) simulations are used to provide insight on the migration of hydrogen and helium within and around a sub-surface cavity in tungsten as a function of bubble size and partial pressures of helium and hydrogen to be compared to theoretical assessments of tritium partitioning to bubbles based on Sievert's Law. In this study, a cavity is created from a lattice of tungsten by removing atoms from inside a centered spherical region 2.5 nm below a free surface and approximately 2 nm in diameter. Hydrogen and helium are inserted into the cavity at random positions and allowed to find their local minimum. MD simulations are then performed for times on the order of nanoseconds for various concentrations of H and He, temperature, and surface orientations. The MD simulations provide quantitative information on the H and He distributions and partitioning amongst the bubbles and surfaces required to further understand the H and He synergies to estimate tungsten divertor lifetimes in ITER due to tritium retention, as well as provide insight into possible approaches to mitigate gas-driven damage to the tungsten divertor. [Preview Abstract] |
Wednesday, November 18, 2015 3:20PM - 3:40PM |
PM11.00005: Large-scale MD simulations investigating H plasma interactions with Tungsten surfaces Mary Alice Cusentino, Brian Wirth Tungsten is a prime candidate material for the divertor in future fusion reactors such as ITER. However, the tungsten divertor will need to be able to withstand high fluxes, on the order of 10$^{\mathrm{24}}$ m$^{\mathrm{-2}}$s$^{\mathrm{-1}}$, of low energy hydrogen. It is crucial to understand both the tungsten surface response as well as the hydrogen retention and recycling for the divertor region. Molecular dynamics (MD) is a useful tool to study these effects. One issue with MD is that implantation fluxes tend to be very high, on the order of 10$^{\mathrm{27\thinspace }}$m$^{\mathrm{-2}}$s$^{\mathrm{-1}}$, due to time and computational limitations. By performing large scale MD on supercomputers, it is possible to reach more realistic fluxes of 10$^{\mathrm{25}}$ m$^{\mathrm{-2}}$s$^{\mathrm{-1}}$. Results will be presented from MD simulations from a 50 nm x 50 nm x 25 nm tungsten box at 1200 K and 2000 K. Hydrogen is implanted every 10 ps based on the 60 eV depth distribution calculated by SRIM, which amounts to a flux of 4 x 10$^{\mathrm{25}}$ m$^{\mathrm{-2}}$s$^{\mathrm{-1}}$. A modified version of the Juslin bond order W-H potential is used to describe the W-H interactions. Preliminary results show an initially high retention of hydrogen that accumulates in a sub-surface region. These simulations provide insight into the early stages of surface deformation as well as hydrogen retention for the tungsten divertor. [Preview Abstract] |
Wednesday, November 18, 2015 3:40PM - 4:00PM |
PM11.00006: Modeling of material erosion and redeposition for dedicated DiMES experiments on DIII-D R. Ding, T. Abrams, C.P. Chrobak, H.Y. Guo, P.B. Snyder, V.S. Chan, D.L. Rudakov, P.C. Stangeby, J.D. Elder, D. Tskhakaya, W.R. Wampler, A. Kirschner, A.G. McLean Erosion and redeposition of plasma facing materials is a key issue for high-power, long pulse tokamak operation. A series of experiments has been carried out on DIII-D in which well-characterized samples of different materials were exposed to divertor plasma using DiMES. Such experiments provide a good benchmark for PMI codes, such as ERO. It was found that the erosion and redeposition are strongly determined by the impurity content in the plasma and sheath properties near the surface. The principal experimental results (net erosion rate and profile, net/gross erosion ratio) are reproduced by ERO simulations to within the uncertainties, indicating that the controlling physics has likely been identified. New techniques suggested by modeling such as external biasing and local gas injection for suppressing material erosion are planned to be tested in DiMES/DIII-D experiments. [Preview Abstract] |
Wednesday, November 18, 2015 4:00PM - 4:20PM |
PM11.00007: Improved Fractal Surface Algorithm for Modeling Evolving Surface Roughness in Dynamic-Composition BCA Codes Jon Drobny, Kyle Lindquist, David Ruzic Fractal TRIDYN (FTRIDYN) is a modified version of the Monte Carlo BCA code TRIDYN that includes an explicit fractal model of surface roughness. Surface roughness plays a significant role in ion irradiation processes such as sputtering; roughness can reduce the maximum yield by as much as a third and can significantly shift the angle of incidence at which yield is maximized. The complete effect of surface roughness is still not completely understood. Fractals provide a consistent and physically realistic method to model rough surfaces using fractal dimension as a single input parameter. FTRIDYN includes a robust fractal surface algorithm that is more efficient than previous fractal codes and which reproduces the available experimental data of sputtering yields from rough surfaces. Surface evolution is handled in FTRIDYN by tracking the position of displaced target atoms and calculating the fractal dimension of the surface modified by ion bombardment. In the presented study, the effects of evolving surface roughness on the angular sputtering yield and sputtering yield versus energy curves for Be and W plasma facing materials are investigated. [Preview Abstract] |
Wednesday, November 18, 2015 4:20PM - 4:40PM |
PM11.00008: Multi-scale modeling to relate Be surface temperatures, concentrations and molecular sputtering yields Ane Lasa, Elnaz Safi, Kai Nordlund Recent experiments [1,2] and Molecular Dynamics (MD) simulations [3] show erosion rates of Be exposed to deuterium (D) plasma varying with surface temperature and the correlated D concentration. Little is understood how these three parameters relate for Be surfaces, despite being essential for reliable prediction of impurity transport and plasma facing material lifetime in current (JET) and future (ITER) devices. A multi-scale exercise is presented here to relate Be surface temperatures, concentrations and sputtering yields. Kinetic Monte Carlo (MC) code MMonCa is used to estimate equilibrium D concentrations in Be at different temperatures. Then, mixed Be-D surfaces -- that correspond to the KMC profiles -- are generated in MD, to calculate Be-D molecular erosion yields due to D irradiation. With this new database implemented in the 3D MC impurity transport code ERO, modeling scenarios studying wall erosion, such as RF-induced enhanced limiter erosion or main wall surface temperature scans run at JET, can be revisited with higher confidence. \\[4pt] [1] D. Nishijima et al., Plasma Phys Contr F 50 (2008) 125007\\[0pt] [2] S. Brezinsek et al., NF 55 (2015) 063021\\[0pt] [3] E. Safi et al., JNM 463 (2015) 805-809 [Preview Abstract] |
Wednesday, November 18, 2015 4:40PM - 5:00PM |
PM11.00009: Mini-Conference Discussion III |
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