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Author: Andris M Dimits
Requested Type: Poster
Submitted: 2017-03-17 13:59:22

Co-authors: B. I. Cohen, A. Friedman, I. Joseph, L. L. LoDestro, M. E. Rensink, T. D. Rognlien, B. Sjogreen, D.P. Stotler, M. V. Umansky

Contact Info:
Lawrence Livermore National Laboratory
7000 East Avenue/PO Box 808
Livermore, California   94550
USA

Abstract Text:
The success of the UEDGE code [1] as a tool for modeling transport in the tokamak edge and scrape-off layer depends on a highly efficient fully implicit solution of coupled fluid-neutrals and plasma models. Because more accurate yet still computationally fast models are needed, we investigate efficient [e.g., quasi-Newton and Jacobian-free Newton-Krylov (JFNK)] methods for implicit coupling of kinetic-neutrals and fluid plasma.
We have verified an implicit coupling of the 2D UEDGE plasma component and the DEGAS-2 Monte-Carlo (MC)-neutrals code [2] against the preexisting explicit one [3]. Work is ongoing on optimization of the coupling to minimize error for a given computational effort. There are discretization, statistical (variance) and nonlinear bias (due to finite Monte-Carlo sampling), and convergence contributions to the overall error. Optimization of the overall error involves balancing those error contributions that cannot easily be made small. We are investigating an existing approach [4] for JFNK iterations with MC noise, the usefulness of which is directly tied to the quality of preconditioners used to aid the convergence of the JFNK iterations. We have therefore developed analyses for approximating preconditioners for the kinetic problem with those from a reduced fluid model and a method for “extracting” the needed preconditioners from those provided by existing coding.
In addition, we have investigated the numerical stability boundaries for the explicit coupling for simplified, but relevant, 1D and 2D test cases.
*Prepared for US DOE by LLNL under Contract DE-AC52-07NA27344 and LDRD project 15-ERD-059, by PPPL under Contract DE-AC02-09CH11466, and supported in part by the U.S. DOE, OFES.
[1] T.D. Rognlien et al., Contrib. Plasma Phys. 34, 362. (1994)
[2] D. Stotler & C. Karney, Contr. Plas. Phys., 34, 392 (1994)
[3] I. Joseph et al., accepted by Nucl. Mater. Energy (2017)
[4] J. Willert, et al., SIAM J. Numer. Anal. 53, 1738 (2015)

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