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Author: Mike F Martin
Requested Type: Poster
Submitted: 2018-02-27 12:45:00

Co-authors: M.Landreman, W.Dorland, P.Xanthopoulos, N.R.Mandell

Contact Info:
University of Maryland
5011 Berwyn Rd.
College Park, MD   20740
United States

Abstract Text:
A new boundary condition in the direction along the magnetic field is derived for flux tube simulations of plasma turbulence in geometries with low global magnetic shear. The calculation applies both to tokamaks and stellarators, assuming either the ends of the tube are separated by an integer number of poloidal turns in axisymmetry or that the flux tube is stellarator-symmetric. The classic ``twist-and-shift'' condition [Beer, Cowley, Hammett textit{Phys. Plasmas} textbf{2}, 2687 (1995)], derived in the axisymmetric limit, involves a shift to the radial wavenumber ${k}_{psi }$ and a quantization of the domain aspect ratio, both of which are proportional to the global magnetic shear. This quantization produces a restrictive condition on low $hat{s}$ devices, like W7-X. The new boundary condition is a generalization of ``twist-and-shift'' that incorporates local magnetic shear information into the shift in $k_{psi}$, by evaluating the difference between the integrated local shear at the two ends of the tube. The dependence of the $k_{psi}$ shift on this quantity allows one to optimize the length of the flux tube to achieve several potentially advantageous conditions. For certain geometries the integrated local shear can cross through zero at a number of locations along a magnetic field line, and if the flux tube length is chosen to end at one of these locations it is possible to use periodic parallel boundary conditions and remove any quantization of the box aspect ratio. The new boundary condition also preserves the continuity of $k_{perp}$ and thus the evolution of the secondary instability across the boundary. Simulations employing periodic parallel boundary conditions using this technique show a dramatic decrease in computation time compared to the conventional boundary condition. Results further indicate that enforcing periodicity textit{without} the justifications of the new boundary condition has a minimal impact on the accuracy of results.