April 15-17

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Author: Nicolas Christen
Requested Type: Poster
Submitted: 2019-02-21 13:18:11

Co-authors: M.Barnes, F.Parra

Contact Info:
University of Oxford
Rudolf Peierls Centre for Theo
Oxford,   00000
United Kingdom

Abstract Text:
Turbulent transport is a limiting factor for tokamak performance. Results show that shear in the background flow perpendicular to the mean magnetic field reduces transport, while shear in the parallel flow can enhance transport [1][2]. It is hence important to include flow shear in simulations.

Local, delta-f gyrokinetic codes such as GS2 [3] include flow shear by approximating time-dependent radial wave numbers with the nearest point on a fixed grid. For experimentally relevant values of flow shear, the wave numbers evolve slowly: this leads to discontinuous jumps occurring between periods where flow shear has no effect. We present an improved approach that avoids this potentially unphysical behavior by treating flow shear continuously in time.

Two alternative algorithms have been implemented in GS2. Linearly, GS2 is implicit in time, using a “response matrix” approach to solve the gyrokinetic equation. With flow shear, this matrix becomes time dependent and recomputing it at every time step is prohibitively slow. Our first algorithm uses interpolation to approximate this time dependence. Our second algorithm makes the response matrix independent of time via the following trick: for every term in the gyrokinetic equation made time dependent by flow shear, the difference between its exact expression and its nearest neighbor approximations is treated explicitly in time. We then present linear and nonlinear simulations comparing the two new approaches with the old code, focusing on cases that we expect to be challenging for the old implementation.

The work of N.Christen is supported by a Berrow Foundation Scholarship. The authors acknowledge EUROfusion, the EUROfusion High Performance Computer (Marconi-Fusion), and the use of ARCHER through the Plasma HEC Consortium EPSRC grant number EP/L000237/1 under the projects e281-gs2.

[1] M.Artun et al, Phys of Fluids B: Plasma Physics, 1992
[2] M.Barnes et al, PRL, 2011.
[3] M.Kotschenreuther et al, Comp Phy

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