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arXiv · 2208.02202

A phase-shift-periodic parallel boundary condition for low-magnetic-shear scenarios

D. A. St-Onge·M. Barnes·F. I. Parra
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Abstract

We formulate a generalized periodic boundary condition as a limit of the standard twist-and-shift parallel boundary condition that is suitable for simulations of plasmas with low magnetic shear. This is done by applying a phase shift in the binormal direction when crossing the parallel boundary. While this phase shift can be set to zero without loss of generality in the local flux-tube limit when employing the twist-and-shift boundary condition, we show that this is not the most general case when employing periodic parallel boundaries, and may not even be the most desirable. A non-zero phase shift can be used to avoid the convective cells that plague simulations of the three-dimensional Hasegawa-Wakatani system, and is shown to have measurable effects in periodic low-magnetic-shear gyrokinetic simulations. We propose a numerical program where a sampling of periodic simulations at random pseudo-irrational flux surfaces are used to determine physical observables in a statistical sense. This approach can serve as an alternative to applying the twist-and-shift boundary condition to low-magnetic-shear scenarios which, while more straightforward, can be computationally demanding.

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D. A. St-Onge, M. Barnes, F. I. Parra. 2022-08-03. A phase-shift-periodic parallel boundary condition for low-magnetic-shear scenarios. https://doi.org/10.1088/1361-6587/aca4f8

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