Gyrokinetic simulations in stellarators using different computational domains

TL;DR

This study compares gyrokinetic simulations in stellarators across different computational domains, revealing how domain choice affects results on zonal flows and ITG modes, with implications for simulation accuracy and efficiency.

Contribution

It systematically evaluates the impact of flux tube length and domain type on gyrokinetic simulation outcomes in stellarators, highlighting configuration-dependent convergence.

Findings

Flux tube length influences convergence of zonal flow relaxation.

Discrepancies between flux tube and full-flux-surface results decrease with longer flux tubes.

Radially global simulations closely match full-flux-surface results.

Abstract

In this work, we compare gyrokinetic simulations in stellarators using different computational domains, namely, flux tube, full-flux-surface, and radially global domains. Two problems are studied: the linear relaxation of zonal flows and the linear stability of ion temperature gradient (ITG) modes. Simulations are carried out with the codes EUTERPE, GENE, GENE-3D, and stella in magnetic configurations of LHD and W7-X using adiabatic electrons. The zonal flow relaxation properties obtained in different flux tubes are found to differ with each other and with the radially global result, except for sufficiently long flux tubes, in general. The flux tube length required for convergence is configuration-dependent. Similarly, for ITG instabilities, different flux tubes provide different results, but the discrepancy between them diminishes with increasing flux tube length. Full-flux-surface and flux tube simulations show good agreement in the calculation of the growth rate and frequency of the most unstable modes in LHD, while for W7-X differences in the growth rates are found between the flux tube and the full-flux-surface domains. Radially global simulations provide results close to the full-flux-surface ones. The radial scale of unstable ITG modes is studied in global and flux tube simulations finding that in W7-X, the radial scale of the most unstable modes depends on the binormal wavenumber, while in LHD no clear dependency is found.