Global linear drift-wave eigenmode structures on flux surfaces in stellarators: ion temperature gradient mode

TL;DR

This paper investigates the linear ion-temperature-gradient eigenmodes in stellarators, revealing their nonuniform, localized structures across flux surfaces and proposing a model to explain their behavior using complex wavenumbers.

Contribution

It introduces a novel analysis of drift-wave eigenmodes in stellarators, combining numerical simulations with a simple model to explain mode localization and surface-global eigenmode construction.

Findings

Eigenmode structures are localized and nonuniform across flux surfaces.

Localization can be explained by the imaginary part of the binormal wavenumber.

Surface-global eigenmodes can be constructed from local codes with analytic continuation.

Abstract

Turbulent transport greatly impacts the performance of stellarator magnetic confinement devices. While significant progress has been made on the numerical front, theoretical understanding of turbulence in stellarators is still lacking. In particular, due to nonaxisymmetry, different field lines couple within flux surfaces, the effects from which have yet to be adequately studied. In this work, we numerically simulate the linear electrostatic ion-temperature-gradient modes in stellarators using the global gyrokinetic particle-in-cell code GTC. We find that the linear eigenmode structures are nonuniform across field lines on flux surfaces and are localized at the downstream of the ion diamagnetic drift. Based on a simple model from Zocco et al. [Phys. Plasmas 23, 082516 (2016); 27, 022507 (2020)], we show that the localization can be explained from the nonzero imaginary part of the binormal wavenumber. We further demonstrate that a localized surface-global eigenmode can be constructed from local gyrokinetic codes stella and GX, only if we first solve the local dispersion relation with real wavenumbers on each field line, and then do an analytic continuation to the complex-wavenumber plane. These results suggest that the complex-wavenumber spectra from surface-global effects are required to understand the linear drift-wave eigenmode structures in stellarators.