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 (2016 Phys. Plasmas23 082516) and (2020 Phys. Plasmas27 022507), 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.
Ion-temperature-gradient modes in stellarator geometry