Abstract Details
Abstracts
Author: Hongxuan Zhu
Requested Type: Consider for Invited
Submitted: 2025-02-20 22:35:51
Co-authors: H. Chen, R. Xu, Z. Lin, A. Bhattacharjee
Contact Info:
Princeton University
Princeton University
Princeton, 08544
USA
Abstract Text:
Turbulent transport significantly impacts the performance of stellarators. Many gyrokinetic codes have been developed to simulate turbulence in stellarators, including global and local codes. Recently, global gyrokinetic simulations of ion-temperature-gradient (ITG) turbulence showed that both the linear eigenmode structure and the nonlinear fluctuation level are nonuniform on flux surfaces. This nonuniformity arises from the coupling of different field lines, which may not be adequately described by the local approach. Zocco et al. (Phys. Plasmas 27, 022507) proposed a simplified model which reproduced the nonuniform eigenmode structures, but a general theoretical understanding is still missing.
In this work, we study the global drift-wave eigenmodes on flux surfaces in stellarators. We find that the nonuniform structures can be explained from the complex poloidal wavenumber. Starting with Zocco’s model, we solve the local dispersion relation, which varies across field lines. Since the global eigenmode frequency is constant, we invert the dispersion relation to solve for the poloidal wavenumber, and solutions can be found only on the complex plane. The mode structures are nonuniform due to the imaginary part of the wavenumber, and they peak at the downstream direction of the ion diamagnetic drift.
Next, we use the global gyrokinetic code GTC to simulate linear ITG eigenmodes in quasi-axisymmetric, quasi-helically symmetric, and quasi-isodynamic configurations. The nonuniform mode structures are present in all three configurations. Using the 5D ion distribution from GTC, we calculate the poloidal wavenumber, which indeed has a nonzero imaginary part. These results can also be qualitatively reproduced using a local code, if we solve the local dispersion relation with real wavenumbers and do an analytic continuation to the complex plane. Therefore, stellarator turbulence may access the complex-wavenumber spectrum, which requires further theoretical analyses.
Characterization: 1.0
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