Author: Benjamin Faber

Publisher:

Published: 2018

Total Pages: 187

ISBN-13:

The ability to optimize stellarator geometry to reduce transport has led to renewed interest in stellarators for magnetic confinement fusion. In this thesis, turbulence in the Helically Symmetric eXperiment (HSX), a quasi-helically symmetric stellarator, is investigated through application of the gyrokinetic code GENE and the new reduced fluid model PTSM3D. Gyrokinetics provides an efficient formalism for high-fidelity plasma turbulence simulations by averaging out the unimportant fast particle gyromotion. Both GENE and PTSM3D are capable of handling three-dimensional stellarator geometries. The first comprehensive simulations of Trapped Electron Mode turbulence in HSX are presented. HSX geometry introduces a complex landscape of unstable eigenmodes, with strongly ballooning modes, non-symmetric modes, and extended modes all coexisting at the same wavelengths. Nonlinear simulations display several characteristics unique to HSX. At long wavelengths, surprisingly large transport is observed despite the corresponding linear growth rates being small, and is attributed to nonlinear mode interactions. Zonal flows are prominent, however the velocity shear is insufficient to be solely responsible for saturation. These linear and nonlinear features are the consequence of the low global magnetic shear of HSX, which allows modes to extend far along field lines and requires simulation domains spanning multiple poloidal turns to properly resolve. Subdominant modes with extended structures play an important role in nonlinear energy transfer at long wavelengths, removing energy from shorter wavelength modes to both drive long-wavelength transport and dissipate energy through transfer to stable modes. Calculations with PTSM3D of triplet correlation times, used to quantify turbulence saturation, support the gyrokinetic results and show geometry plays a crucial role in turbulence saturation. Energy transfer from unstable modes to stable modes through non-zonal modes is the dominant mechanism in quasi-helically symmetric geometry, while zonal modes catalyze transfer in quasi-axisymmetric geometry. PTSM3D triplet correlation time calculations for HSX configurations with different magnetic hill and well depths accurately reproduce the nonlinear simulation trends, demonstrating the suitability of triplet correlation times as the first nonlinearly-derived metric for turbulence optimization.