Author: Jason Cedric Smoniewski

Publisher:

Published: 2021

Total Pages: 0

ISBN-13:

Turbulent transport is responsible for much of the energy and particle losses in present-day fusion plasma experiments, and optimization to reduce turbulence will be a major step towards realizing the benefits of fusion energy. Stellarators, with the flexibility afforded by external coils and three-dimensional geometry, may be able to reduce turbulence through careful shaping of the magnetic field. Such optimization relies on the ability of simulations to accurately predict turbulence in real devices, and validation studies are severely lacking for the stellarator. In this dissertation, the magnetic flexibility of the Helically Symmetric eXperiment (HSX) stellarator is exploited to investigate Trapped Electron Mode (TEM) turbulence in quasi-helically symmetric and degraded-symmetry configurations through experimental measurements and gyrokinetic simulation. This work includes the first comparison of nonlinear simulations in the Quasi-Helically Symmetric (QHS) and Mirror configurations, as well as the first comparison of nonlinear simulations at experimental parameters to experimental measurements. A database of archived HSX plasma discharges has enabled the temperature and density profiles to be matched in QHS and Mirror, showing that thermal transport is larger in the Mirror configuration at the mid-radius. Simulations do not reproduce this difference between geometries, but transport is sensitive to whether turbulence is in a ∇[n]-driven or ∇[T][e]-driven regime. More precise gradient measurements would be required for full validation of this geometry dependence. While linear growth rates are not predictive of overall turbulence, general aspects of experimental transport are captured by nonlinear simulations. In both simulation and experiment, the heat flux and density fluctuation amplitude increase more strongly with the density gradient than the temperature gradient, and the simulated heat flux matches measurements within experimental uncertainties for both configurations. This confirms that ∇[n]-driven TEM turbulence is the dominant driver of anomalous transport in HSX. Zonal flows can be important to TEM turbulence saturation, and are present in all nonlinear simulations of HSX. This work includes the first calculation of the linear collisionless zonal flow damping in quasi-symmetric magnetic geometry. Flux-tube, flux-surface, and full-volume calculations of the zonal flow evolution and residual are compared in the QHS and Mirror configurations, as well as the quasi-axial symmetry of the National Compact Stellarator eXperiment (NCSX). Despite quasi-symmetry, the dynamics of the zonal flow in all three configurations are similar to those in a conventional stellarator. The zonal flow oscillation presents another opportunity for comparison between simulation and experiment, but measurement of the zonal flow is left to future work. This dissertation is only the starting point for a validation study on the HSX stellarator. Significant opportunities exist for updated experimental measurements and a deeper investigation into the nonlinear physics responsible for TEM dynamics.