Speaker: Joseph Duff, Physics PhD Graduate Student
Abstract: The effect of triangularity and sign of geodesic curvature $\mathcal{K}^x$ on ion-temperature-gradient (ITG)-driven turbulence was investigated, covering both extreme positive and negative triangularities. Triangularity had a substantial impact on linear and nonlinear physics, and reversing $\mathcal{K}^x$ significantly impacted turbulence saturation. Negative triangularity reduced peak linear growth rates and broadened the growth rate spectrum as a function of radial wavenumber $k_x$. Positive triangularity increased peak growth rates that shifted to finite $k_x$ and narrowed the growth rate spectrum. Reversing the sign of $\mathcal{K}^x$ slightly lowered linear growth rates, except for $\delta=0.85$, where the growth rates decreased significantly. The effect of triangularity on linear instability properties at low perpendicular wavenumbers can be explained through its impact on magnetic polarization and curvature. The nonlinear heat flux was weakly dependent on triangularity for $-0.5\le\delta\le0$, increasing significantly with extreme $\delta$, regardless of sign. When $\mathcal{K}^x$ was reversed, the heat flux decreased, became weakly dependent on triangularity for $\abs{\delta}\le0.5$, and decreased significantly at extreme triangularity, regardless of sign. Zonal modes play an important role in nonlinear saturation for the configurations studied, and artificially suppressing zonal modes increased the nonlinear heat flux by a factor of at least two and a half, with negative triangularities having a larger increase. When $\mathcal{K}^x$ was reversed, so did the trend of heat flux ratios with triangularity. Proxies for zonal-flow damping and drive suggest that zonal flows are enhanced with increasing positive $\delta$ in both $\mathcal{K}^x$ scenarios. Conventional quasilinear models did not capture the nonlinear heat flux trends, but, by using a reduced three-field fluid model for ITGs, the effect of unstable modes coupling to stable modes via zonal modes was added to the quasilinear model. This three-wave-interaction corrected quasilinear model was only able to capture the nonlinear trends in triangularity when $\mathcal{K}^x$ was reversed. The failure of the modified quasilinear model to estimate the nonlinear trend for the physical equilibria was likely due to the nonlinear heat flux spectra extending into scales where the fluid model is not valid.
Optimizations resulted in two three-dimensionally-shaped magnetic configurations with suppressed trapped-electron-mode (TEM)-driven turbulence. Initial equilibria had flux surface shapes with a helically rotating negative triangularity (NT) and positive triangularity (PT). The optimization targeted quasihelical symmetry and the available energy of trapped particles. In electron-temperature-gradient-driven scenarios, the most unstable linear modes of the TEM-optimized configurations were inconsistent with TEMs, and the nonlinear simulations showed no significant fluctuations at ion scales. When a density gradient was present, the most unstable modes at low $k_y$ were toroidal universal inabilities (UIs) in the NT case and slab UIs in the PT geometry. Nonlinear simulations showed that UIs drove substantial heat flux in the NT and PT configurations. Increasing the ratio of plasma pressure to magnetic pressure to $\beta=4\times10^{-3}$ significantly reduced linear instability at low $k_y$, halved the nonlinear heat flux for the NT case, and almost completely suppressed the turbulence in the PT configuration.