Speaker: Ping-Yu Li , Physics PhD Graduate Student
Abstract: Microturbulence is caused by gyroradius-scale instabilities such as the Ion-Temperature-Gradient-driven (ITG) instability, Trapped Electron Mode (TEM), Kinetic Ballooning Mode (KBM), etc. Understanding how these instabilities saturate and form turbulence is important for the optimization of magnetic confinement fusion devices in the quest for sustained fusion energy. The objective of this thesis is to understand the important factors and mechanisms that saturate ITG turbulence and to utilize said understanding to build reduced models that capture key physical behavior as described by full-physics approach.
Zonal-flow-catalyzed interactions that involve large-scale stable and unstable modes are crucial for the saturation of curvature-driven ITG turbulence. A corresponding saturation theory is built based on a fluid model and implemented and tested numerically. The crudest saturation theory drops the non-zonal interactions and also the nonlinear corrections to frequencies, it also truncates the wavenumber space to obtain scalings for the saturation level with the triplet correlation times with linear frequencies and coupling coefficients. It is then discovered that nonlinear interactions can cause nonnegligible modifications on the mode oscillations for systems with higher turbulence level. Furthermore, the kx direction in wavenumber space needs to be resolved in order to break the symmetry between modes and build up the zonal flow, which is shown in both time-dependent and time-independent research. Constructing a two-predator-prey model with no free parameter inputted base on the saturation theory is also demonstrated. This provides an idea how to build a predator-prey model from the first principle, which has the potential to help understanding the limit-cycle oscillations observed in L-H transition.
The importance of large-scale stable modes and the triplet correlation time derived from the saturation theory are tested in gyrokinetics. Numerical results show that the resonance between the stable and unstable modes through the coupling of zonal flow corresponds to long nonlinear interaction life times, or large triplet correlation times, which increases nonlinear energy transfer and leads to strong turbulence suppression beyond any purely linear estimates.
The triplet correlation time is further used to improve a highly reduced model for fast heat-flux prediction in gyrokinetics, which shows significant improvement in several cases that demonstrate heat-flux onset upshift from the linear critical gradient for gradient scans. The role of the coupling coefficient in gyrokinetics is still under investigation.
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