Finite T_i Effects on Parallel-Velocity-Gradient Driven Instability

Abstract

This study investigates the influence of finite ion temperature on the linear growth rate of the parallel velocity gradient (PVG)-driven instability. Parallel (toroidal) flows are prevalent in magnetically confined fusion plasmas, where external momentum sources such as neutral beam injection (NBI) can act as a potential driver for PVG instabilities in both conventional and spherical tokamaks. Accurate transport modeling using quasi-linear models such as TGLF or QuaLiKiz requires a fundamental understanding of the characteristics of PVG modes in warm plasmas. The linear stability analysis is conducted to include the finite ion temperature effects. Depending on the radial profiles of ion temperature and parallel velocity, the dominant unstable modes can be categorized as either ion temperature gradient (ITG) or PVG-driven modes. These two instabilities are found to be mutually exclusive in their parameter spaces. Both modes become excited when the compression becomes negative. The ITG and PVG modes are each strengthened by the gradient that drives the other, up to the point where the driving gradient exceeds the instability threshold of the respective mode. In the PVG-dominant regime, increasing the temperature ratio enhances the compression and thereby exerts the stabilizing effects, whereas the temperature gradient contributes to PVG growth until the ITG thresholds is reached.