Dynamics of linear ITG modes with flow shear in ballooning space

Abstract

It is widely recognized that in a toroidal nuclear fusion reactor (tokamak), unstable plasma modes driven by a radial ion temperature gradient (ITG) can be stabilized by the effect of rotational flow shear. In this study these rotational modes are solved from the linear gyrokinetic equations and scrutinized in ballooning space, using the Gene code. In ballooning space the effects of flow shear are clearly visible, as well as a difference in stabilization between kinetic and adiabatic electron modes at low magnetic shear s (the latter is quenched at lower flow shear). Mode shapes consistently equilibrate in ballooning space, whereas they can still fluctuate highly in time (Floquet modes). To gain physical insight into the mechanics of stabilization and Floquet fluctuations, a toy model is created. The full rotational ITG solution is decomposed into shearless modes to which flow shear is separately added. The model reproduces general mode structures, Floquet fluctuations and the stabilizing impact of flow shear. However, the difference in stabilization of modes with kinetic and adiabatic electrons at low s is not captured. Besides this main research, a linear quench rule is derived from Gene simulations, approximating the impact of flow shear on ITG modes in a section of 4-dimensional parameter space. This rule can serve as a new dimension in a plasma turbulence neural network based on QuaLiKiz runs (Citrin et al., in progress).