Triangularity effect on the ExB shearing rate in tokamak plasmas

Abstract

It has been shown in TCV and DIII-D experiments that negative triangularity (NT) tokamak plasmas could achieve H-mode-level confinement even without the H-mode transition. It is widely accepted that ExB flow shear suppression of turbulence and transport plays a crucial role in confinement improvement and transport barrier formation in fusion plasmas. In this thesis, a study of the triangularity effects on the ExB shearing rate is performed. I employ Millers magnetic equilibrium model, which contains various shaping effects, including triangularity, elongation, and Shafranov shift. Using Millers model, I derive an analytic expression of the Hahm-Burrell ExB shearing rate, which explicitly shows the contributions of shaping factors. I discuss the isolated effect of shaping factors on the poloidal variation of the shearing rate in terms of flux-squeezing. Two identical discharges in DIII-D are selected for analyses where the triangularity is the only difference; one is NT and the other is positive triangularity (PT). Using kinetic EFIT reconstruction data of these discharges, the ExB shearing rates are evaluated by carefully distinguishing the contribution from the triangularity to those from the radial electric field and magnetic shear. Finally, I discuss the highly anisotropic features of the ExB shearing rate.