Transition of turbulence regime during neutral beam injection on EAST

Abstract

In recent experiments on EAST with neutral beam injection (NBI) and radio frequency (RF) waves (lower hybrid wave (LHW) and electron cyclotron waves) heating, it was observed that an increase in the line-averaged electron density and a decrease in the plasma internal inductance occurred in the duration of NBI. To explain the experimental observation, the simulations are conducted by using the equilibrium code EFIT and the transport codes ONETWO and TGYRO. It is shown that L-H transition is triggered right after neutral beam is injected, leading to the formation of density pedestal, the plasma density gradient in the outer region (0.4 < ρ < 0.75) is thus changed, which leads to an increase in η i (, where and ), where ρ is the normalized square root of the toroidal magnetic flux. A transition from a trapped electron modes (TEM) to an ion temperature gradient (ITG) modes dominant turbulence regime then happens due to the increase of η i in the region (0.4 < ρ < 0.75), which results in an inward particle pinch. As a result of the rise of plasma density, the parallel refractive index and ray trajectories of the LHW are affected and more power is deposited outside ρ > 0.6, which drives more off-axis current and therefore the internal inductance is reduced.