Effect of deposition location of electron cyclotron resonance heating on active control of fishbone modes in the HL-2A tokamak

Abstract

Experiment related to suppression of fishbone activities by electron cyclotron resonance heating (ECRH) has been carried out on HL-2A tokamak. To achieve multiple deposition locations of ECRH, the magnetic field ranges 1.22~1.4<i>T</i> from shot to shot. It is found that the fishbone modes exhibit different characteristics with different radial deposition locations. With the same injected power, the effect of off-axis ECRH is much better than that of on-axis heating. The fishbone modes can be completely suppressed when ECRH is deposited nearby the <i>q</i>=1 ration surface, but would only mitigated in other cases. Further analysis indicate that injection of high power ECRH leads to increase of electron temperature, then changes the pressure gradient and plasma current density, finally causes a change in safety factor and makes the minimum safety factor larger than 1. Meanwhile, M3D-K simulation suggests the growth rate of fishbone mode declines with the increasing of <i>q</i><sub>min</sub>. In other words, the growth of safety factor and disappearance of <i>q</i>=1 ration surface induced by ECRH contribute to the suppression of fishbone activities. The experimental results reported here may not only help to better understand complex effects of ECRH on magnetohydrodynamic instability, but also provide a physics basis for the active control of energetic particle driven modes on the future magnetic confined fusion devices.