The first observation of GAM induced by negative biasing in J-TEXT tokamak

Abstract

Recently, the geodesic acoustic modes (GAMs) induced by negative electrode biasing (EB) are first observed in J-TEXT tokamak, and the electrostatic and magnetic fluctuation characteristics of GAM and ambient turbulence (AT) are measured by combined Langmuir-magnetic probe (CLMP). The experimental results show that with the ramped up negative biasing, the mean flow shear gradually strengthens, and excites a high-frequency GAM (H-GAM) and a low-frequency GAM (L-GAM) with frequencies of 24kHz and 17kHz respectively. With the combined effects of Er×B shear by EB and E ̃r×B shear by GAMs, the amplitude of AT is effectively suppressed, leading to an improvement in plasma confinement. This work investigated the relationship between AT, GAM zonal flows and Er×B mean flow shear, which provides proper mean flow shear can enhance the Reynolds Stress (RS) via enhancement of symmetry breaking and thus enhance the amplitude of GAM, but excessive flow shear can suppress both AT and GAM zonal flows.