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

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 GAMs and ambient turbulence (AT) are measured by combined Langmuir-magnetic probe. Experimental results reveal that with the ramped up negative biasing, the mean flow shear gradually strengthens, and exciting a high-frequency GAM (H-GAM) and a low-frequency GAM (L-GAM) with frequencies of 24 kHz and 17 kHz respectively. With the combined effects of E r × 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 investigates the relationship between AT, GAM zonal flows and E r × B mean flow shear, which provides proper mean flow shear can enhance the Reynolds stress via enhancement of symmetry breaking and thus enhance the amplitude of GAMs, but excessive flow shear can simultaneously suppress AT and GAM zonal flows.