The close correlation between the edge electrostatic fluctuations around the last closed flux surface and the magnetohydrodynamic (MHD) activity in the case of lower hybrid current drive (LHCD) is observed and analysed in the HT-7 tokamak. The turbulent transport is primarily driven by a coherent MHD mode, indicating that the electrostatic fluctuations are strongly correlated with MHD activity. The results supply direct evidence that the edge electrostatic fluctuations in tokamak plasmas can be decreased by suppressing internal tearing mode activity and thus reduce the radial plasma transport. The coherent electrostatic fluctuations may be possibly attributable to the radial plasma transport accompanied by the internal tearing mode activity. Studies show that the smaller electrostatic fluctuations at a frequency of 10 kHz may be mainly ascribed to the suppressed MHD activity, possibly due to the formation of an internal transport barrier and flow shear in the edge region. It is speculated that the change of mode region resulting from the mode coupling/transform, excited by the modified current profile by LHCD, could also provide some contribution to the MHD reduction.
MHD instabilities and their effects on plasma confinement in Large Helical Device plasmas