Observation of Lower Hybrid Current Drive Improved Confinementwith a Graphite Probe at the Last Closed Flux Surface of the HT-7 Tokamak

High time resolution measurements of the electrostatic fluctuations, radial electric field E_r and turbulence-induced electron flux Γ_e have been performed across the transition of lower-hybrid-current-drive-improved confinement with a graphite Langmuir probe array at the last closed flux surface of HT-7 tokamak. The decrease of Γ_e is dominated by the suppression of fluctuation levels, which follows the change of E_r. A reversion of the poloidal propagation direction of turbulence demonstrates that the poloidal propagation is dominated by E_r x B_Ф drift. The enhancement of poloidal coherence accompanies the fluctuation suppression, which suggests the subtle variation of turbulence features.