Experimental Phenomena of Improved Ohmic Confinement Induced by Modulated Toroidal Current on the HT-7 Tokamak

The phenomena of improved Ohmic confinement have been observed during modulation of toroidal current on the Hefei superconducting Tokamak-7 (HT-7). In the experiment, the programming ohmic heating field was modulated. A toroidal frequency-modulated current induced by modulated loop voltage was added on the plasma equilibrium current. The ratio of ac amplitude of the plasma current to the main plasma current is about 12%--30%. These improved plasma confinement phenomena include the facts that the average electron density and the central electron temperature both increase, the D_α radiation from the edge is reduced, the magnetichydrodynamics is obviously suppressed by oscillating plasma current, and the global energy confinement time increases by 27%-45%. It is found that the faster the modulation is, the more effective the improved ohmic confinement phase.