Multi-cycle AC operation was firstly carried out in CT-6B Tokamak

and plasmas in different cycles were compared. The results show that although there are differences in the plasma-wall interactions in the two halves of each AC cycle

the purity of plasma is not significantly affected. The distribution of plasma current density was measured by a set of internal magnetic probe system. It was found that the plasma current is firstly reversed on the low field side

and the local positive and negative currents coexist as the net current passes zero

in which case the particles are efficiently confined by the magnetic surfaces.