EFFECT OF MODE-LOCKING ON TOROIDAL FLOW EVOLUTION

The response of the toroidal flow velocity to being abruptly locked in the vicinity of a magnetohydrodynamic (MHD) tearing-mode-induced magn etic island is examined analytically and numerically using a diffusive transport model in a cylindrical plasma. The resultant toroidal momen tum confinement is shown to degrade significantly after mode locking, as is often observed on many tokamaks and reversed field pinches (RFPs ). The degraded toroidal momentum confinement time in the core and edg e regions of the plasma are evaluated and compared with experimental o bservations. The mode Locking time-scale itself is also investigated b y using a model toroidal torque balance equation. The decrease of mode frequency during mode locking seems to be governed mainly by the elec tromagnetic torque exerted on the resistive layer. For tokamak plasmas , the mode locking time-scale is found to be much shorter than the dif fusion time-scale, which is in general agreement with experimental obs ervations.