Evolution of toroidal flow during, after mode locking

The response of the toroidal flow velocity to the abrupt locking of it in the vicinity of a magnetohydrodynamic (MHD) tearing-mode- induced magnetic island is examined analytically and numerically using a diffusive transport model in a cylindrical plasma model. The resultant toroidal momentum 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 edge regions of the plasma are evaluated and compared to experimental observations, The mode locking time scale itself is also investigated by using a model toroidal torque balance equation. The decrease of mode frequency during mode locking seems to be governed mainly by the electromagnetic torque exerted on the resistive layer. For tokamak plasmas, the mode locking time scale is found to be much shorter than the diffusion time scale, which is in general agreement with experimental observations.