Highly resolved measurements of periodic radial electric field and associated relaxations in edge biasing experiments

Abstract

High time-space resolved measurements of the radial electric field Er and plasma rotations have been performed during edge biasing experiments in the CASTOR tokamak. During polarization, edge sheared Er×B flow is routinely generated, triggering a transition to a global improved confinement and a formation of an edge transport barrier (ETB). Furthermore, on top of the biasing-imposed DC Er, we observed, for the first time, concurrent fast periodic oscillations (dubbed as relaxations) on Er, plasma rotations, edge recycling and the ETB as well. Although the global confinement improvements are not much affected by the relaxation event, the local edge plasma parameters and the ETB are substantially modulated during the oscillating phases. Moreover, throughout the relaxation period a possible link between the modulated radial transport and the toroidal plasma flow is found. The results support the paradigm of the nonlinear dynamical coupling and energy transfer between the turbulence eddies and plasma flows.