Momentum transport from tearing modes in the mst reversed field pinch

Abstract

Summary form only given. Transport of parallel momentum during reconnection events has been investigated in the Madison Symmetric Torus reversed field pinch (MST RFP). The events are characterized by a sudden increase of resistive tearing magnetic fluctuations and change in the plasma rotation. The parallel plasma velocity abruptly decreases in the core and speeds up in the edge which results in the flattening of the parallel momentum profile. The parallel velocity in the core is reconstructed from the poloidal velocity of bulk plasma measured with the Rutherford scattering diagnostic and the toroidal phase velocity of the core resonant resistive tearing modes measured with an edge array of magnetic pickup coils. The edge flow is measured by the Mach probe. The transport of parallel momentum can be understood within the framework of two-fluid turbulent relaxation theory and from detailed MHD calculations of fluctuation induced Maxwell and Reynolds stresses resulting from multiple tearing modes. Previous measurements show the Maxwell stress to be a factor of ten larger than either the inertial or the viscous terms in the momentum balance equation both in the edge and in the core. Recently, measurements of the Reynolds stress have been performed in the edge plasma of MST. The Reynolds stress is shown to balance the Maxwell stress and the difference of the two stresses is approximately equal to the rate of change of plasma momentum. Reduction of magnetic fluctuations, achieved by an inductive control of the plasma current profile, results in a significant decrease of the momentum transport in agreement with previously observed reduction of particle and energy transport. This was established by transiently applying an external torque to the plasma edge with biased electrodes. Application of the edge torque during improved confinement induces a fast edge rotation, but the change in the core rotation remains small, which is an indication of a reduced inward momentum transport.