Observation of SOL current correlated with MHD activity in NBI heated DIII-D tokamak discharges

Abstract

This work investigates the potential roles played by the scrape-off-layer current (SOLC) in MHD activity of tokamak plasmas, including effects on stability. SOLCs are found to be an integral part of the MHD activity, with a propensity to flow in a non-axisymmetric pattern and with magnitude potentially large enough to play a role in the MHD stability. Candidate mechanisms that can drive these SOLCs are identified: (a) non-axisymmetric thermoelectric potential, (b) electromotive force from MHD activity, and (c) flux swing, both toroidal and poloidal, of the plasma column. Other potential magnetic consequences of the SOLC are identified: (a) its error field can introduce complications in feedback control schemes for stabilizing MHD activity, and (b) its non-axisymmetric field can be falsely identified as an axisymmetric field by the tokamak control logic and in equilibrium reconstruction. The radial profile of an SOLC observed during a quiescent discharge period is determined, and found to possess polarity reversals as a function of radial distance.