Local scrape-off layer control using biased electrodes in NSTX

Abstract

An experiment on the National Spherical Torus Experiment (NSTX) was designed to test the theory that biased electrodes can affect the local scrape-off layer (SOL) by creating a strong radial E x B flow (Cohen and Ryutov 1997 Nucl. Fusion 37 621). These electrodes were located near the outer midplane and were biased at up to +/-90V with respect to the local vacuum vessel ground. This biasing caused large changes in the local SOL profiles as measured by an array of Langmuir probes between the electrodes. A theory is presented which at least partially describes the experimental results.