Asymmetric edge biasing for SOL control in NSTX

In theory, the SOL strike location at a divertor plate can be actively controlled using local convective cells created by electrostatic biasing. An experiment was done on NSTX to test this idea using a small set of electrodes in the far-SOL near the outer midplane. Significant changes were observed in the local density profile near these electrodes when they were positively (but not negatively) biased with respect to the vessel ground. This was expected from the theory, as was the observation that sign of these changes reversed with the direction of the local radial ExB drift. Biasing one electrode with respect to another drew a significantly lower current, as expected, but produced a smaller density change. The SOL turbulence motion was also viewed using the GPI diagnostic; however, little convective cell motion was detected $\sim $1 m along B from these electrodes. A planned upgrade to allow the biasing of small electrodes at the divertor plate of NSTX will be described. This work was supported by USDOE Contract DE-AC02-76CHO3073.