Effect of single-legged coil on 3D plasma boundary corrugation in EAST

A single-legged coil behind the lower divertor and covering a 120° toroidal angle is utilized in a recent EAST discharge, for the purpose of increasing the wetted area of the divertor surface by locally modifying the magnetic field near the X-point. The plasma response, in particular, the plasma boundary surface corrugation due to the single-legged coil current, is modeled by the updated MARS-F code, by computing the plasma displacement for all important toroidal harmonics (n = 1, 2, 4 and 5) associated with the partial toroidal coverage by the coil. The plasma response produced by the single-legged coil is found to be non-local and is of the kink-peeling type. For a reference EAST plasma with a lower single-null magnetic configuration, the plasma boundary corrugation near the X-point, produced by the upper single-legged coil, is about twice as large as that produced by the lower single-legged coil, despite the proximity of the latter to the X-point.