Double transport barrier plasmas in alcator C-mod

摘要

Double transport barrier plasmas comprised of an edge enhanced D_α (EDA) H-mode pedestal and an internal transport barrier (ITB) have been observed in Alcator C-Mod. The ITB can be routinely produced in ICRF heated plasmas by locating the wave resonance off-axis near |r/a|～0.5, provided the target plasma average density is above - 1.4 x 10~20 m~-3, and can develop spontaneously in some Ohmic H-mode discharges. The formation of the barrier appears in conjunction with a decrease or reversal in the central (impurity) toroidal rotation velocity.