Double transport barrier plasmas in Alcator C-Mod

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 × 10²⁰ 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. The ITB foot is located near r/a ∼ 0.5, regardless of how the barrier was produced. The ITBs can persist for ∼ 15 energy confinement times (τ_E), but exhibit a continuous increase of the central electron density, up to values near 1 × 10²¹ m^-3 (in the absence of an internal particle source), followed by collapse of the barrier. Application of additional on-axis ICRF heating arrests the density and impurity peaking, which occurs along with an increase (co-current) in the core rotation velocity. Quasi-steady state double barrier plasmas have been maintained for 10τ_E or longer, with a bootstrap fraction of 0.13 near the ITB foot.