DIII-D Experiments and Modeling of Core Confinement in Quiescent Double Barrier Plasmas

QDB-regime plasmas exhibit both a core transport barrier and a quiescent, H-mode edge barrier. These near steady plasma conditions have been run in excess of 4 s (>30 τ_E) and exhibit moderate performance with β_N ∼ 2.5. Recent experiments have been directed at expanding the operating parameters and control of current and pressure profiles. Modeling codes have been used to predict the effects of electron cyclotron heating and current drive (ECCD) on the q-profile. Experiments based on this modeling showed significant modification to the q-profile evolution. The injection of EC power inside the barrier region also reduced the density peaking from n_e(0)/< n_e> = 2.1 to 1.5 and decreased both the core carbon and high-Z impurities. These QDB experiments indicate the potential for sustaining high quality H-mode performance with the transient edge localized modes (ELMs) replaced by a more continuous control of edge pressure achieved with the edge harmonic oscillation (EHO).