DIII–D Quiescent Double Barrier Regime Experiments and Modeling

Discharges characteristic of the quiescent double barrier (QDB) regime [1] are attractive for development of advanced tokamak (AT) scenarios relevant to fusion reactors [2] and they offer near term advantages for exploring and developing control techniques. We continue to explore the QDB regime in DIII–D to improve understanding of formation and control of these discharges and to explore scaling to steady-state reactors. The formation of an internal transport barrier (ITB) provides a naturally peaked core pressure profile. This peaking in density in combination with the H–mode-like edge barrier and pedestal provide a path to high performance. We have achieved βNH89P ~ 7 for several energy confinement times ( ≤ 25 τE). We discuss here a combination of modeling and experiments using electron cyclotron heating (ECH) and current drive (ECCD) to demonstrate steady state, current-driven equilibria and control of the current distribution, safety factor q, and density profile. Experimental conditions leading to formation of the QDB discharge require establishing two distinct and separated barrier regions, a core region near ρ~0.5 and an edge barrier outside ρ>0.95, ρ is the square root of toroidal flux (radial coordinate). A region of higher transport due to a change in polarity of the E×B shearing rate [1] separates the core barrie