Very high- and high-confinement mode limited discharges in DIII-D

The first observations of marginally limited very high confinement mode (VH-mode) discharges have been achieved in DIII-D [Nucl. Fusion Special Supplement: World Survey of Activities in Controlled Fusion Research (International Atomic Energy Agency, Vienna, 1990)] with significant reductions in peak heat flux conducted to plasma facing surfaces. In addition, quasistationary well limited high confinement-mode (H-mode) discharges have been obtained in DIII-D, also with reduced peak heat flux. This demonstration of reduced peak heat flux while maintaining high performance, i.e., high energy confinement time, can be important for the design of fusion ignition devices. Energy confinement enhancements in these high triangularity discharges are comparable to diverted discharges with similar parameters: τ_E/τ_ITER-89P=2.9 for VH-mode and τ_E/τ_ITER-89P=1.8 for quasistationary high confinement mode (H mode), where τ_ITER-89P is the empirically derived low confinement mode (L-mode) energy confinement scaling relation [Nucl. Fusion 30, 1999 (1990)]. Comparisons of the conducted heat flux, particle flux, and radiated power profiles show a shift toward the inner wall as the discharge configuration becomes more limited. In addition to the advantage of reduced peak heat flux in these limiter discharges, such configurations also allow more effective use of the internal vessel volume, providing the potential for higher performance, i.e., higher plasma current at a fixed safety factor, q₉₅.