Development of an ITER relevant advanced scenario at ASDEX Upgrade

The "improved H -mode," realized in ASDEX Upgrade [A. Herrmann and O. Gruber, Fusion Sci. Technol. 44, 569 (2003)] in 1998, demonstrates that advanced requirements beyond the standard H -mode for confinement [confinement enhancement factor H98 (y,2) >1], stability (normalized beta ΒN ~3-3.5) and, at densities close to Greenwald density, exhaust can be simultaneously met and maintained stationary for several resistive diffusion times. The q profile is characterized by low central magnetic shear and axis safety factor q0 >1 that is obtained by particular heating and current ramp-up scenarios and maintained via benign instabilities. Core transport is still governed by drift-wave turbulence with stiff temperature profiles, but density profiles are more strongly peaked and contribute to the increase in confinement. Neoclassical tearing modes remain small, enabling routine operation up to ΒN ~3 at international thermonuclear experimental reactor (ITER) relevant collisionalities, for normalized Lamor radii down to four times the ITER value and for a broad range of q95 =3.2-4.5. Using tailored heat deposition including central wave heating a compromise was found in density peaking for enhanced confinement and limiting the high- Z impurity concentrations even with a tungsten-coated first wall and divertor. As far as the ITER [ITER EDA Documentation Series No. 24, 2002] relevance of this regime is concerned, its compatibility with significant central electron heating, high edge densities, and type-II edge localized modes is of importance. The GLF23 turbulence model predicts still peaked density profiles and sufficient transport to avoid impurity accumulation. The fusion performance in terms of ΒN H98 (y,2) q952 is nearly doubled compared with the ITER base-line scenario at low- q values, while at medium q 's bootstrap current fractions up to 50% and long inductive pulse lengths allow ITER "hybrid" operation.