ITER operation beyond its baseline scenario

Abstract

The ‘improved H-mode’ regime, realized in ASDEX Upgrade in 1998 and confirmed by other devices, demonstrates the integration of advanced performance beyond the standard H-mode for confinement (confinement enhancement factor H98(y,2) ⩽ 1.6), stability (normalized beta βN ∼ 3–3.5) and, at densities close to the Greenwald density, exhaust in stationary discharges longer than 40 confinement times or up to six resistive diffusion times. The q-profile is characterized by low central magnetic shear and axis safety factor q0 > 1 that is obtained by particular discharge and heating ramp-up scenarios and maintained via fishbones or benign higher (m, n) instabilities without using elaborate current control. 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 global confinement. A further contribution manifests itself by enhanced pressures at the edge barrier pedestal top and at the ρ = 0.9 surface both increasing with the input power. (3, 2) NTMs remain small, enabling routine operation up to βN ∼ 3 (limited by (2, 1) NTMs) at ITER relevant collisionalities, for normalized Larmor radii down to four times the ITER value and for a broad range of q95 = 3–5. Tailored heat deposition including central wave heating allows for a compromise in density peaking for enhanced confinement and tolerable high-Z impurity concentrations even with tungsten coated structures.As far as the ITER relevance of this regime is concerned, its compatibility with significant central electron heating, low collisionality and even densities close to the Greenwald density combined with type-II ELMs and βN ∼ 3.5 is of importance. The GLF23 turbulence model still predicts peaked density profiles (R/Ln ∼ 3) and sufficient transport to avoid impurity accumulation. At low q95 ∼ 3 the fusion performance in terms of is more than doubled compared with the ITER baseline scenario (performance factor ∼ 0.2) extrapolating to long Q ≫ 10 pulses on ITER. At medium q's bootstrap current fractions up to 50% and performance factors close to 0.2 can be achieved resulting in long inductive pulse lengths of ∼1 h allowing ITER ‘hybrid’ operation at Q ⩽ 9.