The improved H-mode at ASDEX Upgrade:: a candidate for an ITER hybrid sceanario

A stationary regime with improved confinement (H-98(y,H-2) > 1) and, simultaneously, improved stability (beta(N) > 2.5) compared to standard H-mode has been investigated on ASDEX Upgrade for many years. This so-called 'improved H-mode' is characterized by a q-profile with low central magnetic shear and q(0) >= 1 that is obtained by early heating during the current ramp. Studies of this discharge scenario have been continued. New results are presented concerning the existence domain in the q(95) range, the dependence on the normalized Larmor radius rho(i)* and initial experiments showing that high performance improved H-modes can be obtained with strong central ion cyclotron resonance heating (ICRH). In addition, the present status of understanding of the improved H-mode is reviewed. Improved H-mode plasmas are documented for 3.2 < q(95) < 4.5 and n(e)/n(GW) up to 0.85. At low densities the normalized collisionality nu* is close to the ITER value and within the range of rho(i)*, accessible in ASDEX Upgrade, no dependence of the performance on this parameter was found. When compared with standard H-mode, core transport is still governed by drift-wave turbulence. Impurity control is achieved by central wave heating. The improved stability is due to the q-profile: by q(0) > 1 sawteeth as the main trigger of large amplitude neoclassical tearing modes are avoided. The stability is eventually limited by the occurrence of a mode with poloidal mode number m = 2 and toroidal mode number n = 1 at typically beta(N) similar to 3. As far as the reactor relevance of this regime is concerned, its compatibility with significant central electron heating by ICRH, with high edge densities and low amplitude edge localized modes is of importance. The improved H-mode is, therefore, seen as a candidate for a long pulse ITER 'hybrid' operation.