Neoclassical tearing modes on AUG: Improved scaling laws, high confinement at high βN and new stabilization experiments

The accuracy of the scaling laws derived so far for the normalised beta values at the onset of neoclassical tearing modes is limited as the results depend on the presence and magnitude of seed islands. Therefore power ramp down experiments have been performed on ASDEX Upgrade, allowing to find a scaling law for the critical β value at which the NTMs disappear. For (m,n)=(3,2) NTMs these critical beta values have been found to scale nearly proportional to ρ*. As it has been recently found on ASDEX Upgrade, at high βN values there is a regime in which (3,2) NTMs cause a much smaller confinement degradation as one would expect from the β dependence of the saturated island size. The transition to this regime allows high confinement (H=1) at high beta values (βN>2.3) on ASDEX Upgrade in spite of the presence of (3,2) NTMs. The plasma conditions for the transition into such a high confinement regime are discussed in detail. Furthermore, new results on NTM stabilization by localized ECCD will be presented, showing that NTMs remain stabilized even with increased heating power and thus normalized beta values well above the NTM threshold. (author)