The origin of the reduced electron heat diffusivity χe and the diffusion coefficient D in the core of H-mode plasmas is explored. By statistical analyses of 38 H-mode discharges in ASDEX it is shown that in the L-and H-regimes the electron temperature profile shape is invariant in the outer half of the plasma. On the basis of this profile invariance and the edge temperatures in the L-and H-phases the magnitude, the weak radial dependence and the short time-scale of the reduction of χe in the bulk plasma are explained. The constancy of the Te profile shape yields a nonlocal contribution to the anomalous transport and accounts for the regime dependent factors in the empirical scalings of χe and D used up to now. It is further shown that Te profile invariance can explain the power dependence of χe and of the energy confinement time in the intermediate regime between Ohmic heating (OH) and pure L-mode confinement and the delayed L-OH transition after neutral beam injection.
Transport analysis of the observed Te profile invariance in ASDEX