Study of anomalous inward drift in tokamaks by transport analysis and simulations

The origin of the anomalous inward drift is explored by transport analysis of Ohmic, L- and H-mode discharges in ASDEX Upgrade using a special version of the 1.5-D BALDUR transport code. It is shown that the anomalous particle pinch significantly affects the density profile, in contrast to the Ware pinch. The measured density profiles can be modelled by the anomalous inward drift velocity with Cv equal to 0.2 for H-mode and 1.1 for Ohmic plasmas and by a strongly rising vin/D near the edge. Here, x = ρ/ρw and xs = ρs/ρw with effective radii ρ, ρw and ρs of flux surface, wall contour and separatrix contour, respectively. At low densities, beam fuelling alone yields peaked density profiles. With increasing density the beam fuelling is shifted to the edge which causes the observed density flattening. Evaluation of measured electron density and temperature profiles in deuterium and hydrogen discharges with various heating schemes yields with the electron temperature gradient length. A semi-empirical scaling is set up and validated against ASDEX Upgrade, DIII-D, JET and ASDEX discharges. It is shown to work in the core and edge regions of Ohmic, L- and H-mode plasmas. The ratio vin/D is independent of density, plasma current, toroidal magnetic field, hydrogenic atomic mass number, collisionality and Zeff. The anomalous inward flux is driven by the square of the electron temperature gradient. Simulations of ITER using the new vin scaling predict peaked density profiles for gas puffed scenarios because of central heating due to alpha particles.