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

摘要

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 v{sub}(in) = C{sub}v2xD/ρ{sub}w(x{sub}s){sup}2) with C{sub}v equal to 0.2 for H-mode and 1.1 for Ohmic plasmas and by a strongly rising v{sub}(in)/D near the edge. Here, x =ρ/ρ{sub}w and x{sub}s = ρ{sub}s/ρ{sub}w with effective radii ρ, ρ{sub}w and ρ{sub}s 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 C{sub}v α (L{sub}(T{sub}e)){sup}(-2) with L{sub}(T{sub}e) the electron temperature gradient length. A semi-empirical scaling v{sub}(in) = C{sub}t(ρ{sub}s/L{sub}(T{sub}e)){sup}2D/(ρ{sub}wx) 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 v{sub}(in)/D is independent of density, plasma current, toroidal magnetic field, hydrogenic atomic mass number, collisionality and Z{sub}(eff). The anomalous inward flux is driven by the square of the electron temperature gradient. Simulations of ITER using the new v{sub}(in) scaling predict peaked density profiles for gas puffed scenarios because of central heating due to alpha particles.