Fast-ion transport in the presence of magnetic reconnection induced by sawtooth oscillations in ASDEX Upgrade

Abstract

The transport of beam-generated fast ions has been investigated experimentally at the ASDEX Upgrade tokamak in the presence of sawtooth crashes. After sawtooth crashes, phase space resolved fast-ion D-alpha measurements show a significant reduction of the central fast-ion density—more than 50%—together with an increase at larger radii. The corresponding loss of fast particles, detected by a fast-ion loss detector outside the plasma, is marginal during the crash phase, which proves that the sawteeth internally redistribute fast ions. This is also well reproduced by modelling results in which the fast-ion redistribution is assumed to be in line with the sawtooth-induced change of the magnetic field topology. The simulation, however, underestimates the total amount of redistributed fast ions which can be explained by additional drift effects. In the time intervals between the crashes, a neoclassical fast-ion behaviour is observed. Radial profiles and measured decay rates of the redistributed fast ions show that any anomalous fast-ion diffusion in between sawtooth crashes is well below 0.5 m2 s−1.