Investigation of transport models in ASDEX Upgrade current ramps

Understanding the physics of transport during current ramps in tokamaks is essential in order to prepare adequate ramp-up and ramp-down scenarios for ITER, where constraints on the transformer flux and the internal inductance will be more stringent than in present-day devices. In this paper, five transport models (Coppi–Tang, Neo-Alcator, Bohm/gyro-Bohm, critical gradient model and H98/2 scaling-based model) are used to reproduce the experimental data during the current ramps of ASDEX Upgrade with its metallic wall. The calculated temperature profiles are compared with the experimental temperature profiles under different ramp-up conditions. Our study reveals important differences between boronized and non-boronized walls which are reproduced with variable success by the models. We also investigated ramp-up phases heated centrally by electron cyclotron heating which creates experimental conditions with peaked electron temperature profiles, which are very different from the usual ohmic heated cases. Our study reveals that the models react very differently to this additional heating. (paper)