Transport in high normalized beta discharges on ASDEX Upgrade

An advanced high confinement mode scenario with normalized beta (βN) up to 3.5 in stationary conditions (up to 20 energy confinement times) and βN=3.8 transiently, has been obtained in ASDEX Upgrade. For transport analysis of high βN discharges, two types of simulations have been performed with the automated system for transport analysis code. One is the simulation of the current density profile with experimental data assuming neoclassical electrical conductivity. The other is the simulation of the energy transport and the current density profile using the Weiland transport model (ion temperature gradient and trapped electron mode limit the temperature gradient length). The comparison of the simulations of the q-profiles and temperature profiles with the experimental profiles shows good agreement and demonstrates that for this plasmas regime the temperature profiles are stiff (no internal transport barrier) as in conventional H-mode discharges. The contribution of the Ohmic, bootstrap and neutral beam driven current to the total current density profiles is calculated. The sum of the non-inductively driven plasma current is in the range 50-57% of the total plasma current. Simulations with the Weiland model are performed to investigate the influence of neoclassical tearing modes, which limit confinement and achievable βN in this scenario, by comparing with the measured kinetic data. In addition, the high βN discharges are compared to the improved H-mode discharges in ASDEX Upgrade with respect to the profile stiffness, MHD activity and experimental conditions. (author)