MHD limits and plasma response in high-beta hybrid operations in ASDEX Upgrade

The improved H-mode scenario (or high beta hybrid operations) is one of the main candidates for high-fusion performance tokamak operation that offers a potential steady-state scenario. In this case, the normalized pressure beta(N) must be maximized and pressure-driven instabilities will limit the plasma performance. These instabilities could have either resistive ((m = 2, n = 1) and (3,2) neoclassical tearing modes (NTMs)) or ideal character (n = 1 ideal kink mode). In ASDEX Upgrade (AUG), the first limit for maximum achievable beta(N) is set by the NTMs. The application of pre-emptive electron cyclotron current drive at the q = 2 and q = 1.5 resonant surfaces reduces this problem, so that higher values of beta(N) can be reached. AUG experiments have shown that, in spite of the fact that hybrids are mainly limited by NTMs, the proximity to the no-wall limit leads to amplification of the external fields that strongly influence the plasma profiles. For example, rotation braking is observed throughout the plasma and peaks in the core. In this situation, even small external fields are amplified and their effect becomes visible. To quantify these effects, the plasma response to the magnetic fields produced by B-coils is measured as beta(N) approaches the no-wall limit. These experiments and corresponding modeling allow the identification of the main limiting factors, which depend on the stabilizing influence of the conducting components facing the plasma surface, the existence of external actuators, and the kinetic interaction between the plasma and the marginally stable ideal modes. Analysis of the plasma reaction to external perturbations allowed us to identify optimal correction currents for compensating the intrinsic error field in the device. Such correction, together with the analysis of kinetic effects, will help to increase beta(N) further in future experiments.