Challenges of locked mode avoidance in the low-ν* ITER baseline scenario plasmas in ASDEX upgrade

In AUG, the requirement to explore the low-ν* regime within the same ITER baseline scenario parameters, i.e. a Greenwald fraction of ∼0.85, q 95 ≈ 3, β N ≈ 1.8, and H 98 ≈ 1, has been addressed by inducing density pump-out through n = 2 magnetic field perturbations. This approach required additionally relaxing the high- δ plasma shape to δ ≈ 0.1 to enable sufficiently strong density pump-out. However, at low density also the generation of locked modes was observed which deteriorates the plasma performance. Several strategies were therefore tested, including model-based error field correction combined with different heating schemes, as well as a core–edge decoupling metric derived from the general perturbed equilibrium code. Nevertheless, these attempts were unsuccessful, largely because the gap between the onset of density pump-out and the excitation of the core mode is very small.