Dependence of the L-H power threshold on the alignment of externally applied non-axisymmetric MPs

A series of experiments at ASDEX Upgrade (AUG) has been conducted to study the impact of the alignment of external magnetic perturbations (MPs) with n = 2 toroidal mode symmetry on the power threshold of the transition from L-mode to H-mode (L-H transition). This is interesting for ITER, since its operation will rely on an MP-field configuration that is expected to solidly suppress edge localised modes (ELMs) while avoiding an increase in the L-H power threshold (PLH).

Experiments at AUG show that PLH increases by up to ≈ 80% when the external n = 2 MP field is aligned to maximise the associated plasma response at the edge. This alignment differs from the equilibrium field pitch and is the same needed in H-mode to suppress ELMs by MPs. The relative perturbation amplitude δBr/B has to surpass a critical value, roughly 2×10−4, to cause an increase in PLH. This is above the value needed to sustain ELM suppression at AUG and is favourable for ITER. State-of-the-art diagnostics probing the edge E × B velocity measure a reversal and flattening during the application of MPs. More heating power is needed to get roughly the same shear in the edge ExB velocity profile as without MPs, but with an ExB velocity profile elevated in the ion diamagnetic direction.