Dependence of the L–H power threshold on the alignment of external non-axisymmetric magnetic perturbations in ASDEX Upgrade

Abstract

Experiments at ASDEX Upgrade have 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- to H-mode (PLH). This is interesting for ITER, since its operation will rely on an MP field configuration that securely suppresses edge localized modes (ELMs) while avoiding an increase in the L–H power threshold. PLH can increase up to ≈80% when the external MP field is aligned to maximize the associated plasma response at the edge. This alignment deviates from the equilibrium field pitch and is similar to the one that is required to suppress ELMs by MPs in H-mode. The relative radial magnetic field perturbation δBr/BT has to exceed a critical value, roughly 1.7−2×10−4 (at the q = 5 surface), to cause an increase in PLH. This value is above the one that is required to sustain ELM suppression at ASDEX Upgrade. Combining plasma response calculations with a critical value of δBr/BT captures well the measured dependencies of PLH on the alignment of the applied MP field. The increase in PLH is accompanied by a change in the E×B flow velocity (vE×B) profile. The application of MPs in L-mode flattens and reverses the edge vE×B profile when at least δBr/BT≈1.9×10−4 is applied. This is the same value above which PLH increases. More heating power is needed to roughly achieve the same shear in the vE×B profile at the transition to H-mode as without MPs but with an vE×B profile that is shifted into the ion diamagnetic direction.