Poloidal localization of the explosive onset of edge localized modes

Abstract

In ASDEX Upgrade lower single null H-mode plasmas, the onset of the explosive phase of Edge Localized Modes (ELMs) is poloidally localized in the X-point/outer strike point (X/OSP) region. From this location, ELMs develop on average towards the low-field and high-field sides in typically 100 µs. An associated magnetic activity localized in the X/OSP region is also observed in-between ELMs, at a lower level than at the ELM onset, and interpreted as being due to perturbed currents connected to the divertor target-s. Its broadband spectra typically extend up to 100 kHz–due to short-lived events of variable frequencies–and are dominated by n = 1 toroidal mode numbers rotating in the counter-current direction.