Characteristics of I-phase bursts and their identity to type-III ELMs*

In tokamaks, a regime of regular kHz limit cycle oscillations, the so-called LOCs or bursts, is often observed in the parameter range between L-mode and the fully developed H-mode. It is known under various names as I-phase at ASDEX Upgrade (AUG), M-mode at JET, or IM mode at DIII-D. The main characteristic of this regime is the periodic flattening of temperature and density profiles at the plasma edge region, which is perceived as bursts. Utilizing the high spatiotemporal resolution of the AUG He-beam and the TCV GPI diagnostic, we see the burst as the result of periodically increased transport at the plasma edge, caused by a short-living high-frequency mode. This mode is localized in the confined region close to the separatrix and appears with a frequency of ∼ 50kHz at AUG and ∼ 200kHz at TCV. It is thought to be driven by steep plasma edge gradients. The mode occurrence is connected to each individual burst and vanishes at the maximum impact of each burst, i.e., when the edge pressure gradient is maximally flattened. Once the mode disappears, profile recovery starts, and the next cycle begins. Our study presents these dynamic processes during I-phase and highlights the continuous transition from regular I-phase bursts in the parameter range close to L-mode to more intermittent high-amplitude bursts. These are identified as type-III ELMs and occur deeper in the H-mode regime. The underlying mode and the resulting bursts, however, always seem to be of the same nature, so we conclude the identity of I-phase bursts and type-III ELMs.