Langmuir-magnetic probe measurements of ELMs and dithering cycles in the EAST tokamak

Measurements of the dynamical behavior associated with edge localized modes (ELMs) have been carried out in the Experimental Advanced Superconducting Tokamak (EAST) by direct probing near the separatrix and far scrape-off layer (SOL) using electrostatic as well as magnetic probes. Type-III ELMs and dithering cycles have been investigated near the threshold power for the transition from the low confinement mode (L-mode) to the high confinement mode (H-mode).

A precursor is observed prior to type-III ELM events with chirping frequency (130–70 kHz). It is located inside the separatrix and does not lead to considerable particle transport into the SOL. Distinct from type-III ELMs, no precursor modes precede the dithering cycles. It is evident from our measurements that the absence of precursor activity is a good indicator to distinguish the dithering cycles from type-III ELMs.

A number of distinct current filaments are identified slightly inside the separatrix, both during type-III ELM events and dithering cycles. The characteristic current topology in these filaments is still ambiguous in our investigations. Furthermore, small ELMs are observed in type-I ELMy-like H-mode discharge regimes on EAST, in which solitary monopolar current filaments are observed to propagate in the SOL.