The fine structure of ELMs in the scrape-off layer

Abstract

Edge-localized modes (ELMs) have been observed by diagnostic systems with high spatial and temporal resolution in the ASDEX and JET tokamaks. During the ELMs, substructures exist in the scrape-off layer (SOL) with sizes of 2-5 cm and lifetimes of the order of 20-50 µs. Their poloidal-temporal evolution is directly observed, and the radial transport due to turbulent E × B drift in these substructures is estimated. A comparison is made between 'normal' fluctuations between ELMs and these substructures in terms of poloidal size and velocity and related radial transport. The increased radial transport during ELMs is shown to be mainly due to these substructures. The observed fluctuation amplitudes and velocities of the substructures are found to be compatible with the model of radial transport by plasma 'blobs' and plasma potential gradients generated by the sheath boundary conditions. The transport due to turbulent radial E × B flows is compared during and between ELMs, and the observation of ELM substructures in the main plasma SOL is put into context with the observation of substructures on the target plates.

Intermittent transport in the JET far-SOL

“…High temporal resolution measurements reveal that the ELM has a complex internal structure, being composed of a number of filaments [6][7][8][9][10]. evolution of the parallel ion flux, J sat , the radial velocity, v r , and the time integrated turbulent particle flux during a small Type I ELM event (DW $ 50 kJ) measured at r À r sep % 4 cm (#58641).…”

A Probe Head for Simultaneous Measurements of Electrostatic and Magnetic Fluctuations in ASDEX Upgrade Edge Plasma

“…In present-day divertor tokamaks (which are capable of the H-mode) probes can solely be used on reciprocating probe drives and are mainly restricted to the scrape-off layer (SOL). Nevertheless, probes have up to now brought an overwhelming wealth of valuable data and insight into transport phenomena [4][5][6][7][8][9].…”

Engineering design for the magnetic diagnostics of Wendelstein 7-X

“…If we consider an ELM, it was shown that it consits of filamentary substructures with a size of few centimetres perpendicular to the magnetic field (see, e. g., [48]). In these "blobs", currents of 10-20 A parallel to the magnetic field are expected for a size of 2 cm ⊥ B.…”