Inter-ELM filaments and turbulent transport in the Mega-Amp Spherical Tokamak

Results on edge turbulence in periods separating edge localised modes (ELMs), i.e. inter-ELM periods, in Mega-Amp Spherical Tokamak (MAST) are presented. It is shown through combined measurements of fast camera images and reciprocating Langmuir probes that filamentary structures contribute to transport during these periods. Analysis of Dα light emission reveals that inter-ELM filaments are the lowest amplitude fluctuations in the MAST scrape-off layer (SOL) relative to L-mode and ELM filaments. Physical properties such as size, density and mode numbers have also been characterized, along with measurements of the spatio-temporal evolution: inter-ELM filaments are found to rotate in the vicinity of the last closed flux surface and propagate radially outwards. Motion of these filaments is found to depend strongly on plasma density such that with increasing density, there is an enhancement of the radial transport manifested by an increased number of filaments which leave the edge and travel faster into the SOL. Camera images show that intermittent fluctuations in ion saturation current signals correspond to inter-ELM filaments passing the probe. Measured radial e-folding lengths indicate larger decay lengths at higher densities. Similar trends are also obtained in simulations of a filament propagating radially and losing particles on ion parallel loss timescales. Finally, a discussion is presented on how the radial velocity and Isat measurements reported in this paper are used to test the velocity scalings predicted by different theories.