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

Abstract

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 D α light emission reveals that inter-ELM filaments are the lowest amplitude fluctuations in the MAST scrapeoff 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 I sat measurements reported in this paper are used to test the velocity scalings predicted by different theories.

Ion temperature measurements of L-mode filaments in MAST by retarding field energy analyser

“…This is a similar trend to that obtained from Langmuir probe measurements of inter-ELM filaments in MAST [6]. Figure 8 shows the large filament ion and electron temperatures measured in a 0.6MA low density plasma.…”

Multi-code analysis of scrape-off layer filament dynamics in MAST

“…In general, filaments in the SOL have a rather broad statistical distribution of sizes, amplitudes and velocities [10,11,35]. The filaments we selected are probably representative of the high amplitude, large size part of the population, as these features produce signals that are easier to detect in the camera images.…”

“…Filaments, or blobs as they are sometimes called, were observed experimentally in both linear and toroidal machines and are present in L-mode and in H-mode, both during and between ELMs [7][8][9][10][11]. These transient events play a central role in determining the SOL profiles as they form the building blocks of anomalous transport [12].…”

Recent progress on spherical torus research

“…93, where the fast camera images of edge turbulence can be obtained for the entire plasma. 276,277 The full view turbulence images of (a) inter-ELM, (b) L-mode, and (c) ELM phases from the same discharge are displayed, and (d) the corresponding intensity traces normalized to the peak ELM amplitude are also shown for the three cases. The number of filaments per fixed toroidal angle Du, which is indicative of the quasi-toroidal mode number, is the largest for the Lmode phase, followed by the inter-ELM, and lastly the ELM phase.…”