The poloidal distribution of type-III edge localized modes in the Mega-Ampère spherical tokamak (MAST)

Abstract

This article describes the poloidal plasma particle distribution of type-III edge localized modes (ELMs) in the Mega-Ampère spherical tokamak [R.-J. Akers et al., Phys. Plasmas 9, 3919 (2002)]. A fast imaging camera with 10μs exposure time is used to record the Dα light coming from the entire poloidal cross section. Furthermore, three sets of probes, triggered at the same time, acquired at 1MHz, and located at different poloidal, radial, and toroidal locations in the tokamak are used. ELMs are observed to affect the Dα emission throughout the low-field scrape-off layer; on the high-field side, however, this effect is found to be small. The results obtained by imaging agree with the pointwise measurements using Langmuir probes. The radial propagation is shown to occur at a speed of 250m∕s, whereas the toroidal convection from the top to the bottom of the plasma is shown to be consistent with a transport at the local sound speed. Strong correlation amplitudes are reported among the probes that are poloidally and toroidally separated by several meters. The study of the cross-correlation coefficients as a function of the frequency indicates that this correlation is caused by the low-frequency component of the signal and that the high-frequency part is not correlated. Consequently, the filamentary structures are interpreted as caused by the onset of turbulence during an ELM and do not constitute the ELM itself.