ELM Filaments Research Articles

Experimental investigation into ELM filament formation on MAST

Radial profiles of electron temperature and density through type I ELM filaments have been obtained from a new edge Thomson scattering diagnostic at MAST. The lasers were fired in burst mode, 5 µs apart, to study profile evolution of a single filament as it moves toroidally past the laser beams. The plasma particle and energy loss due to each filament can be deduced from these profiles. As the filaments move out of the plasma, the ne pedestal is seen to collapse locally inwards by as much as 7.5% of the plasma minor radius. Insight into the toroidal structure of the perturbation has been obtained from high time resolution interferometry data. The interferometer data show excursions in line integral density through the midplane of the plasma occurring from 150 µs before the onset of the ELM particle loss. These excursions are due to the evolution of the spatial structure of the plasma during the ELM and indicate that the filaments may develop from broader structures. By combining the toroidal structure information from the interferometer and the radial structure information from the TS system with other diagnostic data on MAST, a two dimensional picture of the ELM phenomenon is obtained.

ELM-wall interaction on JET and ITER

Significant interaction of ELM filaments with the vessel wall is now observed on JET by several diagnostics. To interpret the experimental data, a model of ELM-filament evolution was developed, with the radial velocity taken from experiment and the decay of filament density and temperature calculated based on parallel losses to the divertor targets. The model was found to successfully reproduce radial e-folding lengths of density, electron temperature and energy, λnELM,λTeELM and λWELM, inferred from dedicated outer gap-scan experiment for moderate Type-I ELMs (ΔW/W∼5%, where W is the plasma stored energy). It also showed excellent agreement with observations of far SOL ELM ion energies on JET. For nominal ITER conditions (4keV pedestal, 5cm outer gap), the same model predicts that ∼8% of the ELM energy would be deposited on the main chamber wall (mostly on the upper X-point blanket modules), with the ions carrying most of this power and striking the wall tiles with an impact energy in excess of 1keV.

The dynamics and structure of edge-localized-modes in Alcator C-Mod

Characteristics of discrete ELMs produced in Alcator C-Mod discharges of low edge collisionality (0.2<ν∗<1) and large lower triangularity (δlower∼0.75) are examined. The energy lost per ELM from the H-mode pedestal is ∼10% of the pedestal energy. These ELMs exhibit relatively long-lived precursor oscillations, often with two modes of intermediate toroidal mode number present. At the ELM ‘crash’ multiple plasma filament structures are expelled into the scrape-off-layer. A short-lived high frequency (∼0.5MHz) magnetic oscillation is initiated at the ‘crash’. The initial ELM filaments are large perturbations to the SOL with radial extents of 0.5–1cm and typical radial propagation velocities of 1km/s. Velocities of up to 8km/s have been seen. The poloidal extent of the initial filaments is >4.5cm. The initial filaments are followed (at intervals of ∼100μs) by multiple, less perturbing secondary filaments.