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 (∼0.5 MHz) magnetic oscillation is initiated at the ‘crash’. The initial ELM filaments are large perturbations to the SOL with radial extents of 0.5–1 cm and typical radial propagation velocities of 1 km/s. Velocities of up to 8 km/s have been seen. The poloidal extent of the initial filaments is >4.5 cm. The initial filaments are followed (at intervals of ∼100 μs) by multiple, less perturbing secondary filaments.