The propagation of edge localized modes (ELMs) towards the main chamber could lead to potentially serious consequences in ITER (Federici G et al 2001 Nucl. Fusion 41 1967. This effect was studied in JET by examining the interaction of a large sample (>200) of Type-I ELMs with an array of Langmuir probes on the outboard limiter. High clearance discharges were used with a separatrix to wall distance of 8-18 cm. The average ELM duration was measured as ∼200 ± 50 µs, the average radial propagation velocity of the ELM plasmoid in the scrape-off layer as v ⊥ ∼ 0.45 ± 0.1 km s −1 or as a fraction of the sound speed, v ⊥ /c s ∼ 0.2%, and its diffusion coefficient, D ⊥ , as 500 ± 100 m 2 s −1 -a significant increase over inter-ELM levels. The above velocities are in good agreement with the lower ( loss) limit of the sheathlimited model of plasmoid propagation (

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