Radial propagation of type-I ELMs on JET

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 similar to200+/-50 mus, the average radial propagation velocity of the ELM plasmoid in the scrape-off layer as <v(perpendicular to)> similar to 0.45+/-0.1 km s(-1) or as a fraction of the sound speed, <v(perpendicular to)/c(s)> similar to 0.2%, and its diffusion coefficient, D-perpendicular to, 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 sheath-limited model of plasmoid propagation.