Reciprocating probe measurements of ELM filaments on JET

The ELM filamentary structure in the JET scrape-off layer (SOL) has been investigated using a multi-pin reciprocating Langmuir probe system inserted at the top, low field side of the poloidal cross-section. These measurements clearly demonstrate that both Type I and III edge localized modes (ELMs) are composed of a number of filaments. The filament propagation in the SOL is investigated and the radial velocity estimated using different methods. The large radial velocities observed during Type I ELMs (up to 6 km s-1) imply that filaments have no time to drain significant energy along the field lines, reaching the wall with a reasonable fraction of the pedestal density and temperature. In spite of the time between Type I ELMs being much longer than the ELM duration, most of the particle flux to the main chamber plasma-facing components occurs in the latter period. Furthermore, filaments in Type I ELMs are significantly larger and faster than those observed in Type III ELMs. During Type I ELMs a strong parallel flow (M ∼ 0.4) is measured towards the inner divertor, which increases during filaments. These observations are in agreement with the ballooning nature of the ELM losses.