Fast visible imaging of ELM-wall interactions on JET

Abstract

We present and discuss recent measurements of the fast imaging diagnostic in JET. We focus on wide angle observations of few large ELM events (Δ W ELM ∼ 700 kJ) in plasmas with high current, magnetic field and stored energy (∼3 T, 3 MA, 8 MJ). Measurements were taken at recording speeds around 30 kHz. The ELM-wall interaction shows a filamentary structure that appears together with the radiation increase in the divertor, implying similar times of flight for ELM radial and parallel transport in the SOL, τ || ∼ τ r, within a time resolution of 30 μs. Two high and low triangularity cases are compared. For high triangularity the ELM-wall interaction is seen to start in the upper dump plate reaching the outer limiters some 70 μs later. In the low triangularity cases interaction generally starts in the outer wall but clear evidence of radiation increase in the upper dump plate is seen shortly after (30 μs) or together with the first filament impact on the outer limiters. A poloidal drift of the filament strike point on the limiters is sometimes visible which allows an estimation of the perpendicular filament velocity (∼1 km/s).