Structure of MARFEs and ELMs in NSTX

The fast-evolving structure of MARFEs and ELMs is observed in the National Spherical Torus Experiment (NSTX) using a fast-framing camera. It is shown that Type V ELMs, the small ELM regime in NSTX, is characterized by a poloidally propagating ionization front close to the separatrix position. The ELMs also interact with the main chamber MARFE observed in NSTX coupling its movement and partial burn-through with that of the ELM cycle. During this cycle a toroidally localized MARFE precursor is seen propagating upward along field lines at speed of the order of 15 km/s. This interaction between ELM activity and MARFEs may be responsible for slow heat pulse propagation speeds seen in the divertor region.

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