Session CO3 - NSTX and Other ST.
ORAL session, Monday afternoon, November 15
Room 204/205, SCC
A high performance regime with small edge-localized modes (ELMs) has been observed in the National Spherical Torus Experiment (NSTX). The ELMS (“type V”) are consistent with high bootstrap current operation with line average density approaching Greenwald scaling, and provide enhanced edge particle transport compared with ELM-free discharges. Each individual ELM has no measurable impact on stored energy, The ELM perturbation is observed via ultra-soft X-rays (USXR) to typically originate near the lower divertor region and propagate with a poloidal component toward the outer midplane and often to the top of the machine. An electromagnetic signature is observed before the USXR perturbation on separate poloidal and toroidal Mirnov arrays, indicating a toroidal mode number of n=1, propagation counter to the plasma current, and persistence for up to two toroidal transit times. These type V ELMs are distinct from both type I and type III ELMs, which have also been observed in NSTX, and can co-exist with type I ELMs. When the type V ELMS are present, there is no evidence of an edge harmonic oscillation as observed in DIII-D quiescent H-mode discharges, or of an edge quasi-coherent mode observed in the PDX forced density rise scenario and the C-MOD Enhanced D-alpha H-mode scenario.