Plasma Physics Seminar ( Phys 769)

Dr. Rajesh Maingi, PPPL/Oak Ridge National Laboratory

Characteristics of Small ELMs in NSTX

There has been a substantial international research effort in the fusion community to identify tokamak operating regimes with either small or no periodic bursts of particles and power from the edge plasma, known as edge localized modes (ELMs). These studies are critical for confidence in the projected operational scenarios of the International Thermonuclear Experimental Reactor (ITER), which will require small or no ELM regimes for divertor integrity. While several candidate regimes have been presented in the literature, very little has been published on the characteristics of the small ELMs themselves. One such small ELM regime, also known as the Type V ELM regime, has been identified in the National Spherical Torus Experiment(NSTX). In this talk, the characteristics of the Type V ELMs is contrasted with those of other ELM types observed in many tokamaks. The composite picture of the Type V ELM is of an instability with one or two filaments that rotate toroidally at ~ 5-10 km/sec, in the direction opposite to the plasma current and neutral beam injection. The toroidal extent of Type V ELMs is typically ~5m, whereas the cross-field (radial) extent is typically ~ 10 cm (3 cm), yielding a portrait of an electromagnetic, ribbon-like perturbation aligned with the total magnetic field. The filaments comprising the Type V ELM appear to be destabilized near the top of the H-mode pedestal and drift radially outward as they rotate toroidally. After the filaments come in contact with the open field lines, the divertor plasma perturbations are qualitatively similar to other ELM Types, albeit with only one or two filaments in the Type V ELM vs. more filaments for Type I or Type III ELMs. Preliminary stability calculations eliminate pressure driven modes as the underlying instability for Type V ELMs, and work is continuing to determine if current driven modes are responsible for destabilization.

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