Observation of a high performance operating regime with small edge-localized modes in the National Spherical Torus Experiment

We report the observation of a high performance scenario in the National Spherical Torus Experiment with very small edge-localized modes (ELMs). These ELMs, individually, have no measurable impact on the stored energy and are observed by several diagnostics. The small ELMs have clear differences as compared with the ELM types reported in the literature, and this operating mode has distinct features compared with other high performance tokamak scenarios with little or no ELMs. The ELM is termed as 'type V', and it has a short-lived n=1 magnetic precursor oscillation rotating counter to the plasma current and a distinct signature on the soft x-ray system. If we could extrapolate it, this scenario would provide an attractive operating regime for next step fusion experiments. (author)