Recent progress on spherical torus research

The spherical torus or spherical tokamak (ST) is a member of the tokamak family with its aspect ratio (A = R-0/a) reduced to A similar to 1.5, well below the normal tokamak operating range of A >= 2.5. As the aspect ratio is reduced, the ideal tokamak beta beta (radio of plasma to magnetic pressure) stability limit increases rapidly, approximately as beta similar to 1/A. The plasma current it can sustain for a given edge safety factor q-95 also increases rapidly. Because of the above, as well as the natural elongation kappa, which makes its plasma shape appear spherical, the ST configuration can yield exceptionally high tokamak performance in a compact geometry. Due to its compactness and high performance, the ST configuration has various near term applications, including a compact fusion neutron source with low tritium consumption, in addition to its longer term goal of an attractive fusion energy power source. Since the start of the two mega-ampere class ST facilities in 2000, the National Spherical Torus Experiment in the United States and Mega Ampere Spherical Tokamak in UK, active ST research has been conducted worldwide. More than 16 ST research facilities operating during this period have achieved remarkable advances in all fusion science areas, involving fundamental fusion energy science as well as innovation. These results suggest exciting future prospects for ST research both near term and longer term. The present paper reviews the scientific progress made by the worldwide ST research community during this new mega-ampere-ST era. (C) 2015 AIP Publishing LLC.