Spherical torus concept as power plants—the ARIES-ST study

Recent experimental achievements and theoretical studies have generated substantial interest in the spherical torus concept. The ARIES-ST study was undertaken as a national US effort to investigate the potential of the spherical tokamak concept as a fusion power plant. This 1000 MWe fusion power plant conceptual design has an aspect ratio of 1.6, a major radius of 3.2 m, a plasma elongation (at 95% flux surface) of 3.4 and triangularity of 0.64. This configuration attains a plasma βT of 50% (which is 90% of theoretical limit). While the plasma current is 28 MA, the almost perfect alignment of bootstrap and equilibrium current density profiles results in a current-drive power of only 28 MW. The on-axis toroidal field of 2.1 T and the peak field at the TF coil of 7.4 T led to 329 MW of Joule losses in the normal-conducting TF system. The power core uses an advanced ‘dual-cooled’ breeding blanket with flowing PbLi breeder and He-cooled ferritic steel structures that can achieve a thermal conversion efficiency of ∼45%. The ARIES-ST study has highlighted many areas where trade-off among physics and engineering systems are critical in determining the optimum regime of operation for ST power plants.