High-beta spherical tokamak research achieving the second stability

Recently, high-beta operations of Spherical Tokamak (ST) have received increased attention due to the compatibility achieved between high-beta and a long confinement time. In addition to the high normalized current I/aB achievable at a low aspect ratio, an important advantage of high-beta ST is its direct access to second stability for ideal ballooning modes. The mega-ampere class experiments. NSTX and MAST, renewed the record beta values up to βT - 35%, increasing confinement time up to τE - 0.1 sec. Small-scale experiments produced the second-stable STs with diamagnetic toroidal field/absolute minimum-B by transforming oblate FRCs. Major issues for the second stable STs are achieving a stable startup/profile control for kink and the ballooning stabilities and a concrete approach to high-ratio pressure driven currents. (author)