Steady state operation of spherical tokamaks

For a fusion power plant to be economically and technologically attractive, it should be as compact as possible and capable of 'steady state' operation. One approach is based upon the spherical tokamak (ST) concept. This configuration features many of the qualities of the conventional aspect ratio tokamak, such as good confinement and MHD stability, together with a number of highly promising features for the realization of a cost effective, steady state fusion power core. In particular, it allows high beta capability due to the high I/B achievable at low A and strong, natural shaping of the configuration, together with the possibility of approaching 100% pressure driven currents through high natural elongation and access to second stability to ideal ballooning modes. The physics insights gained from experiments on START are discussed and the theoretical modelling work summarized, describing how the beneficial properties of STs can be combined in steady state ST power plant designs such as the Culham STPP concept. Where appropriate, we shall refer to the two new mega-amp machines MAST and NSTX, both designed for a plasma duration approaching the plasma current relaxation time and also other new, smaller, STs, designed to expand the knowledge gained so far.