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' operatio n. One approach is based upon the spherical tokamak (ST) concept. This conf iguration features many of the qualities of the conventional aspect ratio t okamak, such as good confinement and MHD stability, together with a number of highly promising features for the realization of a cost effective, stead y state fusion power core. In particular, it allows high beta capability du e to the high I/B achievable at low A and strong, natural shaping of the co nfiguration, together with the possibility of approaching 100% pressure dri ven 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, describ ing 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 al so other new, smaller, STs, designed to expand the knowledge gained so far.