Toroidal field coil design for the spherical tokamak power plant

The design of the toroidal field (TF) coil system of a spherical tokamak (ST) power plant has considerable influence on the design and performance of the entire plant due mainly to its electrical power requirements which are dominated by the centre rod conductor. The low aspect ratio and high TF coil current combined with the effects of neutron irradiation on the structures lead to compromises in the centre column dimensions and choice of materials. Since the electrical conductivity of metals increases markedly with falling temperatures this offers a route for reducing the resistive power dissipation in the coils. However the coils now need refrigerating which also consumes power which adds to the total dissipation. The optimum for a water cooled copper centre conductor appears to be around room temperature and the optimum for a helium cooled aluminium conductor appears to be around 30 K. Centre column and TF coil designs based on these two options are being developed and the current status of the designs is presented here. The water cooled copper option uses a dispersion strengthened copper alloy for the centre rod with minimal shielding giving a simple design. The cryogenically cooled option uses pure aluminium and offers advantages in terms of power consumption but requires significant shielding and more complex structural supports.