“PROCESS”: Systems studies of spherical tokamaks

With a reduced aspect ratio, spherical tokamaks have a number of attractive features for a fusion power plant. This can be studied using systems codes, which allow for the rapid conceptual study of power plants covering everything, from the plasma through to electricity generation. In this paper, we describe models in the systems code PROCESS that have been added specifically for spherical tokamaks. Within PROCESS an alternative relation for the plasma current is included which accounts for the increased ratio of I_p/aB. We have tested this against a series of equilibria created with the free boundary equilibrium code FIESTA, and additionally performed our own fit. We also outline the engineering changes that can be made to the device and describe a water-cooled copper centrepost model. To test our models we recreate the published designs for the Fusion Nuclear Science Facility (FNSF) and a High Temperature Superconducting Pilot Plant (HTS-PP) and find good agreement. We conclude by highlighting the efficiencies needed to produce net electricity from small fusion devices.