Prospects for Pilot Plants based on the Tokamak, ST, and Stellarator

Full text: A potentially attractive next-step towards fusion commercialization is a 'pilot plant', i.e. a device which produces net electricity as quickly as possible and in as small a facility as possible in a configuration directly scalable to a power plant. The pilot plant approach could accelerate the commercialization of magnetic fusion by both demonstrating net electricity production while also carrying forward a high neutron fluence component testing mission needed to ultimately achieve high availability in fusion systems. This paper will explore three configurations for a pilot plant: the advanced tokamak (AT), spherical tokamak (ST), and compact stellarator (CS). These configurations are considered because: the tokamak presently has the most well-developed physics basis, the ST offers simplified maintenance schemes and reduced size and cost, and the CS offers disruption-free operation with low recirculating power. Several noteworthy trends have been found. First, AT pilot plant scenarios have been identified with field, current, normalized beta and fusion power similar to proposed ITER fully non-inductive scenarios but with reduced H98 and in a device 30% smaller in major radius and requiring advanced magnet technology with increased average TF current density. For the ST, fusion power and Q required are roughly 2 times the AT values with similar dependence on thermal conversion efficiency. Not unexpectedly, the ST has the smallest major radius and highest average neutron wall loading of all configurations assessed with wall loading values at or above those previously proposed for nuclear component testing. Finally, for the CS, to minimize device size, increase neutron wall loading, and utilize physics assumptions closest to the tokamak, a quasi-axisymmetric (QAS) design with low aspect ratio = 4.5 is considered. CS scenarios with fusion power ∼500 MW similar to the AT but with 5 times higher fusion Q and engineering Q up to 2.6 have been identified resulting from elimination of external current drive power. Given the importance of high thermal efficiency blankets, pilot plants should arguably be designed to incorporate progressively more advanced blankets over the course of operation. Improved magnet technology is also critical for all configurations. These and other issues will be discussed. (author)