The spherical tokamak path to fusion power

The low-aspect-ratio tokamak or spherical torus (ST) approach offers the two key elements needed to enable magnetic confinement fusion to make the transition from a government-funded research program to the commercial marketplace: a low-cost, low-power small-size market entry vehicle and a strong economy of scale in larger devices. Within the ST concept, a very small device (A 1.4, major radius similar to 1 m, similar size to the DIII-D tokamak) could be built that would produce similar to 800 MW(thermal), 200 MW(net electric) and would have a gain, defined as Q(PLANT) = (gross electric power/recirculating power), of similar to 2. Such a device would have all the operating systems and features of a powerplant and would therefore be acceptable as a pilot plant, even though the cost of electricity would not be competitive. The ratio of fusion power to copper toroidal field (TF) coil dissipation rises quickly with device size (like R-3 to R-4, depending on what is held constant) and can lead to 4-GW(thermal) power plants with Q(PLANT) = 4 to 5 but which remain a factor of 3 smaller than superconducting tokamak power plants. Large ST power plants might be able to burn the advanced fuel D-He-3 if the copper TF coil is replaced by a superconducting TF coil and suitable shield. These elements of a commercialization strategy are of particular importance to the U.S. fusion program in which any initial nongovernment financial participation demands a low-cost entry vehicle. The ability to pursue this line effusion development requires certain advances and demonstrations that are probable. Stability calculations support a specific advantage of low aspect ratio in high beta that would allow simultaneously beta(T) similar to 60% and 90% bootstrap current fraction (I-p similar to 15 MA, kappa = 3). Steady-state current drive requirements are then manageable. The high beta capability means the fusion power density can be so high that neutron wall loading at the blanket, rather than plasma physics, becomes the critical design restriction.