The Spherical Tokamak Path to Fusion Power

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Abstract
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 ~1 m, similar size to the DIII-D tokamak) could be built that would produce ~800 MW(thermal), 200 MW(net electric) and would have a gain, defined as QPLANT = (gross electric power/recirculating power), of ~2. Such a device would have all the operating systems and features of a power plant 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 R3 to R4, depending on what is held constant) and can lead to 4-GW(thermal) power plants with QPLANT = 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-He3 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 of fusion 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 βT ~ 60% and 90% bootstrap current fraction (Ip ~ 15 MA, κ = 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.