THE SPHERICAL TOKAMAK PATH TO FUSION POWER

The low-aspect-ratio tokamak or spherical torus (ST) approach offers t he two key elements needed to enable magnetic confinement fusion to ma ke the transition from a government-funded research program to the com mercial marketplace: a low-cost, low-power small-size market entry veh icle and a strong economy of scale in larger devices. Within the ST co ncept, a very small device (A 1.4, major radius similar to 1 m, simila r size to the DIII-D tokamak) could be built that would produce simila r to 800 MW(thermal), 200 MW(net electric) and would have a gain, defi ned as Q(PLANT) = (gross electric power/recirculating power), of simil ar to 2. Such a device would have all the operating systems and featur es of a powerplant and would therefore be acceptable as a pilot plant, even though the cost of electricity would not be competitive. The rat io of fusion power to copper toroidal field (TF) coil dissipation rise s 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 tok amak power plants. Large ST power plants might be able to burn the adv anced fuel D-He-3 if the copper TF coil is replaced by a superconducti ng TF coil and suitable shield. These elements of a commercialization strategy are of particular importance to the U.S. fusion program in wh ich any initial nongovernment financial participation demands a low-co st entry vehicle. The ability to pursue this line effusion development requires certain advances and demonstrations that are probable. Stabi lity calculations support a specific advantage of low aspect ratio in high beta that would allow simultaneously beta(T) similar to 60% and 9 0% bootstrap current fraction (I-p similar to 15 MA, kappa = 3). Stead y-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 c ritical design restriction.