Development path of low aspect ratio tokamak power plants

Abstract

Recent advances in tokamak physics indicate the spherical tokamak may offer a magnetic fusion development path that can be started with a small size pilot plant and progress smoothly to larger power plants. Full calculations of stability to kink and ballooning modes show the possibility of greater than 50% beta toroidal with the normalized beta [ p~= p~/ ( h b ) ] as high as 10 and fully aligned 100% bootstrap current. Such beta values coupled with 2-3 T toroidal fields imply a pilot plant about the size of the present DEI-D tokamak could produce -800 MW thermal, 160 M W net electric, and would have a ratio of gross electric power over recirculating power (&PLANT) of L9. The high beta values in the ST mean that ExB shear stabilization of turbulence should be 10 times more effective in the ST than in present tokamaks, implying that the required high quality of confinement needed to support such high beta values will be obtained. The anticipated beta values are so high that the allowable neutron flux at the blanket sets the device size, not the physics constraints. The ST has a favorable size scaling so that at 2-3 times the pilot plant size the QPLANT rises to 4-5, an economic range and 4 GW thermal power plants result. Current drive power requirements for 10% of the plasma current are consistent with the plant efficiencies quoted. The unshielded copper centerpost should have an adequate lifetime against nuclear transmutation induced resistance change and the low voltage, high current power supplies needed for the 12 turn TF coil appear reasonable. The favorable size scaling of the ST and the high beta mean that in large sizes, if the copper TF coil is replaced with a superconducting TF coil and a shield, the advanced fuel D-He3 could be burned in a device with Q p~m -4. If the anticipated physics of the ST regime can be proven in near term experiments and engineering challenges (such as the high power density to be exhausted and centerpost neutronics issues) can be met, then the ST offers the possibility of a magnetic fusion development path with a minimal cost initial step and exciting further possibilities.

Recent progress on spherical torus research

“…This distinguishes a ST from other high b compact toroids such as the spheromak and field reversed configuration as noted in Appendix C. Earlier advanced ST configuration reactor studies with high bootstrap current fractions have motivated the ST fusion development path. [57][58][59][60] As the aspect ratio is reduced, there are a number of important changes in the device characteristics. Perhaps the most significant is the toroidal coil current I TF compared to the plasma current I p .…”

“…In Ref. 59, it was shown theoretically that the high b N regime for STs is indeed accessible through plasma shape and plasma profile control. Another insight for higher b N can be seen in Fig.…”

Spherical torus concept as power plants—the ARIES-ST study

“…It had been argued [7] that the resulting increase in fusion power density ( Â/b 2 B 4 ) more than compensates for inherently larger recirculating power fraction in a ST, leading to power plants that are more economical compared with advanced tokamak power plants with superconducting coils. Assuming advanced ST equilibria that require very little current-drive power, the optimization of ST naturally leads to a trade-off between the power density and the Joule losses in the TF coils.…”

Optimization of magnetic field system for glass spherical tokamak GLAST-III

“…A spherical tokamak (ST) is a magnetic confinement device whose aspect ratio (A=R/a) is equal to or less than 2, where 'R' is major radius and 'a' is minor radius of the tokamak. ST research is attractive due to natural elongation, high β (∝1/A) and being economical [1]. Although several results achieved on STs are specific to the spherical design, there are many features which are common between STs and conventional tokamaks [2,3].…”