Spherical tokamaks are tokamaks with very small aspect ratio, which is the ratio of major radius to minor radius. They are fat doughnuts with a very small hole in the middle. These are hard to make, but they have advantages in stability. They are described in Chap. 10. Peng et al. [25] have designed a fusion development facility using a spherical tokamak (FDF-ST) with an aspect ratio of 1.5. This is shown in Fig. 9.33. The magnetic coils are normal-conducting copper, even the narrow center leg going through the small central hole. With major radius only 1.2 m, the machine is much smaller than other designs and yet can generate a neutron wall loading of 1.0 or even 2.0 MW/m2. The toroidal field is 1.2 T, and the plasma current is

8.2 MA. The fusion power is only 7.5 MW or 2.5 times the input power. The machine can accommodate 66 m2 of blanket area. If this can be engineered, it would be the least costly nuclear test facility to prepare for DEMO.