Figure 1 illustrates the different plasma aspect ratio of an spherical tokamak compared to a conventional tokamak.
Figure 1. Comparison of Spherical Tokamak shape with conventional Tokamak.
Two private companies and one public organization have plans for Spherical Tokamaks:
Tokamak Energy (TE) has built a desktop ST prototype, ST25, which operated in 2012. Another prototype ST40, with radius 40 cm, operated in early 2022 and now is being upgraded. Despite minimal available details on designs and results Tokamak Energy advertises a pilot power plant in the early 2030s.The interest of spherical tokamaks is that the modified plasma aspect ratio gives improved plasma confinement parameters compared to that of a conventional tokamak. Unfortunately, the changed aspect ratio presents engineering difficulties for the ST because of the smaller central bore diameter which leads to several practical problems. In a conventional tokamak, the central bore contains neutron shielding, breeder blanket, the toroidal magnet coils and the central solenoid. A spherical tokamak design must either eliminate these elements or make them much thinner.