page_title: Spherical tokamak
section_title: Design
hierarchical_section_title: Spherical tokamak / Design
caption_attribution_description: English: Left: engineering drawing of the ST40 tokamak of Tokamak Energy Ltd, Culham Science Centre, showing the steel support rings above and below the vessel, and the merging-compression coils which provide an initial high current, hot plasma without need of the central solenoid.
context_page_description: A spherical tokamak is a type of fusion power device based on the tokamak principle. It is notable for its very narrow profile, or aspect ratio. A traditional tokamak has a toroidal confinement area that gives it an overall shape similar to a donut, complete with a large hole in the middle. The spherical tokamak reduces the size of the hole as much as possible, resulting in a plasma shape that is almost spherical, often compared with a cored apple. The spherical tokamak is sometimes referred to as a spherical torus and often shortened to ST.
  The spherical tokamak is an offshoot of the conventional tokamak design. Proponents claim that it has a number of substantial practical advantages over these devices. For this reason the ST has generated considerable interest since the late 1980s. However, development remains effectively one generation behind traditional tokamak efforts like JET. Major experiments in the ST field include the pioneering START and MAST at Culham in the UK, the US's NSTX-U and Russian Globus-M.
  Research has investigated whether spherical tokamaks are a route to lower cost reactors. Further research is needed to better understand how such devices scale.
context_section_description: Tokamak reactors consist of a toroidal vacuum tube surrounded by a series of magnets. One set of magnets is logically wired in a series of rings around the outside of the tube, but are physically connected through a common conductor in the center. The central column is also normally used to house the solenoid that forms the inductive loop for the ohmic heating system (and pinch current).
  The canonical example of the design can be seen in the small tabletop ST device made at Flinders University, which uses a central column made of copper wire wound into a solenoid, return bars for the toroidal field made of vertical copper wires, and a metal ring connecting the two and providing mechanical support to the structure.
caption: Design of the ST40 spherical tokamak with a major radius of 0.4 m.
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