The first tokamaks in the early 1960's held the plasma in a toroidal shape with a large hole in the centre, rather like a car tyre. Tokamaks like JET, with fatter shaped plasmas, have followed.

The START experiment at the Culham Science Centre began operation in 1991, and was designed to take this trend in plasma shape to its limits. START produced an even more compact plasma, like a cored apple. Within a cylindrical vacuum vessel 2 metres in diameter and 2 metres high, plasmas were produced at aspect ratio as low as 1.25, compared to values of 3 or more in conventional machines like JET.

Built initially as a low cost experiment to test the theoretical predictions made for tight aspect ratio tokamak systems, START quickly became the world's leading spherical tokamak experiment. Many fusion scientists from countries all over the world, including Brazil, Japan, USA and Russia have visited Culham to take part in research on START. The high temperatures acheived (more than 10 million °C) in combination with its small size and low construction cost created world-wide interest in this novel approach, such that many similar devices are now being built around the World.

In the future, as the figure shows, the ST could provide an alternative path to a power plant, instead of the conventional route through high aspect-ratio devices.