History of the Spherical Tokamak

The first tokamaks in the early 1960's had fairly high aspect ratio A, where A is the ratio of the major to minor radii of the plasma torus. Lower aspect ratio designs followed (see diagram, right), in particular JET at A ~2.5 which demonstrated the advantages of elongated and 'D' shaped plasmas. The START experiment at Culham, UK, which began operation in 1991 explores the limit of very low aspect ratio. It employs a cylindrical vacuum vessel 2m diameter and 2m high, and plasmas are obtained at aspect ratios as low as 1.25. START has provided the world's first experimental results on hot (Te ~ 500eV or 5 million ºC) spherical tokamak plasmas.