 In the early 1970s, studies at Princeton into the use of high-power superconducting magnets in future tokamak designs examined the layout of the magnets.They noticed that the arrangement of the main toroidal coils meant that there was significantly more tension between the magnets on the inside of the curvature where they were closer together.Considering this, they noted that the tensional forces within the magnets would be evened out if they were shaped like a D, rather than an O.