Aspect ratio

In a magnetic confinement device, the ratio of the major radius, the distance from the machine's central axis to the centre of the plasma, to the minor radius, the radius of the plasma's own cross-section. A conventional tokamak has a relatively high aspect ratio, a wide doughnut with a thin ring of plasma: ITER's is about 3.1 (6.2 metres divided by 2.0 metres). A spherical tokamak is squeezed to an aspect ratio below about 2, closer to a cored apple, which holds plasma pressure with less magnetic field but leaves very little room at the centre for the central column and its shielding.

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