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A rotational surface, generated by the rotation of a circle in three-dimensional space about an axis that is coplanar with the circle, and has no common points with it. The resulting shape is also called a ring torus. A typical example of a torus is a donut or a tyre. A torus can be described by an aspect ratio, which is the ratio of the major radius to the minor radius. A torus with a large aspect ratio is for example a hula hoop ring, a torus with a small aspect ratio is for example a thick donut with a small central hole. In thermonuclear fusion research, a torus-shaped magnetic vessel is used in which an external magnetic field holds a toroidal ring of hot plasma.