aspect ratio is the ratio of a torus's major radius — the distance from the machine's central axis to the center of the 📝plasma — to its minor radius, the radius of the plasma cross-section itself, making it the most basic descriptor of a tokamak's geometry.
The term is borrowed from the general vocabulary of proportion: screens and photographs have an aspect ratio of width to height, aircraft wings one of span to chord. In magnetic confinement fusion the ratio compares the two radii of the doughnut, and the number encodes the machine's whole silhouette. A conventional 📝Tokamak has an aspect ratio of roughly 2.5 to 4 — ITER's is about 3.1, with a major radius of 6.2 meters and a minor radius of 2 meters — producing the familiar doughnut with a wide central hole. Push the ratio down toward 1.5 and the hole nearly closes, yielding the compact, cored-apple shape of the spherical tokamak.
Aspect ratio matters because it trades off against nearly everything else in the design. A fatter plasma (lower ratio) uses its magnetic field more efficiently and tolerates higher plasma pressure, but leaves little room in the center for the solenoid and the inboard legs of the field coils; a slimmer plasma (higher ratio) gives the engineering room to breathe at the cost of field utilization. 📝SPARC, with a major radius of 1.85 meters and a minor radius of 0.57 meters, sits near 3.2 — a conventional aspect ratio made compact not by shape but by the strength of its magnetic field.