Aspect Ratio (A)

The ratio of a torus's major radius to its minor radius; low A means a 'fat', compact plasma.

Aspect ratio A = R₀/a compares the distance from the machine centre to the plasma centre (major radius) with the plasma's own radius (minor radius). Conventional tokamaks sit near A = 3; a spherical tokamak pushes A down toward 1.5–2.

HYPERION runs at A = 2.5 (R₀ = 1.2 m, a = 0.48 m). Low aspect ratio raises the achievable plasma pressure for a given field and improves stability, at the cost of a tight, highly-loaded central column.

Aspect ratio is the master geometric lever of a tokamak. Dropping from a conventional A of about 3 to HYPERION's 2.5 sharply raises the achievable plasma pressure per unit field and improves natural shaping, which is what lets a small machine reach reactor-relevant performance. The penalty is paid at the centre: the inboard column has almost no room for the solenoid, toroidal-field conductor and neutron shielding, making it the defining engineering focus of the breeder.