The small tight aspect ratio tokamak experiment

Low‐aspect‐ratio tokamaks offer both the economic advantage of smaller size and a number of physics advantages which are not available at conventional aspect ratio. The Small Tight Aspect Ratio Tokamak (START) [Fusion Technology 1990, edited by B. E. Keen, M. Huguet, and R. Hemsworth (North‐Holland, Amsterdam, 1991), Vol. 1, p. 353] was conceived as a first substantial test of tokamak plasma behavior at low aspect ratio. It has achieved plasma currents up to 200 kA, peak densities of ∼2×1020 m−3 and central electron temperatures of ∼500 eV at an aspect ratio of 1.3–1.5. Central beta values of ∼13% have been measured and the volume‐averaged beta 〈β〉 can approach the Troyon limit. Plasmas are naturally elongated (κ≲2.0) and are vertically stable without feedback control. Major disruptions have not been observed at low aspect ratios (A≤2.0).