High-beta performance of the START spherical tokamak

Using additional heating provided by neutral-beam injection, the START spherical tokamak at UKAEA Fusion Culham has achieved high-beta (ratio of volume average plasma pressure to vacuum magnetic-field pressure) values of beta(T) greater than or equal to 30%, more than twice the value previously obtained in a tokamak. These plasmas reach normalized beta values of beta(N) = beta%/(I/aB) similar to 4 at values of auxiliary heating power comparable to the ohmic power. Operation at high normalized current I-N = I-p/aB(T) similar to 8 is observed, so that the plasma current exceeds the central rod toroidal-field current for the first time in a hot tokamak.