H-Mode Operation in the START Spherical Tokamak

$H$-mode operation has been achieved in high current $(I_p>200\mathrm{}\mathrm{kA})$ plasmas in the START spherical tokamak for both neutral-beam-injection-heated and Ohmic discharges. The transition to $H$ mode features the development of well-defined edge pedestals in density and temperature, which signifies the formation of an edge-transport barrier, and associated edge-localized modes. Recent operation at plasma currents exceeding 250 kA shows that these features are accompanied by increases in energy confinement time. This is the first clear demonstration of the $H$-mode regime in a spherical tokamak.