Edge-localized modes and edge transport in spherical tokamaks

Turbulent energy transport in numerical simulations of the edge region of spherical tokamaks with small toroidal aspect ratio A=1.5 is directly compared to that in conventional tokamaks with larger aspect ratio A=3. Depending upon the aspect ratio of the torus, the energy flux can vary greatly. While a tokamak plasma with aspect ratio A=3 is in the high (H) mode where the transport rate is small, an otherwise identical tokamak plasma with small aspect ratio A=1.5 exhibits extremely poor confinement. The magnitude of the energy flux in these two cases differs by a multiplicative factor larger than 50. However, good H mode confinement in small aspect ratio spherical tokamaks can be obtained by a further increase in the edge density gradient. Confinement is then dominated by bursts of ballooning modes which periodically transport energy to large major radius. The periodic bursts in the numerical simulations are similar to the periodic radiation bursts that characterize edge-localized modes in tokamaks.