A complex interaction between turbulence driven $E\times B$ zonal flow oscillations, i.e., geodesic acoustic modes (GAMs), the turbulence, and mean equilibrium flows is observed during the low to high ($L\mathrm{\text{−}}H$) plasma confinement mode transition in the ASDEX Upgrade tokamak. Below the $L\mathrm{\text{−}}H$ threshold at low densities a limit-cycle oscillation forms with competition between the turbulence level and the GAM flow shearing. At higher densities the cycle is diminished, while in the $H$ mode the cycle duration becomes too short to sustain the GAM, which is replaced by large amplitude broadband flow perturbations. Initially GAM amplitude increases as the $H$-mode transition is approached, but is then suppressed in the $H$ mode by enhanced mean flow shear.

• Received 23 July 2010

DOI:https://doi.org/10.1103/PhysRevLett.106.065001