Turbulent transport regimes in the tokamak boundary and operational limits

Two-fluid, three-dimensional, flux-driven, global, electromagnetic turbulence simulations carried out by using the GBS (Global Braginskii Solver) code are used to identify the main parameters controlling turbulent transport in the tokamak boundary and to delineate an electromagnetic phase space of edge turbulence. Four turbulent transport regimes are identified: (i) a regime of fully developed turbulence appearing at intermediate values of collisionality and β, with turbulence driven by resistive ballooning modes, related to the L-mode operation of tokamaks, (ii) a regime of reduced turbulent transport at low collisionality and large heat source, with turbulence driven by drift-waves, related to a high-density H-mode regime, (iii) a regime of extremely large turbulent transport at high collisionality, which is associated with the crossing of the density limit, and (iv) a regime above the ideal ballooning limit at high β, with global modes affecting the dynamics of the entire confined region, which can be associated with the crossing of the β limit. The transition from the reduced to the developed turbulent transport regime is associated here with the H-mode density limit, and an analytical scaling law for maximum edge density achievable in H-mode is obtained. Analogously, analytical scaling laws for the crossing of the L-mode density and β limits are provided and compared to the results of GBS simulations.