Abstract:  A new global 3D two-fluid code, GDB, based on the drift-reduced Braginskii model has been developed and tested to study the turbulent transport across the entire tokamak edge region: from plasma sources in the inner core to plasma sinks in the outer-most scrape-off layer (SOL). In this code, profiles of plasma density, electron and ion temperature, electric potential, magnetic flux and parallel flow are evolved self-consistently. Milliseconds-long simulations are carried out in a shifted-circle magnetic configuration with realistic Alcator C-Mod tokamak inner wall limited (IWL) discharge parameters.

The resistive ballooning instability is identified as the predominant driver of edge turbulence in the L-mode regime. Simulations show, in agreement with experimental observations, as the simulation moves towards density limit regime by increasing density, the turbulent transport is drastically enhanced and the plasma profiles are relaxed; on the other hand, as the simulation approaches to the H-mode regime by increasing temperature, the turbulent transport is suppressed and plasma profiles are steepened with a pedestal-like structure forming just inside of the separatrix. Radial transport level and turbulence statistics of these simulations also qualitatively match the experimental measurements.

Spontaneous ExB rotation in the electron diamagnetic drift direction in the closed flux region are observed in all cases. It can be explained based on the steady state ion continuity relation div(nv)~0. ExB rotation in the closed flux region is found mostly cancels the ion diamagnetic drift as H-mode-like regimes are approached, and exceeds it by a factor of two or more at lower temperatures due to parallel ion flows.