Investigation of turbulent transport dynamics in scrape-off-layer (SOL) and divertor heat flux width prediction is performed for ITER. Both BOUT++ transport and BOUT++ turbulence codes are applied to capture the physics on different temporal scales. Simulations start with an ITER 15MA baseline scenario profile generated by CORSICA (Kim et al 2015 Paper ITER_D_R9T8J9 v1.1). In BOUT++ transport code, the plasma parameters (, , ) and radial electric () profiles are evolved to steady state. The initial plasma profiles inside the separatrix are taken from CORSICA scenario studies. Transport coefficients are calculated by inverting the plasma profiles inside the separatrix. SOL transport coefficients are assumed to be constants connected to the separatrix. A parametric scan for the anomalous thermal diffusivity (, ) in the SOL is performed separately with and magnetic drift included, and without any drift effects. The results show that when the diffusivity is smaller than a critical , the heat flux width remains almost unchanged, which is roughly consistent with Goldston’s heuristic drift model (Goldston 2012 Nucl. Fusion 52 013009). Otherwise, it increases as a scaling resulting in a larger . BOUT++ six-field/two-fluid turbulence code is used to study pedestal and SOL turbulence dynamics and corresponding transport. In the turbulence simulation, pedestal is found to be peeling-ballooning unstable, which results in a larger . Pedestal structure is also found to be important in determining the effective thermal diffusivity and could lead to changes in the divertor heat flux width.
The BOUT + + code has been used to simulate edge plasma electromagnetic (EM) turbulence and transport, and to study the role of EM turbulence in setting the scrape-off layer (SOL) heat flux width λq. More than a dozen tokamak discharges from C-Mod, DIII-D, EAST, ITER and CFETR have been simulated with encouraging success. The parallel electron heat fluxes onto the target from the BOUT + + simulations of C-Mod, DIII-D, and EAST follow the experimental heat flux width scaling of the inverse dependence on the poloidal magnetic field. Further turbulence statistics analysis shows that the blobs are generated near the pedestal pressure peak gradient region inside the separatrix and contribute to the transport of the particle and heat in the SOL region. Transport simulations indicate two distinct regimes: drift dominant regime and turbulence dominant regime. Goldston's heuristic drift-based (HD) model yields a consistent divertor heat flux width in the drift dominant regime. For C-Mod enhanced Dα H-mode discharges, drifts and turbulence are competing in setting the divertor heat flux width, possibly due to its compact machine size and good pedestal confinement. The simulations for ITER and CFETR indicate that divertor heat flux width λq of the future machines may no longer follows the 1/Bpol,OMP HD-based empirical (Eich) scalings and the HD model gives a pessimistic limit of divertor heat flux width. The simulation results show a transition from a drift dominant regime to a turbulence dominant regime from current machines to future machines such as ITER and CFETR for two reasons. (1) The magnetic drift-based radial transport decreases due to large CFETR and ITER machine sizes. (2) The SOL turbulence thermal diffusivity increases due to larger turbulent fluxes ejected from the pedestal into the SOL when operating in a different pedestal structure, from an ELM-free H-mode pedestal regime to a small and grassy ELM regime.
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