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 λ_q. 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 show two distinct regimes: drift dominant regime and turbulence dominant regime. For current tokamak H-mode discharges, drift and turbulent transport both compete in setting the 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 q of the future machines may no longer follows the 1/Bpol,MP experimental empirical scaling and the Goldston 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/grassy ELM regime.
Experimental observations on C-MOD and ASDEX-Upgrade have shown a similar parametric dependence for λ_q in I-mode and L-mode discharges to that previously found in H-mode, with consistently larger width and weaker current I_p^(-α) scaling as turbulence transport increases. But turbulence seems not strong enough to dominate the radial transport by drifts in I-mode and L-mode discharges, and hence to break the experimental scaling (Eich scaling), in which λ_q∝1/I_p (or 1/Bpol). Further researches show that grassy ELM operation yields larger turbulence transport with H-mode level performance across a range of collisionalities. A stationary, high-confinement small grassy-ELM regime has been demonstrated in EAST with fELM~2.6kHz and q┴<2MW/m2 on divertor target, which is less than 10% of that of type-I ELM. Furthermore, recent DIII-D grassy ELM experiments show a consistent divertor heat flux width broadening and amplitude reduction, just as BOUT++ simulations demonstrated in the grassy ELM regime. The comparison of divertor heat flux width will be given for quasi-steady inter-ELMs H-mode, grassy ELM and type-I ELM discharges.
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