Small ELM dynamics and its impact on the SOL width scaling

Simultaneous control of large ELMs and divertor heat loads in H-mode plasma is crucial for steady-state operation of a tokamak fusion reactor. BOUT++ turbulence simulations are performed to capture the physics of the small ELM characteristics with different pedestal density profiles achieved via controlling strike points from vertical to horizontal divertor plates in EAST discharges. When the strike point shifts from the vertical to horizontal divertor plates, the separatrix density increases, and the pedestal pressure gradient and bootstrap current decrease. Linear simulations show that the most unstable modes change from high-n ideal ballooning modes to the intermediate-n peeling-ballooning modes and eventually to peeling-ballooning stable plasmas in the pedestal as separartrix density increases. Nonlinear simulations show that both the fluctuation level and the elm size decrease as the separatrix density increases, leading to small ELMs. To project from the current tokamak to ITER relevant parameters, pedestal collisionality scans are performed with a fixed pedestal pressure. The ELM size increases as the collisionality decreases for type-I ELMs, which shows a good agreement with an experimental scaling. However, starting from a stable pedestal plasma at high collisionality, small ELMs can be easily triggered either by high-n ballooning modes with very weak pedestal density gradient or by low-n peeling modes with relatively large pedestal density gradient by decreasing the normalized collisionality to <0.1. The SOL width is calculated as a function of separatrix density fluctuation intensity flux from the pedestal to the SOL, showing positive correlations between them which is consistent with the theory of turbulence spreading. Operating in H-mode with small ELMs will solve two critical problems: reducing the ELM size and broadening the SOL width.