The simulation of ELMs mitigation by pedestal coherent mode in EAST using BOUT++

A general phenomenon that the edge localized modes (ELMs) can be effectively mitigated/suppressed by the enhanced coherent mode (CM) intensity has been observed on EAST. For this phenomenon, the experimental statistical analysis and the electromagnetic (EM) simulations have been performed. There is a threshold value of the CM intensity in the experiments, which plays a key role in the ELM mitigation. Using the ELITE and BOUT++ analysis, we found that the pedestal is located outside the P-B boundary when the ELM mitigation and the enhanced CM co-exit. It is inconsistent with the experiment. Therefore, the effects of the CM are considered to explain the mitigation/suppression of ELM.

Modifying the three-filed reduced model in BOUT++, an imposed perturbation is added as the CM effect. The simulation results indicate that: without considering the CM, the ELM size belongs to the large ELM region; after considering the CM effect, the ELM is effectively mitigated and the ELM size is reduced by about 44.5%. Analysis shows that the CM can enhance the three-wave nonlinear interactions in the pedestal and reduce the phase coherence time (PCT) between the pressure and potential, which lead the perturbation to tend to be ‘multiple-mode’ coupling. The multiple modes compete with each other for the free energy, resulting in no obvious filament structure generation and decreased energy loss. The above reveals that there is a competitive relationship between turbulence and ELMs, and the CM can effectively regulate the edge energy loss. In addition, through the parameter scanning, we found that there is a threshold of the amplitude, which is consistent with the statistical results in the experiments.