Author: Nami Li
Requested Type: Consider for Invited
Submitted: 2022-03-02 17:27:56
Co-authors: X.Q. Xu, Y. F. Wang, Ning Yan, J.Y. Zhang, J.P. Qian and D.Z. Wang
7000 East Ave.
Livermore, CA 94550
The grassy ELMs, one of small ELMs, are characterized by a high frequency and spatially localized quasi-periodic collapse in the bottom of pedestal near the separatrix. To understand the mechanism for the grassy ELMs and its impact on the divertor heat flux width broadening, BOUT++ turbulence simulations are conducted for a 60s steady-state long pulse high βp EAST grassy ELM discharge. BOUT++ linear simulations show that the unstable mode spectrum coves a range of toroidal mode numbers from low-n (n=10~15) peeling-ballooning modes (P-B) to high-n (n=40~80) drift-Alfvén instabilities. Nonlinear simulations show that the fluctuation is generated at the peak pressure gradient position and radially spread outward into the SOL. The ELM crash is trigged by the peeling modes at low-field side, even though the drift-Alfvén instabilities dominate the linear growth phase with a wide n-spectrum and the fluctuation peaks on high-field side. Drift-Alfvén turbulence delays the onset of the grassy ELM and enhances the energy loss with the fluctuation extending to pedestal top region. Simulations further show that if the peeling drive is removed, the fluctuation amplitude drops by an order of magnitude and the ELM crashes disappear. The divertor heat flux width is ~2 times larger than the estimates based on the HD model and the Eich’s ITPA multi-tokamak scaling due to the strong radial turbulence transport. The temporal evolution of the power loading shows no obvious decay for a grassy ELM period from the maximum of the ELM heat flux and the elm size is small (< 1%), indicating that the heat load is quasi-continuous due to its small enough bursts and high enough frequency.