Nonlinear study of local ballooning mode near the separatrix

TL;DR

This paper investigates the nonlinear behavior of local ballooning modes near the separatrix in tokamak plasmas, revealing their role in small ELM formation and matching experimental observations through numerical simulations.

Contribution

It introduces a detailed numerical study of local ballooning modes near the separatrix, linking mode dynamics to small ELMs and experimental data.

Findings

Localized collapse near the separatrix observed in simulations

Mode structure constrained by ExB shear

Simulation results match experimental QCE observations

Abstract

Small edge-localized-mode (ELM), similar to the quasi-continuous exhaust (QCE), has been achieved by increasing the density at the separatrix. Starting from the Type-I ELM experimental data in EAST, we have performed a numerical separatrix density scan to study the formation of the small ELM using BOUT++ 6-field 2-fluid module. In the high separatrix density case, localized collapse near the separatrix has been found. The corresponding ELM size, dominant mode number, and filament transport match the experimental observations of the QCE. Local ballooning mode near separatrix has been identified in the nonlinear simulation. The mode is driven by the local pressure gradient and the mode structure is constrained by the ExB shear.