  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.
