A model is proposed for the edge harmonic oscillation, in which the stationary coherent mode is sustained in the almost linear phase as has been observed in JT-60U. We study the coupled dynamics of the peeling mode amplitude and edge pressure gradient. The limit cycle oscillation is predicted. The peeling mode (which is almost in the linear phase) is in a dynamical stationary state with amplitude modulation. In this model, the time scales for the change of parameters that specify magnetic structures (such as magnetic shear and edge plasma current) are assumed to be much slower, so that are decoupled from the limit cycle dynamics. The condition that the limit cycle state appears is shown. The oscillation frequency of the modulation is given by the hybrid mean of the typical growth rate of the peeling mode and the additional loss rate of pressure gradient by the peeling mode.
The role of edge current-driven modes in ELM activity