Numerical analysis of key factors for the appearance of grassy ELMs in tokamak plasmas

Aiba, Nobuyuki; Oyama, Naoyuki

In this paper, we developed a simplified model that is used for the MHD stability analysis with the kinetic effects, and applied this to the numerical analysis of small amplitude ELM, called grassy ELM. As the result, we found that the stabilizing effect due to the ion diamagnetic drift can be canceled by taking into account the plasma compressibility and the electron diamagnetic drift, and this cancelation moves the stability boundary of small ELM closer to the ideal ballooning stability boundary. This trend is consistent with the experimental results of JT-60U grassy ELM plasma, and implies that the short-wavelength electromagnetic instability like ideal ballooning mode is thought as the origin of grassy ELM.