Asymmetry of collapse of density pedestal by type I ELM in JT-60U

摘要

The edge plasma density at the high-field side (HFS) and low-field side (LFS) was measured using a FIR interferometer and an O-mode reflectometer simultaneously in order to study the asymmetry of collapse of the density pedestal by type I ELM. From fast time series analysis using the same digitizer system for the reflectometer, D_α intensity, interferometer and magnetic probe, experimental evidence for poloidal asymmetry of the collapse was clearly observed in JT-60U ELMy H-mode discharges for the first time. The collapse of the pedestal density is manifested as a fast inward displacement of the cutoff layer at the LFS, the increase of D_α intensity and large magnetic oscillations. No immediate response, however, was observed in the line-integrated density at the HFS of the plasma (FIR-U1). Instead, the signal showed a small reduction after a time delay of about L_(‖)/C_s, which was attributed to the parallel flow from the inside to outside in order to fill the density gap as a result of localized particle expulsion from the LFS. Furthermore, no density increase in the HFS scrape-off layer (SOL) was observed during the horizontal plasma sweep experiments corresponding to particles expelled from the HFS midplane to the SOL directly, at the onset of density collapse and before the increase of D_α intensity. Together, these results indicate that the collapse of the density pedestal during type I ELMs is localized at the LFS midplane.