Pedestal characteristics in JFT-2M HRS H-mode plasma

Abstract

Recent experiments in the JFT-2M tokamak have concentrated on studies of the access conditions for H-mode operation without any large ELMs, namely the ‘high recycling steady’ (HRS) H-mode regime, in terms of the pedestal parameters. The HRS regime is more likely to occur at higher edge density and lower edge temperature, while the ELMy H-mode having large ELMs appears at lower edge density and higher edge temperature. It is found that the ELMy/HRS operational boundary occurs at the normalized electron collisionality of in the plasma edge region, depending slightly on the safety factor at 95% flux surface q95. A key feature of the HRS H-mode is the presence of coherent magnetic fluctuations in the frequency range of the order of 10–100 kHz. It is suggested that the edge MHD activities may keep the edge pressure below a certain level needed to induce a large ELM.