Characteristics of boundary plasmas after the removal of divertor baffle plates and boronization on JFT-2M

A dense and cold divertor (nediv∼2×1019 m−3, Tediv∼6 eV) during H-mode, was realized with the closed divertor (CD2) on JFT-2M. By removing all baffle plates, such a condition was compared between open (OD) and closed configurations. When a strong gas-puff is introduced into the divertor region during L-mode, the baffling effect of CD2 is verified definitely by the reduction of neutral particle backflow and the enhanced radiation localization in the divertor region. Introducing a strong gas-puff during H-mode for the OD case, H-mode is terminated in an early stage when the radiation loss fraction exceeds a critical level. A dense and cold divertor, as shown in the CD2 case, is not obtained. This is attributed to the lack of particle-handling capability for the OD compared with the CD2 configuration. As an initial result, the radiation loss fraction was reduced by half after the boronization which prevents the H–L transition.