Clear transition to high-T_e state with an electron internal transport barrier creation in EC heated LHD plasmas

A transition to a high electron temperature state in the core accompanied by an electron internal transport barrier (eITB) was clearly observed in neutral beam (NB) sustained plasmas with strongly focused on-axis electron cyclotron heating (ECH) in Large Helical Device (LHD). During stepwise injection of ECH power, the core electron temperature started to build up spontaneously and became high temperature in the core (high-T_e state), while the temperature outside of ρ ≃ 0.3 drastically decreased. The transition dynamics of the core temperature is analyzed by the time evolution of electron temperature profiles and heat flux changes based on the data from multi-channel electron cyclotron emission. Effect of an initial island size (m/n=2/1) on the high-T_e transition was experimentally investigated by controlling the width by an external coil field. The results show that the island size possibly affects a threshold value of ECH power and collisionality on the high-T _e transition.