Fusion Science and Technology / Volume 46 / Number 1 / July 2004 / Pages 106-114
Technical Paper / Stellarators
The electron internal transport barrier (ITB) is formed with centrally focused electron cyclotron resonance heating superposed on plasmas heated by neutral beam injection in the Large Helical Device. The electron transport is investigated for the electron ITB plasmas observed in various magnetic axis positions of Rax = 3.6, 3.75, and 3.9 m, and it turns out that the core electron transport is reduced with suppression of the anomalous transport in all three magnetic axis positions. In the theoretical calculations, positive radial electric fields are generated in the improved transport region, implying that the electron ITB formation is correlated with the neoclassical electron root. At an outer-shifted configuration of Rax = 3.9 m, where the helical ripple is large, the thermal diffusivity is decreased with decreasing collisionality, suggesting the reduction of the ripple transport by the radial electric field. The temperature and density conditions for the ITB formation are consistent with the theoretical density dependence of the transition temperature to the neoclassical electron root from the ion root.