Core electron-root confinement (CERC) in helical plasmas

The improvement of core electron heat confinement has been realized in a wide range of helical devices such as CHS, LHD, TJ-II and W7-AS. Strongly peaked electron temperature profiles and large positive radial electric field, E r , in the core region are common features for this improved confinement. Such observations are consistent with a transition to the ‘electron-root’ solution of the ambipolarity condition for E r in the context of neoclassical transport, which is unique to non-axisymmetric configurations. Based on this background, this improved confinement has been collectively dubbed ‘core electron-root confinement’ (CERC). The thresholds for CERC establishment are found for the collisionality and electron cyclotron heating power. The magnetic configuration properties (e.g. effective ripple and magnetic islands/rational surfaces) play important roles for CERC establishment.