Towards improved confinement: Analysis of the radial electric field in LHD

The radial electric field ( E r ) properties in LHD have been investigated to give guidance towards improved confinement with a possible E r transition and bifurcation. The ambipolar E r is calculated from the neoclassical flux on the basis of analytical formulas. This approach is appropriate to clarifying the E r properties over a wide parameter range in a more transparent way. A comparison between calculated E r and the experimental values has shown a qualitatively good agreement, for example, for the threshold density for the transition from the ion root to the electron root. The calculations also reproduce well the experimentally observed tendency that the electron root is possible by increasing temperatures even for higher density and the ion root is enhanced for higher density. On the basis of the usefulness of this approach, calculations over a wide range have been performed to clarify the parameter region where the E r transition and bifurcation are possible. This gives a comprehensive understanding of the parameter region of interest, which is valuable for the promotion of experiments towards improved confinement.