An impurity hole observed in the large helical device (LHD) is a hollow density profile of an impurity ion species formed in the core plasma where the negative (inward-pointing) ambipolar radial electric field (Er) exists. Although local neoclassical models have predicted that the sign of Er in impurity hole plasmas is negative for the entire minor radius, an experimental measurement of an impurity hole plasma has shown that the Er changes the sign from negative to positive along the minor radius. In the present work, we investigate neoclassical impurity transport in an impurity hole plasma using a global neoclassical simulation code FORTEC-3D. The variation of electrostatic potential on each flux surface (Φ1) is evaluated from the quasi-neutrality condition in multi-ion-species plasma by the global simulation. The ambipolar Er and neoclassical fluxes are determined in solving a global drift-kinetic equation including the effect of Φ1. By the global simulation, we show that an Er which changes the sign along the radius is obtained as a solution of the ambipolar condition and with such an Er, impurity carbon flux can be outwardly directed even where Er < 0 and the carbon density profile is hollow around the magnetic axis. Furthermore, it is found that the outward carbon flux is only a factor 2–3 from balancing the modeled inward turbulent flux. Our result indicates that we have moved one step closer to reproducing the impurity transport in impurity hole plasmas by kinetic simulation.
A review of impurity transport characteristics in the LHD