Study on impurity hole plasmas by global neoclassical simulation

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 (E (r)) exists. Although local neoclassical models have predicted that the sign of E (r) in impurity hole plasmas is negative for the entire minor radius, an experimental measurement of an impurity hole plasma has shown that the E (r) 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 (phi(1)) is evaluated from the quasi-neutrality condition in multi-ion-species plasma by the global simulation. The ambipolar E (r) and neoclassical fluxes are determined in solving a global drift-kinetic equation including the effect of phi(1). By the global simulation, we show that an E (r) which changes the sign along the radius is obtained as a solution of the ambipolar condition and with such an E (r), impurity carbon flux can be outwardly directed even where E (r) < 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.