Study on impurity hole plasmas by global neoclassical simulation

TL;DR

This study uses global neoclassical simulations to analyze impurity hole plasmas in LHD, revealing that the radial electric field can change sign along the radius, influencing impurity transport and flux directions.

Contribution

The paper demonstrates that a global neoclassical simulation can reproduce the sign change of the radial electric field and impurity flux behavior observed in impurity hole plasmas.

Findings

The radial electric field can change sign along the minor radius.

Impurity carbon flux can be outward even with negative $E_r$.

Outward flux is within a factor of 2-3 of turbulent inward flux.

Abstract

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 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 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.