Study on impurity hole plasmas by global neoclassical simulation

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.

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Cited by in corpus (4)

• A quasi-isodynamic configuration with good confinement of fast ions at low plasma $\beta$
• Turbulent transport of impurities in 3D devices
• Electron root optimisation for stellarator reactor designs
• Direct optimization of neoclassical ion transport in stellarator reactors