On neoclassical impurity transport in stellarator geometry

TL;DR

This paper investigates how electrostatic potential variations on flux surfaces influence impurity transport in stellarators, using numerical simulations to improve understanding of impurity accumulation issues in devices like LHD.

Contribution

It demonstrates the significant impact of flux surface potential variations on neoclassical impurity transport in stellarators, employing advanced numerical tools and exploring self-consistent potential calculations.

Findings

Impurity transport is strongly affected by flux surface potential variations.

Numerical simulations with EUTERPE and GSRAKE reveal the importance of $ ilde{ heta}$ in impurity dynamics.

Preliminary self-consistent potential calculations are presented.

Abstract

The impurity dynamics in stellarators has become an issue of moderate concern due to the inherent tendency of the impurities to accumulate in the core when the neoclassical ambipolar radial electric field points radially inwards (ion root regime). This accumulation can lead to collapse of the plasma due to radiative losses, and thus limit high performance plasma discharges in non-axisymmetric devices.\\ A quantitative description of the neoclassical impurity transport is complicated by the breakdown of the assumption of small $\mathbf{E}\times \mathbf{B}$ drift and trapping due to the electrostatic potential variation on a flux surface $\tilde{\Phi}$ compared to those due to the magnetic field gradient. The present work examines the impact of this potential variation on neoclassical impurity transport in the Large Helical Device (LHD) stellarator. It shows that the neoclassical impurity transport can be strongly affected by $\tilde{\Phi}$. The central numerical tool used is the $\delta f$ particle in cell (PIC) Monte Carlo code EUTERPE. The $\tilde{\Phi}$ used in the calculations is provided by the neoclassical code GSRAKE. The possibility of obtaining a more general $\tilde{\Phi}$ self-consistently with EUTERPE is also addressed and a preliminary calculation is presented.