Compressible impurity flow in the TJ-II stellarator

TL;DR

This study investigates impurity flow in the TJ-II stellarator, revealing that impurity flow is not incompressible as traditionally assumed, and explores the effects of impurity redistribution and electrostatic potential variations.

Contribution

It provides the first detailed measurement of impurity flow deviations from incompressibility in the TJ-II stellarator and compares these with impurity redistribution models.

Findings

Impurity flow is inconsistent with the assumption of incompressibility.

Calculated impurity return flows do not fully explain observed flow variations.

Electrostatic potential inhomogeneities have limited impact on impurity redistribution.

Abstract

Fully-ionised carbon impurity flow is studied in ion-root, neutral beam heated plasmas by means of Charge Exchange Recombination Spectroscopy (CXRS) in the TJ-II stellarator. Perpendicular flows are found to be in reasonable agreement with neoclassical calculations of the radial electric field. The parallel flow of the impurity is obtained at two locations of the same flux surface after subtraction of the calculated Pfirsch-Schl\"uter parallel velocity. For the medium density plasmas studied, $\bar{n}_{\rm e}\in(1.2-2.4)\times 10^{19}$ m$^{-3}$, the measured impurity flow is found to be inconsistent with a total incompressible flow, i.e. $\nabla\cdot{\bf u}_z\ne0$, thus contradicting the usual assumption of a constant density on each flux surface. The experimentally observed velocity deviations are compared with the parallel return flow calculated from a modelled impurity density redistribution driven by ion-impurity friction. Although the calculated return flow substantially modifies the incompressible velocity pattern, the modifications do not explain the in-surface variations of impurity parallel mass flow at the precise locations of the CXRS measurements. Small inhomogeneities of the electrostatic potential in a surface are also shown to affect the impurity redistribution but do not provide a better understanding of the measurements.