Possible mechanism responsible for observed impurity outward flow under radio frequency heating

TL;DR

This paper investigates how poloidal impurity asymmetry influences impurity transport in tokamaks, revealing mechanisms that can cause outward impurity flow under radio frequency heating conditions.

Contribution

It introduces a detailed analysis of impurity asymmetry effects on transport, highlighting conditions that lead to impurity outward flow, especially under RF heating.

Findings

In-out impurity asymmetry reduces the peaking factor.

Large asymmetry can reverse impurity flux direction.

Ion parallel compressibility affects impurity peaking factors.

Abstract

The effect of poloidal asymmetry of impurities on impurity transport driven by electrostatic turbulence in tokamak plasmas is analyzed. It is found that in the presence of in-out asymmetric impurity populations the zero-flux impurity density gradient (the so-called peaking factor) is significantly reduced. A sign change in the impurity flux may occur if the asymmetry is sufficiently large. This may be a contributing reason for the observed outward convection of impurities in the presence of radio frequency heating. The effect of in-out asymmetry is most significant in regions with low temperature gradients. In the trapped electron mode dominated case also an up-down asymmetry can lead to a sign change in the peaking factor from positive to negative. The effect of ion parallel compressibility on the peaking factor is significant, and leads to positive peaking factors in regions with high temperature gradients, even in the presence of in-out asymmetry.