Full orbit simulations of collisional impurity transport in spherical tokamak plasmas with strongly-sheared electric fields

TL;DR

This study uses full orbit simulations to analyze how strongly-sheared electric fields influence impurity ion transport in spherical tokamak plasmas, revealing enhanced confinement effects dependent on electric field polarity and impurity charge.

Contribution

It introduces a full orbit simulation approach to quantify impurity transport under strong electric field shear, extending beyond standard drift approximations.

Findings

Sheared electric fields enhance impurity confinement.

Inward drift occurs with negative electric fields, outward with positive.

Confinement increase is proportional to impurity charge number.

Abstract

The collisional dynamics of test impurity ions in spherical tokamak plasmas with strongly-sheared radial electric fields is investigated by means of a test particle full orbit simulation code. The strength of the shear is such that the standard drift ordering can no longer be assumed and a full orbit approach is required. The effect of radial electric field shear on neoclassical particle transport is quantified for a range of test particle mass and charge numbers and electric field parameters. It is shown that the effect of a sheared electric field is to enhance the confinement of impurity species above the level observed in the absence of such a field. The effect may be explained in terms of a collisional drag force drift, which is proportional to particle charge number but independent of particle mass. This drift acts inwards for negative radial electric fields and outwards for positive fields, implying strongly enhanced confinement of highly ionized impurity ions in the presence of a negative radial electric field.