The monoenergetic approximation in stellarator neoclassical calculations

TL;DR

This paper critically evaluates the monoenergetic approximation used in stellarator neoclassical transport calculations, revealing it underestimates trapped particle fractions especially at high electric fields, which questions the reliability of some existing results.

Contribution

The study assesses the validity of the monoenergetic approach by analyzing particle trajectories, highlighting its systematic underestimation of trapped particles in certain regimes.

Findings

Monoenergetic codes under-predict trapped particle fractions.

Error magnitude is significant at large electric fields.

Inaccuracy is independent of collision operator approximations.

Abstract

In the standard "monoenergetic" approach to numerical calculation of stellarator neoclassical transport, to expedite computation, ad-hoc changes are made to the kinetic equation so speed enters only as a parameter. Here we examine the validity of this approach by considering the effective particle trajectories in a model magnetic field. We find monoenergetic codes systematically under-predict the true trapped particle fraction, with the error in the trapped ion fraction being of order unity when the electric field is large, suggesting some results of these codes may be unreliable in this regime. This inaccuracy is independent of any errors introduced by approximation of the collision operator.