Validating modelling assumptions of alpha particles in electrostatic turbulence

TL;DR

This paper analyzes the behavior of alpha particles in electrostatic turbulence within tokamaks, confirming their confinement, highlighting modeling inaccuracies with Maxwellian assumptions, and proposing corrections for flux estimates.

Contribution

It introduces a non-Maxwellian equilibrium model for alpha particles, evaluates their response to turbulence, and provides a correction method for flux calculations.

Findings

Alpha particles are well-confined for several seconds.

Using a Maxwellian distribution leads to incorrect flux estimates.

Alpha effects on turbulence are negligible at low concentrations.

Abstract

To rigorously model fast ions in fusion plasmas, a non-Maxwellian equilibrium distribution must be used. In the work, the response of high-energy alpha particles to electrostatic turbulence has been analyzed for several different tokamak parameters. Our results are consistent with known scalings and experimental evidence that alpha particles are generally well-confined: on the order of several seconds. It is also confirmed that the effect of alphas on the turbulence is negligible at realistically low concentrations, consistent with linear theory. It is demonstrated that the usual practice of using a high-temperature Maxwellian gives incorrect estimates for the radial alpha particle flux, and a method of correcting it is provided. Furthermore, we see that the timescales associated with collisions and transport compete at moderate energies, calling into question the assumption that alpha particles remain confined to a flux surface that is used in the derivation of the slowing-down distribution.