Pitch-angle scattering of energetic particles with adiabatic focusing

TL;DR

This paper derives a modified quasi-linear theory for pitch-angle scattering of energetic particles in magnetized plasmas with adiabatic focusing, revealing significant differences from classical models depending on focusing conditions.

Contribution

A new derivation of the quasi-linear pitch-angle scattering coefficient that accounts for adiabatic magnetic field focusing effects.

Findings

Fokker-Planck coefficient is significantly altered by adiabatic focusing.

Results agree with classical theory in the limit of no focusing.

The model improves understanding of particle transport in non-uniform magnetic fields.

Abstract

Understanding turbulent transport of charged particles in magnetized plasmas often requires a model for the description of random variations in the particle's pitch angle. The Fokker-Planck coefficient of pitch-angle scattering, which is used to describe scattering parallel to the mean magnetic field, is therefore of central importance. Whereas quasi-linear theory assumes a homogeneous mean magnetic field, such a condition is often not fulfilled, especially for high-energy particles. Here, a new derivation of the quasi-linear approach is given that is based on the unperturbed orbit found for an adiabatically focused mean magnetic field. The results show that, depending on the ratio of the focusing length and the particle's Larmor radius, the Fokker-Planck coefficient is significantly modified but agrees with the classical expression in the limit of a homogeneous mean magnetic field.