Velocity space compression from Fermi acceleration with Lorentz scattering

TL;DR

This paper introduces a modified Fermi acceleration model involving Coulomb collisions, leading to phase space non-conservation and potential energy distribution peaks, with implications for fusion and astrophysics.

Contribution

It presents a novel variation of Fermi acceleration incorporating Coulomb scattering, revealing phase space non-conservation and energy peak formation in particle distributions.

Findings

Collision-induced stochastic effects alter acceleration profiles

Energy distributions can develop peaks upon compression

Phase space non-conservation observed in the model

Abstract

The Fermi acceleration model was introduced to describe how cosmic ray particles are accelerated to great speeds by interacting with moving magnetic fields. We identify a new variation of the model where light ions interact with a moving wall while undergoing pitch angle scattering through Coulomb collisions due to the presence of a heavier ionic species. The collisions introduce a stochastic component which adds complexity to the particle acceleration profile and sets it apart from collisionless Fermi acceleration models. The unusual effect captured by this simplified variation of Fermi acceleration is the non-conservation of phase space, with the possibility for a distribution of particles initially monotonically decreasing in energy to exhibit an energy peak upon compression. A peaked energy distribution might have interesting applications, such as to optimize fusion reactivity or to characterize astrophysical phenomena that exhibit non-thermal features.