Mixed diffusive-convective relaxation of a broad beam of energetic particles in cold plasma

TL;DR

This paper introduces a new nonlinear dynamical model for the relaxation of broad energetic particle beams in cold plasma, revealing a mixed diffusive and convective relaxation process influenced by nonlinear wave-particle interactions.

Contribution

It develops a self-consistent nonlinear model that extends quasi-linear theory to account for broad beams with internal structure, highlighting the importance of convective relaxation.

Findings

Numerical simulations show mixed diffusive-convective relaxation features.

Nonlinear wave-particle interactions significantly influence particle transport.

Including stable linear modes is crucial for accurate evolution modeling.

Abstract

We revisit the applications of quasi-linear theory as a paradigmatic model for weak plasma turbulence and the associated bump-on-tail problem. The work, presented here, is built around the idea that large-amplitude or strongly shaped beams do not relax through diffusion only and that there exists an intermediate time scale where the relaxations are convective (ballistic-like). We cast this novel idea in the rigorous form of a self-consistent nonlinear dynamical model, which generalizes the classic equations of the quasi-linear theory to "broad" beams with internal structure. We also present numerical simulation results of the relaxation of a broad beam of energetic particles in cold plasma. These generally demonstrate the mixed diffusive-convective features of supra-thermal particle transport; and essentially depend on nonlinear wave-particle interactions and phase-space structures. Taking into account modes of the stable linear spectrum is crucial for the self-consistent evolution of the distribution function and the fluctuation intensity spectrum.