How it cramps the flow: three regimes for the saturation of parallel ion-beam instabilities

TL;DR

This paper develops a theoretical framework and simulations to understand the saturation regimes of parallel ion-beam instabilities, highlighting their role in shock formation and particle acceleration.

Contribution

It introduces a comprehensive analysis of three distinct saturation regimes of ion-beam instabilities, expanding understanding of their nonlinear evolution and effects.

Findings

Identification of three saturation regimes: nonresonant, beam-gyroresonant, and mixed-turbulence.

Clarification of the role of background ion gyromotion in nonresonant instability.

Insights into the implications for shock formation and Fermi acceleration processes.

Abstract

Motivated by recent advances in laboratory experiments on parallel ion-beam instabilities, we present a theoretical framework for and simulations of their evolution towards shock formation and Fermi acceleration. After reviewing the theory of beam instabilities with a focus on the so-called nonresonant or Bell instability, which we show to be due to the gyromotion of background ions, we contrast the saturation of three parameter regimes: (I) the left-handed "nonresonant" regime, (II) the right-handed beam-gyroresonant regime, (III) the balanced, mixed-turbulence regime.