Growth of beam-plasma instabilities in the presence of background inhomogeneity

TL;DR

This study investigates how background plasma inhomogeneity affects beam-plasma instability growth, finding that inhomogeneities slow growth but do not suppress the instability, challenging previous claims about their damping effect.

Contribution

The paper demonstrates through simulations that inhomogeneities cause local growth of instabilities and do not prevent the overall development of beam-plasma instabilities in astrophysical contexts.

Findings

Inhomogeneities slow but do not suppress instability growth.

Instability growth remains within the linear regime longer with inhomogeneity.

Beam energy loss at non-linear saturation is similar to uniform cases.

Abstract

We explore how inhomogeneity in the background plasma number density alters the growth of electrostatic unstable wavemodes of beam plasma systems. This is particularly interesting for blazar-driven beam-plasma instabilities, which may be suppressed by inhomogeneities in the intergalactic medium as was recently claimed in the literature. Using high resolution Particle-In-Cell simulations with the SHARP code, we show that the growth of the instability is local, i.e., regions with almost homogeneous background density will support the growth of the Langmuir waves, even when they are separated by strongly inhomogeneous regions, resulting in an overall slower growth of the instability. We also show that if the background density is continuously varying, the growth rate of the instability is lower; though in all cases, the system remains within the linear regime longer and the instability is not extinguished. In all cases, the beam loses approximately the same fraction of its initial kinetic energy in comparison to the uniform case at non-linear saturation. Thus, inhomogeneities in the intergalactic medium are unlikely to suppress the growth of blazar-driven beam-plasma instabilities.