Multiple electron beam propagation and Langmuir wave generation in plasmas

Abstract

The propagation of multiple electron beams in a plasma and the generation of Langmuir waves via a streaming instability is investigated numerically using quasilinear theory. The generation of waves by two equal copropagating beams injected at different times is studied in detail. The two beams are observed merging into one far from the injection points. Meanwhile, waves are enhanced in the vicinity of the mean beam speed of the leading beam, and are suppressed in a localized region after the injection of the trailing beam. Effects of beam injection parameters on the generation of the waves are studied. In particular, for the injection of two beams, the temperature, initial number density, and location of the injected particles are found to be relevant to fine structures in wave levels. It is also observed that the mechanism of beam merging is via interactions between beam particles and associated waves, i.e., fast particles in a trailing beam lose energy to waves generated initially by the leading beam, while slow particles in the leading beam absorb energy from waves driven by the trailing beam, which eventually leads to the elimination of systematic speed differences between the two beams. This mechanism of energy exchange generalizes the version studied in previous works, in which fast particles in a single beam lose energy that is later reabsorbed by slower particles. The characteristics of wave generation for multiple beam injections are found to be similar to the basic case of two beams. Finally, the applicability of this work to type III solar radio storms and shock associated type III-like bursts is commented upon.