Electron velocity distributions during beam–plasma interaction

Abstract

It is well known that low-frequency Alfvén waves can be excited due to an ion/ion instability when a tenuous ion beam streams through a background plasma along a magnetic field. In this article, using a one-dimensional particle-in-cell code, the consequence of this beam–plasma interaction process is investigated. Emphasis is placed on the nonlinear effects of enhanced Alfvén waves on beam electrons. In the simulation, the speed between the beam plasma and ambient plasma is considered to be 10 VA (where VA is the Alfvén speed), the ratio of beam–plasma density to background plasma density is nb/n0=0.006 (nb and n0 are the beam and total plasma densities). For the case βi=4×10−4 (βi being the ratio of kinetic pressure of the ions to magnetic pressure), the Alfvén waves begin to grow exponentially at about t=32 Ωi−1, and they saturate at about t=88 Ωi−1. The excited waves are nearly monochromatic, which satisfies the resonant condition, and the perpendicular velocity (the velocity component whose direction is perpendicular to the ambient magnetic field) distribution of the beam electrons peaks away from its origin with a maximum radius about 2.5 VA at the saturation stage. Then, the amplitude of the excited waves decreases and the higher-frequency waves are also excited. A quasi-equilibrium stage is reached at about t=100 Ωi−1, and the radius of the ring in the perpendicular velocity distribution is about 0.7 VA. For the case βi=0.04, the situation is similar except that the radius of the ring in the perpendicular velocity distribution of the beam electrons is smaller, and the ring almost disappears at the quasi-equilibrium stage. Another point is that both the beam and background electrons can be heated by the excited Alfvén waves. The heating effect is more significant for the beam electrons than the background electrons, and their final thermal speeds are anticorrelated with the parameter βi.