Ion acceleration, magnetic field line reconnection, and multiple current filament coalescence of a relativistic electron beam in a plasma

Results are presented from analytical theory and from two-dimensional particle-in- cell simulations of the relaxation of a relativistic electron beam in a plasma. When the electron beam enters the plasma, it generates a return current which is carried by the plasma electrons. In a collisionless plasma, the relaxation mechanism is related to the onset of an electromagnetic filamentation instability, which leads to the generation of a quasistatic magnetic field associated with many small-scale current filaments. Each filament consists of a direct and of a return electric current which repel each other. This produces a strong electric field which accelerates the ions in the radial direction. In the long time evolution, the successive coalescence of the small-scale current filaments is seen. This process is accompanied by the reconnection of the magnetic field lines, by the formation of current sheets, and by strong ion acceleration inside these sheets.