Plasma heating by relativistic electron beams. II. Return current interaction

Abstract

The combined interactions of the relativistic two-stream instability and return current driven Buneman/ion sound instability are investigated by numerical simulation for a beam-to-plasma density ratio ≈10−1 and γ=2−8. In this regime the evolution of the two-stream instability is relatively unaffected by the return current instability. The growth rate of the Buneman instability drops sharply when trapping of plasma electrons by the unstable Buneman waves occurs, but wave growth continues at a much reduced rate until other processes, e.g.,ion acceleration, wave scattering, and nonlinear Landau damping set in. Ions are heated by the return current and this heating is almost independent of the initial beam γ for constant return current. The bulk of the electron distribution is heated, whereas in the absence of a return current the two-stream instability primarily produces only energetic tails. The total plasma electron energy after several hundred plasma periods is proportional to the beam energy. The effective collision frequency of the plasma electrons due to microturbulence is typically 0.2?0.4 ωi, where ωi is the ion plasma frequency. In the strong turbulence created by the electron beam and the return current, the wave energy spectrum scales as ‖Ek‖2≈k−2.