Simulations of electron/electron instabilities: Electromagnetic fluctuations

Electron/electron instabilities arise in collisionless plasmas when the electron velocity distribution consists of two distinct components with a sufficiently large relative drift speed between them. If the less dense beam component is not too tenuous and sufficiently fast, the electron/electron beam instability is excited over a relatively broad range of frequencies. This instability is often studied in the electrostatic limit, which is appropriate at ωe/|Ωe|≫1, where ωe is the electron plasma frequency and Ωe is the electron cyclotron frequency, but is not necessarily valid at ωe/|Ωe|∼1. Here linear Vlasov dispersion theory has been used and fully electromagnetic particle-in-cell simulations have been run in a spatially homogeneous, magnetized plasma model at βe≪1 and 0.5 ⩽ωe/|Ωe|⩽4.0. Theory and simulations (run to times of order 100ωe−1) of the electron/electron beam instability show the growth of appreciable magnetic fluctuations at ωe/|Ωe|<2; these waves bear right-hand elliptical magnetic polarization. The simulations reproduce the well-known slowing and heating of the beam; at ωe/|Ωe|<1 this heating is predominantly parallel to the background magnetic field, but as ωe/|Ωe| becomes greater than unity the perpendicular heating of the beam increases. The simulations also demonstrate that, for ωe/|Ωe|∼1, electromagnetic fluctuations impart to the more dense electron core component significant heating perpendicular to the background magnetic field.