Self-consistent nonlinear kinetic simulations of the anomalous Doppler instability of suprathermal electrons in plasmas

Abstract

Suprathermal tails in the distributions of electron velocities parallel to the magnetic field are found in many areas of plasma physics, from magnetic confinement fusion to solar system plasmas. Parallel electron kinetic energy can be transferred into plasma waves and perpendicular gyration energy of particles through the anomalous Doppler instability (ADI), provided that energetic electrons with parallel velocities v||≥(ω+Ωce)/k|| are present; here Ωce denotes electron cyclotron frequency, ω the wave angular frequency, and k|| the component of wavenumber parallel to the magnetic field. This phenomenon is widely observed in tokamak plasmas. Here, we present the first fully self-consistent relativistic particle-in-cell simulations of the ADI, spanning the linear and nonlinear regimes of the ADI. We test the robustness of the analytical theory in the linear regime and follow the ADI through to the steady state. By directly evaluating the parallel and perpendicular dynamical contributions to j·E in the simulations, we follow the energy transfer between the excited waves and the bulk and tail electron populations for the first time. We find that the ratio Ωce/(ωpe+Ωce) of energy transfer between parallel and perpendicular, obtained from linear analysis, does not apply when damping is fully included, when we find it to be ωpe/(ωpe+Ωce); here ωpe denotes the electron plasma frequency. We also find that the ADI can arise beyond the previously expected range of plasma parameters, in particular when Ωce>ωpe. The simulations also exhibit a spectral feature which may correspond to the observations of suprathermal narrowband emission at ωpe detected from low density tokamak plasmas.