Previous theoretical considerations of electron beam relaxation ininhomogeneous plasmas have indicated that the effects of the irregular solarwind may account for the poor agreement of homogeneous modelling with theobservations. Quasi-linear theory and Hamiltonian models based on Zakharov'sequations have indicated that when a level of density fluctuations is above agiven threshold, density irregularities act to de-resonate the beam-plasmainteraction, restricting Langmuir wave growth on the expense of beam energy.This work presents the first fully kinetic particle-in-cell (PIC) simulationsof beam relaxation under the influence of density irregularities. We aim toindependently determine the influence of background inhomogeneity on thebeam-plasma system, and to test theoretical predictions and alternative modelsusing a fully kinetic treatment. We carry out 1D PIC simulations of abump-on-tail unstable electron beam in the presence of increasing levels ofbackground inhomogeneity using the fully electromagnetic, relativistic EPOCHPIC code. We find that in the case of homogeneous background plasma density,Langmuir wave packets are generated at the resonant condition and thenquasi-liear relaxation leads to a dynamic increase of wavenumbers generated. Noelectron acceleration is seen - unlike in the inhomogeneous experiments, all ofwhich produce high-energy electrons. For the inhomogeneous experiments we alsoobserve the generation of backwards propagating Langmuir waves, which is showndirectly to be due to the refraction of the packets off the density gradients.Our fully kinetic PIC simulations broadly confirm the findings of quasi-lineartheory and the Hamiltonian model based on Zakharov's equations. Strong densityfluctuations modify properties of excited Langmuir waves altering theirdispersion properties.
展开▼
