Evolution of electron hole propating in inhomogeneous plasma

Abstract:

The evolution of electron hole (EH) propagation in the presence of plasma density inhomogeneity is investigated by both particle-in-cell (PIC) simulation and theory. At the beginning of simulation, EH is generated by plasma blob injection. When EH passes inhomogeneous plasma region, the EH potential (1^st order) increases monotonically in proportion to the background plasma density (0^th order). With this 1^st order growth, 2^nd order back and forth oscillations of the potential are observed in faster time scale compared to EH propagation time scale. A bulk motion of trapped electrons due to density gradient produces these potential oscillations and the frequency of 2^nd order oscillation is found to be comparable to the bouncing frequency of these trapped particles. Using the theoretical technique introduced by Schwarzmeier, we discovered a normal mode localized in the 1^st order EH potential with frequency comparable to average bounce frequency of trapped particle. Such mode has an odd symmetry that is exactly consistent with simulation result. Furthermore, ions are pushed backward in coherence with 2^nd order potential in simulation. These coherent ions form into a train of backward propagating ion acoustic solitary waves (IASWs) that evolve to ion hole in simulation. Finally, EH starts to dissipate by nonlinear Landau damping and converge to another stable EH as it reaches high-density, homogenous region.