Growth of nonlinear intermittent fluctuations in linearly stable and unstable simulation plasma

Abstract

Simulations of nonlinear instability in a one-dimensional Vlasov plasma with a relative electron-ion drift velocity have been extended to parameter ranges more extensive than previously reported [Phys. Fluids 28, 155 (1985)]. Linearly stable runs, with Te/Ti =1, mi/me =100, and a system length of L=512λD show nonlinear hole instability. As in the case of mi/me =4, and L=32λD, the hole depth, double-layer potential structure, acceleration, and growth rate agree well with a theory of hole growth that results from momentum exchange from reflected particles. For a linearly unstable plasma (Te/Ti ≫1), the fluctuations and their time evolution are related to the linearly stable case, but are somewhat more complex. A spatially intermittent distribution of ‘‘wavelike fluctuations’’ grows to large amplitude and then evolve into trapped ion phase-space hole structures. The hole potentials are of order eφ/Te =1, as in the linearly stable (Te/Ti =1) runs, but because Te/Ti ≫1, are of large amplitude (eφ/Ti ≫1) relative to the ions. The hole structure and dynamics agree well with the large-amplitude extension of the small eφ/Ti hole theory.