Nonlinear dynamics of electrons accelerated by resonant fields in nonuniform plasmas

Abstract

A numerical and analytical study is presented of the nonlinear phase-space trajectories of particles accelerated by resonant electric fields in a nonuniform plasma. The fields are represented by a driven Airy pattern characteristic of resonant excitation, and thus the results have relevance to several topics of current interest. The acceleration processes can be classified into three regimes depending upon the initial energy of the particles. The first type is a diffusive interaction which occurs for high-energy particles. The second type is slowing down due to temporary trapping inside potential wells having a spatially decreasing phase velocity. The third type is a phase-independent acceleration of slow particles. The comparison velocity used to classify these regimes is ωL/(kDL/√3)2/3, where ω is the resonant frequency, L the gradient scale length, and kD the Debye wave number. Simple analytic expressions are found that explain the basic features of numerical test particle orbits and illuminate the existence of the three categories of interaction.