Two-dimensional modeling image of space charge migration in a needle-like electron beam plasma

Abstract

Charge neutralization in a continuous electron beam plasma (EBP) in a half-open space at intermediate and high pressures is an important physical issue. A two-dimensional numerical simulation was performed to illustrate the spatial and temporary evolution of the needle-like EBP from the beginning of electron beam injection to the quasi-steady state. The temporary evolution of the space charge separates into three phases and the involved respective physical processes controlling different phases were identified. The first phase lasts for less than 1 ns, where the space charge comprising the beam electrons is mainly near the exiting entrance. In the second phase, a significant ring-shaped distribution of space charge appears because of broad differences in the energy and charge distributions. Moreover, the space potential first increases and then decreases, a result of competition between the charge accumulation by the injection of beam electrons and the charge migration of plasma electrons. The second phase lasts from 1 ns to about 3 μs. In the third phase, the EBP reaches quasi-equilibrium, where the spatial potential is a result of ambipolar diffusion and is therefore only correlated with the distribution of plasma electrons. As a result, the spatial and temporary evolution of the continuous EBP is governed by energy and charge deposition, whereas the parameter values in the quasi-steady state are mainly determined by the energy deposition.