Simulation of Ion-Acoustic Wave and Pseudowave Generation in a Plasma

Abstract

The unexpected effects of a grid in attempts to launch ion-acoustic waves are discussed. A time varying potential on a grid was found to inherently produce ion bursts (pseudowaves) at all voltages and frequencies used. The voltages φ ranged from low (eφ=κTi), to medium (eφ=κTe), to high (eφ≫κTe) in cases where the electron-to-ion temperature ratio was large (Te/Ti=30). The frequencies f used ranged from below to above the ion plasma frequency fpi(0.25fpi≤f≤2fpi). Ion-acoustic waves, however, could be clearly observed only for low excitation voltages and for frequencies at and below the ion plasma frequency. In the computer simulation, the grid is treated as a one-dimensional source of potential, and particle absorption and scattering by the grid wires is neglected. A one-dimensional ion distribution function F(x, v, t) is found by numerically integrating the nonlinear ion Vlasov-Poisson equations by a finite difference method for phase space. Electrons are treated as a massless neutralizating fluid, and the electron density is computed by balancing the electric field against the thermal electron pressure in a self-consistent fashion [ne=n0 exp (eφ)(κTe)−1].