Particle-in-cell simulation of Buneman instability beyond quasilinear saturation

TL;DR

This paper uses particle-in-cell simulations to study the evolution of Buneman instability beyond quasilinear saturation, confirming theoretical predictions and revealing the formation of coupled hole-soliton structures.

Contribution

The study provides detailed numerical verification of the dependence of saturation energy ratios on mass ratio and drift velocity, and characterizes the nonlinear phase involving hole-soliton structures.

Findings

Quasilinear saturation energy ratio scales as (m/M)^{1/3}.

Growth beyond saturation follows an algebraic time scaling.

Electron phase space holes form coupled with ion solitary waves.

Abstract

Spatio-temporal evolution of Buneman instability has been followed numerically till its quasilinear quenching and beyond, using an in-house developed electrostatic 1D particle-in-cell simulation code. For different initial drift velocities $k_{L}v_{0}/\omega_{pe} \approx 0.1 \, - \, 1$ and for a wide range of electron to ion mass ratios (m/M), growth rate obtained from simulation agrees well with the numerical solution of the fourth order dispersion relation. Quasi-linear saturation of Buneman instability occurs when ratio of electrostatic field energy density ($\sum\limits_{k} |E_{k}|^{2}/8{\pi}$) to initial electron drift kinetic energy density ($W_{0} = \frac{1}{2} n_{0}m v^{2}_{0}$) reaches up to a constant value, which as predicted by Hirose [Plasma Physics 20, 481(1978)], is independent of initial electron drift velocity but depends on electron to ion mass ratio m/M as $\sum\limits_{k} |E_{k}|^{2}/16{\pi}W_{0} \approx (m/M)^{1/3}$. This result stands verified in our simulations. Growth of the instability beyond the first saturation (quasilinear saturation ) till its final saturation [Ishihara et. al., PRL 44, 1404(1980)] follows an algebraic scaling with time. In contrast to the quasilinear saturation, the ratio of final saturated electrostatic field energy density to initial kinetic energy density, is relatively independent of electron to ion mass ratio and is found to depend only on the initial drift velocity. Beyond the final saturation, electron phase space holes coupled to large amplitude ion solitary waves, a state known as coupled hole-soliton , are seen in our simulations. The propagation characteristics ( amplitude - speed relation ) of these coherent modes is found to be consistent with the theory of Saeki et. al. [PRL 80, 1224(1998)].