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Register a new userThe separation of ions and fluxes in nonlinear ion-acoustic waves
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The multi-species plasma of natural or laboratory origin is often considered to host nonlinear ion-acoustic waves. We present calculations of ion fluxes induced by nonlinear ion-acoustic waves in a plasma consisting of multiple ion populations, electrons, and dust. The following plasma models are considered: an electron-ion plasma with cold ions, a bi-ion plasma with two types of warm positively charged ions, and a dusty bi-ion plasma. It is found that in the electron-ion plasma, the wave-induced ion flux is directed oppositely to the phase speed of the nonlinear ion-acoustic wave. In the bi-ion plasma, there are two modes of ion-acoustic waves which are fast and slow waves. In the nonlinear fast ion-acoustic wave, the fluxes of both types of ions are found to be co-directed and drift against the wave. In a slow wave, the nonlinear fluxes of ions are directed in opposite directions. This result demonstrates the possibility to use these nonlinear wave-induced ion fluxes for effective separation of ions in the plasma. In a dusty bi-ion plasma, the ion separation process can be intensified by a super-nonlinear regime of slow ion-acoustic waves.
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A model of the fluid nonlinear frequency shift of ion acoustic waves (IAWs) in multi-ion species plasmas is presented, which considers the effect of ion temperature. Because the thermal ion exists in plasmas in inertial confinement fusion (ICF) and also solar wind, which should be considered in nonlinear frequency shift of IAWs. However, the existing models [Berger et al., Physics of Plasmas 20, 032107 (2013); Q. S. Feng et al., Phys. Rev. E 94, 023205 (2016)] just consider the cold ion fluid models. This complete theory considering multi-ion species and thermal ions will calculate the frequency of the large amplitude nonlinear IAWs more accurately, especially the slow mode with high ion temperature, which will have wide application in space physics and inertial confinement fusion.
Excitation of nonlinear ion acoustic wave (IAW) by an external electric field is demonstrated by Vlasov simulation. The frequency calculated by the dispersion relation with no damping is verified much closer to the resonance frequency of the small-amplitude nonlinear IAW than that calculated by the linear dispersion relation. When the wave number $ klambda_{De} $ increases, the linear Landau damping of the fast mode (its phase velocity is greater than any ions thermal velocity) increases obviously in the region of $ T_i/T_e < 0.2 $ in which the fast mode is weakly damped mode. As a result, the deviation between the frequency calculated by the linear dispersion relation and that by the dispersion relation with no damping becomes larger with $klambda_{De}$ increasing. When $klambda_{De}$ is not large, such as $klambda_{De}=0.1, 0.3, 0.5$, the nonlinear IAW can be excited by the driver with the linear frequency of the modes. However, when $klambda_{De}$ is large, such as $klambda_{De}=0.7$, the linear frequency can not be applied to exciting the nonlinear IAW, while the frequency calculated by the dispersion relation with no damping can be applied to exciting the nonlinear IAW.
The properties of the nonlinear frequency shift (NFS) especially the fluid NFS from the harmonic generation of the ion-acoustic wave (IAW) in multi-ion species plasmas have been researched by Vlasov simulation. The pictures of the nonlinear frequency shift from harmonic generation and particles trapping are shown to explain the mechanism of NFS qualitatively. The theoretical model of the fluid NFS from harmonic generation in multi-ion species plasmas is given and the results of Vlasov simulation are consistent to the theoretical result of multi-ion species plasmas. When the wave number $klambda_{De}$ is small, such as $klambda_{De}=0.1$, the fluid NFS dominates in the total NFS and will reach as large as nearly $15%$ when the wave amplitude $|ephi/T_e|sim0.1$, which indicates that in the condition of small $klambda_{De}$, the fluid NFS dominates in the saturation of stimulated Brillouin scattering especially when the nonlinear IAW amplitude is large.
We photoionize laser-cooled atoms with a laser beam possessing spatially periodic intensity modulations to create ultracold neutral plasmas with controlled density perturbations. Laser-induced fluorescence imaging reveals that the density perturbations oscillate in space and time, and the dispersion relation of the oscillations matches that of ion acoustic waves, which are long-wavelength, electrostatic, density waves.
Head-on collisions of ion-acoustic solitary waves in a collisionless plasma consisting of cold ions and Boltzmann electrons are studied using the particle-in-cell simulation. It is shown that the collision of solitary waves can occur under different scenarios. Solitary waves preserve or do not preserve their amplitudes and shapes after a collision, depending on their initial amplitudes. The range of initial amplitudes, at which a solitary wave preserves its identity after collisions, is established. The use of a diagram of initial amplitudes of colliding solitary waves to consider possible collision scenarios is discussed. The characteristic regions in the diagram of the initial amplitudes corresponding to different collision scenarios are determined, and a classification of head-on collisions of ion-acoustic solitary waves in a plasma is proposed.
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