The asymptotic behavior of the solutions of the second order linearized Vlasov-Poisson system around homogeneous equilibria is derived. It provides a fine description of some nonlinear and multidimensional phenomena such as the existence of Best frequencies. Numerical results for the $ 1D\times1D $ and $ 2D\times2D $ Vlasov-Poisson system illustrate the effectiveness of this approach.
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Figure 2. Time evolution of $ |\Re(\widehat{E}_{1,num})(t)| $ (mode 1), $ |\Re\left(\widehat{E}_{1,num}(t)- ze^{-i\omega_1t}\right)| $ (mode 1 - leading mode 1) and $ |\Re\left((z_1+t_jz_2)e^{-i\omega_b t_j}\right)| $ (Best), for coarse $ 128\times256\times0.1 $ and refined $ 2048\times4096\times0.00625 $ grids, the latter being referred as (ref) in the legend. The parameters $ [t_{\min},t_{\max}] = [17.5,35] $ and $ [\tilde{t}_{\min},\tilde{t}_{\max}] = [1.75,17.5] $ are used for the least square procedures
Figure 3.
Time evolution of
● $ |\Re(\varepsilon^{-2}\widehat{E}_{1,num})(t)| $ (simu) vs $ |z_1e^{-i\omega_{0.9}t}| $ (approx1),
● $ |\Re\left(\varepsilon^{-2}\widehat{E}_{1,num}(t)- z_1e^{-i\omega_{0.9}t}\right)| $ (simu1) vs $ |\Re\left((z_2t+z_3)e^{-i0.9\omega_1t}\right)| $ (approx2),
● $ |\Re\left(\varepsilon^{-2}\widehat{E}_{1,num}(t)- z_1e^{-i\omega_{0.9}t}-(z_2t+z_3)e^{-i0.9\omega_1t}\right)| $ (simu2) vs $ |\Re\left(z_4e^{-i(\omega_{1}+\omega_{0.1,-})t}+z_5e^{-i(\omega_{1}+\omega_{0.1,+})t}\right)| $ (approx3),
for coarse (top) $ 128\times256\times0.1 $ and refined (bottom) $ 2048\times4096\times0.00625 $ grids. The parameters for the least square procedure is $ [t_{\min},t_{\max}] = [0,30] $ for the coarse grid and $ [t_{\min},t_{\max}] = [0,35] $ for the refined grid.
Figure 4.
A 2D-case with Best frequency: time evolution of
● $ |\Re(\varepsilon^{-2}\widehat{\rho}_{1,1,num})(t)| $ (simu)
● $ |\Re\left(z_1e^{-i\omega_{\sqrt{2}/10}t}+(z_2t+z_3)e^{-i\frac{1}{3}\omega_{3\sqrt{2}/10}t}\right)| $ (approx)
● $ 10^{-3}|\Re\left(z_1e^{-i\omega_{\sqrt{2}/10}t}\right)| $ (main mode /1e3)
● $ 10^{-3}|\Re\left((z_2t+z_3)e^{-i\frac{1}{3}\omega_{3\sqrt{2}/10}t}\right)| $ (Best mode /1e3)
The parameters $ \lambda = 0.09 $ and $ [t_{\min},t_{\max}] = [0,60] $ are used for the least square procedure to fit (simu) by (approx) and leads to $ z_1\simeq 0.036159+0.042602i $, $ z_2\simeq -0.0031761-0.00089598i $ and $ z_3\simeq 0.010351-0.046355i $.
Figure 5.
A 2D-case without Best frequency: time evolution of
● $ |\Re(\varepsilon^{-2}\widehat{\rho}_{1,1,num})(t)| $ (simu)
● $ |\Re\left(z_1e^{-i\omega_{\sqrt{2}/10}t}\right)| $ (first approx)
● $ |\Re\left(\varepsilon^{-2}\widehat{\rho}_{1,1,num}(t)-z_1e^{-i\omega_{\sqrt{2}/10}t}\right)| $ (simu - first approx)
● $ |\Re\left(z_2e^{-i(\omega_{\sqrt{5}/10,+}+\omega_{\sqrt{13}/10,-})t}+z_3e^{-i(\omega_{\sqrt{5}/10,+}+\omega_{\sqrt{13}/10,+})t}\right)| $ (approx2)
The parameters $ \lambda = 0.05 $ and $ [t_{\min},t_{\max}] = [0,240] $ are used for the least square procedure leads to $ z_1\simeq 0.052836+0.049810i $, $ z_2\simeq -0.032921-0.0010657i $ and $ z_3\simeq -0.013703-0.0050901i $.
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An illustration of the geometrical constructions introduced in the proof of Lemma 3.3
Time evolution of