Electromagnetic ghosts in pair plasmas

TL;DR

The paper addresses how weakly nonlinear EM pulses collide in pair plasmas to create long-lived electromagnetic ghosts. It employs 1D and 2D PIC-like simulations with the EPOCH code to study how density granulation traps EM energy and how magnetic field, temperature, and polarization affect the phenomenon. Key findings show that ghosts persist long after collision with trapped energy density around $10^{-4}$ of the pulse peak; ghost formation is suppressed by moderate to strong guide fields (e.g., $b_0$ up to $4$) and decoheres when the thermal parameter $\Theta$ exceeds $\Theta_0 = a_0^2$. The results reveal a two-stage trapping mechanism—initial Anderson-like localization followed by nonlinear evolution of density walls—leading to a granular pair plasma and offering insights into laser-plasma interactions and high-energy astrophysical plasmas.

Abstract

Collisions of two weakly nonlinear, $a_0 \ll 1$, counter-propagating EM pulses in pair plasma leave behind a long-surviving collection of localized waves, {\it an electromagnetic ghost}. Waves are trapped (localized) by the random large density fluctuations created by the beat between the pulses. The process is similar to random plasma density grating and/or Anderson-like wave localization. Structures survive for long, mesoscale times, while the EM energy slowly bleeds through high density walls of the density trap. Large guide magnetic field, $ω_B \geq $ few $ω$, suppresses the formation of the ghosts.

Figures (8)

• Figure 1: Colliding EM pulses in pair plasma. Columns (left to right): snapshots at time $50$ fs (before collision), $150$ fs (soon after collision), $1000$ fs (long after collision). Rows (top to bottom): Poynting flux, density, energy density. Poynting flux and energy density are normalized to peak values, except in the right column where they are multiplied by $10^5$. All plotted quantities are averaged over one wavelength. Peak nonlinearity $a_0=10^{-2}$, pulses' duration full width at half max is 50 fs. $n_x=100,\, n_p=100$. (Results for $n_x=30,\, n_p=30$ look qualitatively similar.)
• Figure 2: Evolution of the structure of the ghost with time: profiles of energy density (top panel) and plasma density (bottom panel). With time the ghost becomes wider. With time the central density depletion increases. In these simulations $n_x=n_p=100$.
• Figure 3: Dependance of the ghost on guide magnetic field. For $b_0=0, \, 0.5, \,2$ the curves are almost coincident. For $b_0=4$ the ghost nearly disappears. In these simulations $n_x=n_p=100$.
• Figure 4: Effect of temperature. At $\Theta = 0.5 \times a_0^2 = 5\times 10^{-5}$ the ghost is nearly gone (and even weaker for larger temperatures).
• Figure 5: Density structure after collision of 5+5 pulses, at time $t=4000$ fs, compared with the undisturbed plasma. Density is averaged over one wavelength. Slight overall increase is due to ponderomotive push from the pulses.