Two-dimensional PIC simulation of collective Thomson scattering in a beam-plasma system

TL;DR

This work uses self-consistent two-dimensional PIC simulations to study collective Thomson scattering (CTS) in beam-plasma systems. By analyzing the full 2D $k$-space spectrum and its projection for specific scattering directions, it shows that beam-driven instabilities cause characteristic asymmetric or distorted CTS features, explained by resonance with Langmuir, ion-acoustic, and beam modes. The study demonstrates that Buneman instability enhances electron features while ion-acoustic instability enhances ion features, with asymmetric spectra persisting even in weak-beam regimes, offering a framework for interpreting ionospheric ISR and laboratory CTS measurements. Limitations include the chosen frequency ratio, reduced ion-to-electron mass ratio, and the need for carefully considering magnetic effects, but the results provide a foundation for diagnosing nonequilibrium plasmas through CTS.

Abstract

Collective Thomson scattering (CTS) in a beam-plasma system is reproduced by using 2D PIC simulations and the characteristics of the scattered wave spectrum are examined. By formulating the geometric shape of the scattered wave spectrum in wave number space, where the velocity vector of the beam component and the wave vectors of the incident and scattered waves are arbitrary, it is demonstrated that the spectrum in 2D wave number space becomes asymmetric. The spectrum of scattered waves propagating in a specific direction is presented as a function of wavelength to show that the electron (ion) feature is amplified and becomes asymmetric or distorted when Buneman (ion acoustic) instability occurs. An additional simulation is conducted for a weak, linearly stable beam-plasma system with a hot beam, and confirmed that the obtained scattered wave spectrum shows asymmetric feature. The results are expected to be applicable to the interpretation of radar observations of ionospheric plasmas as well as CTS measurements in laboratory plasmas.

Figures (7)

• Figure 1: Initial conditions of simulations. (a) $E_z$ and (b) $B_y$ components of incident wave packet (color scale) and beam direction (arrows). Ion (black lines) and electron (gray lines) distribution functions of (c) Run 1: two-component equilibrium plasma, (d) Run 2: strong ion-beam plasma, and (e) Run 3: weak ion-beam plasma. Note that the electron distribution in panel (e) is scaled by a factor of 10 due to the large difference in peak values between ions and electrons.
• Figure 2: Energy time history of (a) Run 1, (b) Run 2, and (c) Run 3.
• Figure 3: Linear dispersion relation of (a) Run 2 and (c) Run 3. The solid and dashed lines denote real and imaginary parts of frequency. $\omega-k_y (k_x=0)$ spectra of $E_y$ in the time range $0 < \omega_{pe}t < 71$ for (b) Run 2 and (d) Run 3.
• Figure 4: $\omega-k_y (k_x=0)$ spectra in the time range $216 < \omega_{pe}t < 287$ for (a,d) Run 1, (b,e) Run 2, and (c,f) Run3. The upper and lower panels show $E_y$ and $E_z$ components.
• Figure 5: Time averaged $k_x-k_y$ spectra of $E_z$ for (a) Run 1, (b) Run 2, (c) Run 3, and (d,e,f) their interpretation.