Strongly Enhanced Stimulated Brillouin Backscattering in an Electron-Positron Plasma

Stimulated Brillouin backscattering of light is shown to be drastically enhanced in electron-positron plasmas, in contrast to the suppression of stimulated Raman scattering. A generalized theory of three-wave coupling between electromagnetic and plasma waves in two-species plasmas with arbitrary mass ratios, confirmed with a comprehensive set of particle-in-cell simulations, reveals violations of commonly held assumptions about the behavior of electron-positron plasmas. Specifically, in the electron-positron limit three-wave parametric interaction between light and the plasma acoustic wave can occur, and the acoustic wave phase velocity differs from its usually assumed value.

Figure 1

Dispersion relations for the Langmuir and acoustic modes as $\beta =m_e/m_i$ is varied between 0 and 1 for $T_e=T_i$. ${\lambda }_D^2=T_e/4\pi e^2{n}_e=C_e^2/{\gamma }_e^2{\omega }_e^2$.

Figure 2

(a) Amplified seed pulses of different wavelength (${\lambda }_{\text{seed}}$) after passage through a 0.8 mm plasma with $n_{e,i}=10^{19}{\mathrm{cm}}^{-3}$ and $m_i=10m_e$. Initial counterpropagating pump and seed intensities are $10^{14}\mathrm{W}/{\mathrm{cm}}^2$ and $T_{e,i}=70\mathrm{eV}$. (b) Maximum final intensity of an amplified seed at varied wavelength and ion mass $m_i$ with the same other parameters as (a). The dashed line indicates the initial seed intensity.

Figure 3

Seed wavelengths (${\lambda }_{\text{seed}}$) at which maximum Raman or Brillouin amplification is observed in PIC simulations (points) at varied plasma temperature ($T_{e,i}=20$, 70, 240 eV) and ion mass ($\beta =m_e/m_i$), found by varying ${\lambda }_{\text{seed}}$ at fixed ${\lambda }_{\text{pump}}=800\mathrm{nm}$. The solid lines are calculated from Eq. (5). For the Brillouin mode, the upper and lower lines at each temperature correspond to ${\gamma }_{e,i}=3$ and ${\gamma }_{e,i}=1$, respectively. The 0.8 mm long plasma has a density $n_{e,i}=10^{19}{\mathrm{cm}}^{-3}$ and $I_{\text{pump},0}=I_{\text{seed},0}=10^{14}\mathrm{W}/{\mathrm{cm}}^2$. The gray circles are calculated at a temperature of 70 eV and a higher pump intensity ($10^{15}\mathrm{W}/{\mathrm{cm}}^2$) and overlap the lower intensity points for the Raman mode. The bars give uncertainty due to the finite size of changes in ${\lambda }_{\text{seed}}$ between simulations.

Figure 4

Perturbation growth rate ($\mathrm{\Gamma }$) for Raman and Brillouin scattering from the analytic dispersion relation (a) and found with PIC simulations below the saturation regime (b) at $T_e=T_i=20\mathrm{eV}$ and $n_e=10^{19}{\mathrm{cm}}^{-3}$. In (b), the initial seed intensity is $10^{11}\mathrm{W}/{\mathrm{cm}}^2$. (c) Simulated change in seed pulse intensity ($I_{\text{seed},0}=10^{14}\mathrm{W}/{\mathrm{cm}}^2$) after passage through 0.4 mm of plasma ($T_e=T_i=70\mathrm{eV}$, $n_e=10^{19}{\mathrm{cm}}^{-3}$).