Paper detail

Slowdown of interpenetration of two counterpropagating plasma slab due to collective effects

The nonlinear evolution of electromagnetic instabilities driven by the interpenetration of two $e^-\,e^+$ plasma clouds is explored using {\it ab initio} kinetic plasma simulations. We show that the plasma clouds slow down due to both oblique and Weibel generated electromagnetic fields, which deflect the particle trajectories, transferring bulk forward momentum into transverse momentum and thermal velocity spread. This process causes the flow velocity $v_{inst}$ to decrease approximately by a factor of $\sqrt{1/3}$ in a time interval $Δt_{αB} ω_p \sim c/(v_{fl}\sqrt{α_B})$, where $α_B$ is the magnetic equipartition parameter determined by the non-linear saturation of the instabilities, $v_{fl}$ is the initial flow speed, and $ω_p$ is the plasma frequency. For the $α_B$ measured in our simulations, $Δt_{αB}$ is close to $10 \times$ the instability growth time. We show that as long as the plasma slab length $L > v_{fl} Δt_{αB}$, the plasma flow is expected to slow down by a factor close to $1/\sqrt{3}$.