Particle-in-cell simulations of collisionless shock formation via head-on merging of two laboratory supersonic plasma jets

摘要

We describe numerical simulations, using the particle-in-cell (PIC) andhybrid-PIC code Lsp [T. P. Hughes et al., Phys. Rev. ST Accel. Beams 2, 110401(1999)], of the head-on merging of two laboratory supersonic plasma jets. Thegoals of these experiments are to form and study astrophysically relevantcollisionless shocks in the laboratory. Using the plasma jet initial conditions(density ~ 10^14-10^16 cm^(-3), temperature ~ few eV, and propagation speed ~20-100 km/s), large-scale simulations of jet propagation demonstrate thatinteractions between the two jets are essentially collisionless at the mergeregion. In highly resolved one- and two-dimensional simulations, we show thatcollisionless shocks are generated by the merging jets when immersed in appliedmagnetic fields (B ~ 0.1-1 kG). At expected plasma jet speeds of up to 100km/s, our simulations do not give rise to unmagnetized collisionless shocks,which require much higher velocities. The orientation of the magnetic field andthe axial and transverse density gradients of the jets have a strong effect onthe nature of the interaction. We compare some of our simulation results withthose of previously published PIC simulation studies of collisionless shockformation.