  We describe numerical simulations, using the particle-in-cell (PIC) and
hybrid-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. The
goals of these experiments are to form and study astrophysically relevant
collisionless 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 that
interactions between the two jets are essentially collisionless at the merge
region. In highly resolved one- and two-dimensional simulations, we show that
collisionless shocks are generated by the merging jets when immersed in applied
magnetic fields (B ~ 0.1-1 kG). At expected plasma jet speeds of up to 100
km/s, our simulations do not give rise to unmagnetized collisionless shocks,
which require much higher velocities. The orientation of the magnetic field and
the axial and transverse density gradients of the jets have a strong effect on
the nature of the interaction. We compare some of our simulation results with
those of previously published PIC simulation studies of collisionless shock
formation.
