Particle-in-cell simulations of nonneutral magnetic vortex dynamics

Summary form only given. Non-neutral magnetic vortices have been observed in electromagnetic particle-in-cell (PIC) simulations of plasmas where density gradients exist. When plasma dynamics on the length scale of the electron inertial length (c/ω_pe) and the time scale of electron motion (t~1/ω_pe) is considered, charge separation and electron inertia effects become important. Relativistic effects may also be important in a regime where very large current densities exist (|J|>>v_Te/en_e). This can arise when a large magnetic field pushes on an initially unmagnetized plasma (e.g., in a plasma opening switch, dense plasma focus, etc). One phenomenon which can occur in this regime is the formation of magnetic vortices, in which a strong azimuthal current sustains an axial magnetic field. An associated charge separation gives rise to a radial electric field, and the electron motion is primarily an E×B drift in the azimuthal direction. For short times (t <;<;1/ω_pi), the ions are motionless and the vortex is a steady-state solution to the two-fluid approximation to the kinetic equations. Gradients in the background plasma density can cause the vortex to propagate along density contours, and can transport magnetic fields into an unmagnetized plasma. Theory indicates that field penetration must be accompanied by energy dissipation and the balance between electromagnetic energy, thermal, and directed kinetic energy can be verified. On longer time scales (t ≈1/ω_pi), the strong electric fields in the vortex can accelerate the ions radially outward from the plasma disrupting the vortex and giving rise to a bi-modal ion velocity distribution function. In plasmas with multiple ion species, this can lead to a spatial separation of the ion species. In the fixed-ion limit, the two fluid equations for a magnetic vortex reduce to a fairly simple form which ha- a semi-analytic solution. This solution has been used as an initial condition in a PIC code, in order to study the dynamics of vortex propagation and the effect of gradients in the background plasma density.