The electrodynamics of railgun plasma armatures

A single-fluid, one-dimensional, time-dependent model of an unconfined plasma armature is discussed. The temporal evolution of the pressure and current density distributions is shown to be controlled by a combination of magnetic diffusion, shear, ablation, and magnetoacoustic wave propagation within the plasma. Different parameter regimes are explored, and the characteristic time constants and length scales are established for these various physical processes. It is shown that, when the plasma slip-velocity is low, the plasma may fragment or disperse by virtue of a diffusion-driven instability. Conversely, when ablation is high and the slip-velocity is large, the plasma will form a parasitic current sheet at the tail of the armature, which may detach from the primary plasma. The first of these instabilities is initiated by a decrease in the input current. The second occurs above a certain critical combination of magnetic field strength and ablation coefficient