Physics of solid armature launch transition into arc mode

The railgun launch of a solid armature is conventionally divided into two stages, namely, the initial stage, where a metallic contact is retained between the armature and the rails, and the second, starting at a velocity ~0.3-2 km/s, where the contact occurs through the arc plasma. While a variety of reasons for the transition were considered, no universally accepted physical picture supported by experiment has thus far been formulated. We showed experimentally that under usual launch parameters, 3D MHD instabilities form in the contact gap because of the absence of shear resistance, which result in the formation, collapse, and, eventually, explosion of pinch waists (see E.M. Drobyshevski et al. Tech. Phys. Letts., vol.25 no.3, p.245-7, 1999). The destruction of the pinch waists gives rise not only to termination of current flow from the armature to the rail through the metal and transformation of the metallic contact in the armature/rail interface into an arc gap, but to ejection of the liquid metal and dusty plasma from the gap forward and back into the railgun bore as well. Even in the absence of MHD instabilities, armature material can be shed due to very inhomogeneous distribution of fields of thermal, electric, and dynamic parameters in the armature body. Appearance of easily-ionized matter in the bore results in the formation of shunting arcs there, which naturally reduce the armature launching efficiency. The new physics revealed by the authors in the railgun solid-armature launch transition to the arc regime provides a basis for a search for ways of increasing the launch efficiency