Electrodynamic transition in solid armature railguns

An important research problem for solid armature railguns is understanding the sudden transition from low-voltage sliding electrical contact between the rails and armature to high-voltage contact involving a plasma arc. Although an armature can fail in many ways and thereby “transition,” two distinct transition mechanisms currently limit the performance of solid armature electromagnetic (EM) launchers. One mechanism, broadly termed “wear-induced transition,” results from uneven or excessive loss of material from the contact interface. The other mechanism is associated with a rapid reduction in driving current (negative dI/dt). A series of experiments have shown the waveform-induced transition mechanism is independent of the state of wear of an armature. This paper explores this mechanism, which we call “electrodynamic transition,” and seek to explain the cause of transition as an electromechanical instability that disrupts the liquid film interface between armature and rail. Using 3-D finite element analyses (FEA), we observed the development of localized forces at the edges of the armature as the driving current drops rapidly. This behavior, we believe, may cause molten material to be ejected from the armature-rail contact region