On the design and testing of solid armatures for rail accelerator applications

Two different armature designs for rail accelerator applications have been studied during electromagnetic launch experiments. The designs investigated are an aluminum multifinger monobloc armature and a copper fiber brush armature. The experimental set-up used and the results obtained together with an electrothermal model that describes the armature interface behavior during the acceleration process, are presented. The results obtained lead to conclusions which are summarized. The aluminum solid monobloc armatures launched with the 3 m rail accelerator of rather loose tolerance in borewidth do not provide the expected solid-solid electrical sliding contacts. Loss of rigidity, mainly due to thermal loading of the finger tips, results in vigorous arcing, evaporation of the armature, and aluminium deposit on the rails. The copper fiber brush armatures launched with the 1 m rail accelerator with tight tolerance in borewidth provide integer solid-solid, current carrying sliding contacts in the initial phase of the acceleration process, followed by a transition to a hybrid form where plasma layers and the solid armature body constitute the current conducting interfaces. At the transition region, armature resistance increases by two orders of magnitude. A major portion of the electrical energy commutated into the rail accelerator is dissipated by ohmic heating of the rails. An analytical model to simulate electrothermal aspects of the armature which can predict the onset of transition from a solid to a hybrid armature adequately has been developed