Specific problems of sliding contact in railgun. The influence of hall-effect in the micro-plasma of contact zone

The problem of pulsed current density distribution at the contact zone of railgun is one of the key problems on the path to successful acceleration of macrobodies up to velocity 1 up 2 km/s and more. Current concentration at the trailing edge of armature (called as “velocity skin-effect) creates a damage for destruction of rails contact surface in result of melting micro-zone at the points of current concentration. Electrical contact through the liquid micro-bridges of metal is unstable and it trends to transform into contact via micro-plasma volumes. The presence of current carriers in plasma with increased mobility along the surface of contact brings new features into current distribution. Magnetic field in the contact zone is strong enough (10 – 15 T and more), so this plasma is magnetized with appearance of longitudinal electric field along the contact border due to Hall-effect. In addition Hall-effect reduces the effective electrical conductivity across the contact layer. These facts have visible influence on the field diffusion velocity along the contact surface and yields to re-distribution of current density. Non-symmetry of transition resistances of contact between armature and each rails (reasons of this non-symmetry are considered in paper) causes the transversal electromagnetic force on the armature what is not favorable for the contact operation. At the same time Hall-effect can slightly reduce the “velocity skin-effect”. The finite simulation of contact process in railgun at the tensor conductivity of medium in the contact gap illustrates the specifics of current density distribution along the contact surface at such conditions. Another great problem of sliding contact (and reason of “velocity skin-effect” existence) is a big difference between the armature velocity and the speed of electromagnetic field diffusion into the material of rail. Due to original approach to simulation of contact process the possible ways for resolution of this problem have been shown in the paper by the mathematical simulation methods. Preliminary results of investigation say about principal ability to avoid the extra-concentration of current density at the sliding contact at the velocity of metallic armature 2 up 10 km/s. On the base of simulation results the main demands to design and material of rails can be postulated to provide a permissible current density at high velocity of armature motion.