Materials evaluation for use as plasma brush electrodes with hybrid armatures

Abstract

AWucf-There is a strong motivation to develop a railgun armature which will function as a non-arc, solid contact armature throughout the complete projectile acceleration to launch process. Solid contact armatures are more ef'ficient in converting electrical to kinetic energy and generally cause less erosion of the rail. Considerable effort is being directed towards designing armatures capable of reaching 3+km/s while avoiding transition from a solid to an arcing contact. While nearly 2 W s has been achieved in a non-arcing contact, it may be inevitable that an arcing contact will be unavoidable during the finial acceleration to full desired velocities. At present most solid armatures are fabricated from aluminum or copper. While aluminum or copper might be good choices for solid contacts, this paper introduces the possibility that a different material will better serve as an electrode in an arcing contact. For example upon transition, the'conduction of current through a plasma brush results in rapid erosion of aluminum armatures. When transition to an arcing contact occurs, we have found that magnetic pressure can be used to minimize the length of the plasma brush, which helps minimize the mass of the armature and improves the overall effiiency of energy transfer to the projectile payload. However, Short length brushes result in high current density, and high erosion rates. We have measured the erosion of aluminum surfaces when used as a cathode and as an anode while magnetically containing the brushes. We have also measured the erosion of a thoriated tungsten (W-Th) cathode when used in the same manner. The erosion of the W-Th was more about a factor of 100 less than the A.! This paper presents these results and a discussion of electron emission for AI, Cu, W and W-Th.