Measurements on plasma-hybrid armatures

Summary form only given. Measurements were conducted on the HERA railgun facility to observe the properties of hybrid armatures as a function of total current and fuse thickness. The hybrid armatures were designed as simple 'thick' fuses at the rear of the projectile. Depending on the thickness, the fuse was observed to transition completely to a plasma armature or to remain partially intact. Preliminary data indicate that the velocity of the armature is affected by fuse thickness. Several other measurements were made on both plasma and hybrid armatures to observe the structure of the armature. Microwave interferometry was used to determine how much plasma was in the 'tail' of the projectile, and how long conducting material remained in the bore. It was determined that a significant amount of plasma remained near the breech end of the gun. Measurement were also made using a b-dot probe to determine the symmetry of the armature in the rail-to-rail plane. This was done on a 0.7" square bore gun with a free arc. It was observed that, at higher velocities, the arc became asymmetric in the plane of interest.