The current melt-wave model

It is generally accepted that the velocity skin effect concentrates the current crossing the armature/rail interface at the rear corners of a solid armature. Ohmic heating melts these corners, and the molten metal is removed by being entrained on the rail. As the resistivity of the resulting gap exceeds that of the solid phase, the current is deflected forwards into the solid region. The mechanism results in a “melt wave” moving from the back to the front of the armature, leaving a high-resistance plasma gap of width h between the armature and the rails. Transition to a high-contact voltage is therefore a likely outcome when the melt wave reaches the front of the armature, although if the armature is initially compressed by the rails, several successive meltings along the armature surface will be required to reach transition. In this paper we give a theory for the speed ν _m at which the melt wave advances relative to the armature. Our model differs from those proposed by other authors in that the magnetic diffusivity in the rails is accurately represented. One outcome is that h depends only on the electrical skin depth in the rail and since νm∝h^-1, our value for νm is different. An expression for the armature speed V_c at which transition can occur is also derived. To test the theory, more detailed experiments are required, but it does yield plausible values for the IAT rail gun