Arc motion in railgun plasma armatures

Recent in-bore optical measurements in the 1 cm square bore UTSI railgun have revealed that a dark, non-emitting and presumably cold buffer region can separate the driving arc from the projectile by as much as 5 bore widths. This is consistent with the common observation that the back sides of railgun projectiles suffer no ablation damage from the arc, even when substantial ablation from the rails and insulator surfaces is observed. This dark zone is not predicted by current time dependent or steady-state theoretical models. There is some experimental evidence that the zone may be formed, initially, during the fusing transient, but reexamination of a theory due to Maecker suggests that there are physical mechanisms that can cause the arc to move away from the projectile even in a quasi-steady-state. Maecker's general theory for arc motion and displacement has been applied to the pressure dependent plasmas and geometry appropriate for railgun arcs, and it was found that the motion of the arc relative to the material in which it is imbedded is controlled by the relative magnitude of inhomogeneous absorption from the electromagnetic field and the diffusion of energy through radiation and thermal conduction. The pressure dependence of the thermodynamic and transport properties determine the relative importance of these two mechanisms, and these have been evaluated for copper plasmas under conditions typical of their experiments