Numerical investigation of an ablation-dominated plasma armature

The main plasma armature in a small bore rail launcher is investigated using a one-dimensional numerical model that is based on experiments at very high rail current densities. These experiments revealed an excellent momentum efficiency of the driving arc (i.e., the main plasma armature) in spite of heavy rail and insulator ablation. The numerical work focuses on the ablation process and the effects caused by ablated material entering the plasma armature. These investigations reveal a significant influence of wall ablation on the thermal properties of a plasma armature. Although swept up only partially by the armature, the ablated material has a strong cooling and dissipative effect. The acceleration drag caused by ablated material exceeds the viscous plasma friction and is a major process reducing the electromagnetic pressure of the driving arc. The simulation shows that the convective heat flux must not be neglected, although thermal radiation is the dominant heat transfer mechanism in a railgun. In the leading edge of the armature, where the loss mechanisms prevail, a pressure decrease towards the projectile base can be observed. The region of decreasing pressure grows with increasing velocity or higher armature current. This development may lead to a buffer zone between the armature and the projectile.