Performance loss due to wall ablation in plasma armature railguns

on results that illustrate the importance of wall ablation. Section 111 develops a simple model of a plasma armature incorporating wall ablation. The model is applied first to the analysis of a constant current railgun to show how ablation affects railgun behavior and then to the experimental data of Section 11. Section IV develops an extension to the simple ablation model that includes viscous drag on the arc plasma. Again comparisons with the data of Section I1 support the inclusion of viscous drag as an important process in railguns. Finally, in Section V the future of plasma armature railguns is examined with particular emphasis on scaling to large devices and techniques to eliminate Parametric measurements have been performed on a plasma armature railgun at Las Alamos. The railgun is extensively instrumented for studies of the projectile motion and its interaction with the plasma armature. The most important parameter, driving current, was varied from 100 kA to 400 kA. Additional parameters investigated include current waveform, injection velocity, injection gas, wall insulation material, and initial pressure. For all combinations of parameters investigated, the measured performance was substantially below theoretical or control the ablation process. predictions. A strong correlation was found between performance loss and abnormal plasma armature features such as multiple arc formation, or separation of the armature from the projectile. In extreme cases, the plasma armature was observed to come to rest inside the railgun. A plasma armature model has been developed which successfully accounts for the measured performance loss and for many of the abnormal plasma armature features. By incorporating the ablation of wall material into the armature plasma thismodel predictstwo parasitic forces that dominate the motion of the armature at high velocity. One force is the inertial drag term mav. The other is frictional drag between the hot, turbulent plasma and the walls, which increases as m,v2. Simple scaling relations, which incorporate the plasma I. Experimental Apparatus The experimental results reported below were obtained with the HYVAX Phase I (HYVAX-I) railgun. The HYVAX railgun is a high performance modular device designed for distributed operation. It has a design length of 13 m and a theoretical velocity capability of 25 km/s at a design current of 500 kA. HYVAX-I consists of two 1.22 m long barrel modules and three 0.30 m long current connection modules from the H W A X device. The modules are assembled as a single stage railgun with an effective lenmh of2.5 m. A cross section ofthe barrel is shown ~ armature model, rho; that velocities >IO km/s wil l he ex. tremely dificult to achieve with plasma armature railguns unless the effects ofablation are eliminated or carefull, controlled.