Simulation of the Eddy Current Effects on the Inductance Gradient of Railgun

Inductance gradient of railgun is one of the most important parameters to railgun launch efficiency. The inductance gradient of simple railgun without metal container (tube) can be calculated via Kerrisk´s approximate formula when the structure of rails is fixed. However, the metal container would be necessary if the repulsion on the rails is great enough in mega Ampere current railgun. Then eddy current will be induced in metal container because of the variable magnetic flux density. In this paper, the effects eddy current putting on the inductance gradient of railgun are investigated with Maxwell 3D modular of Ansoft Corp.. Models of railgun with metal-insulated combination containment (laminated containment) are simulated and compared with solid metal containment railgun and air containment railgun. The eddy current density and Lorentz force put on amature of each model are calculated. It is found that the inductance gradient fluctuates with the armature position relative to containment and the fluctuating amplitude decreases with the electrical conductivity. The reduction of inductance gradient is very little when the metal segments of containment are not wider than caliber. It´s notable that the inductance gradient decreases rapidly near muzzle.