3-D MHD plasma armature railgun simulations

The goal of a plasma armature railgun is to accelerate the projectile to hypervelocities (i.e., to velocities beyond 5 km/s). Despite extensive research, projectile velocities achieved in the plasma armature railgun experiments were under 6-8 km/s - unfortunately, far less than predicted theoretical values. Experimental and numerical studies did not bring a full understanding of the factors limiting performance of the plasma railgun. The numerical studies so far have been limited to 1-D and 2-D computer models. In this dissertation, it is demonstrated that these models inadequately predict the main physical features of railgun plasma flow. To understand the railgun physics, 3-D magnetohydrodynamic (MHD) modelling is necessary. To perform a 3-D MHD time-dependent computer simulation of the plasma armature railgun, a new code MAP3 (MHD Arc Plasma) was developed at University of Tennessee Space Institute (UTSI). MAP3 uses an efficient numerical method to solve Maxwell's equations and Navier-Stokes equations to develop a complex time-dependent electromagnetic and velocity vector field distribution in the railgun. The importance of MAP3 numerical scheme that uses a staggered grid to solve Maxwell's equations, is demonstrated. MAP3 provides the first qualitative and quantitative understanding of 3-D physical phenomena in the plasma armature railgun. The results of the 3-D computer simulation for 1-cm and 2-cm bore railguns with ablating walls, are presented. A profound influence of the inherently 3-D nature of the railgun electromagnetic field on the plasma flow is demonstrated. A strong spatial nonuniformity of the electromagnetic force generates a flow of plasma towards the projectile near the rail and away from the projectile along the center of the bore. This plasma flow is exactly the opposite to flow provided by the previous 2-D numerical models. A zone of high-shear flow near the rail surfaces can increase viscous losses, which are not accounted for in the usual performance estimates. A direct simulation of the particular experiment is the logical step to develop this work further. A satisfactory qualitative agreement was demonstrated between numerically obtained B-dot signal (armature magnetic field) and typical experimental data. MAP3 can be used to study secondary arc formations. The 3-D flow has a tendency to elongate the plasma or arc armature, creating a current conducting tail. This process may play an important role in the formation of secondary arcs. MAP3 can be extended to analyze conditions believed to be accountable for the development of the secondary arcs and to provide reliable quantitative simulation. These conditions should include current that changes in time, and higher ablation rates.