A solution for the three dimensional rail gun current distribution and electromagnetic fields of a rail launcher

Abstract

A bstruct-Rail guns generate electromagnetic signatures that contain frequencies extending from quasi-dc to tens of kHz.The c h a r a c t e r i z a t i o n of these fields for electromagnetic compatability concerns remains, however, largely unexplored. Accordingly, this paper includes a discussion of the theoretical models used to predict the inductance gradient, the transient behavior of the currents produced in the rail gun structure, the dynamical generation of the external fields, and a comparison of the theoretical model with experimental data.The predicted rail inductance gradient of LL = 0.52 p H / m compares very well with the measured value of 0.522 p H / m . The existence of an inductance gradient efficiency factor, E,,, is demonstrated, with a derived value of 0.75. This produces an effective inductance gradient of L' = E ,,LL = 0.39 p H / m which leads to a predicted muzzle velocity of 525 m/s that is within 5% of the measured value.Predicted magnetic field waveshapes a r e in good agreement with observations close to the bore center.For radial distances greater than a foot, measured peak fields exceed predictions by a factor of two to three. This issue is being investigated. a limited comparison with experimental data. Companion paprs by Cobum et al. [1]-[2] address the observations and experimental configuration in detail.The incorporation of a rail gun into a weapon system requires a quantification of the relationship between the power source, rail gun dynamics, and electromagnetic fields generated by the rail and armature currents. In order to ensure EMC with other equipment, it is necessary to not only characterize the dominant low-frequency (dc to tens of kHz) fields generated by the moving armature, but also the higher frequency emissions generated by continuum arcing along the rails, and the larger arc produced at the muzzle.For predicting the low-frequency magnetic fields it appears that the circuit approximation to the rails, which has traditionally been used to study armature acceleration, is probably adequate. On the other hand, a modal analysis of the rail system model viewed as a transmission line is required at higher frequencies in order to model arcing contributions. In addition, it may be necessary to incorporate the effects of electromagnetic shielding in the evaluation of EMC for an actual system. This paper is the first in a series of reports that eventually will quantify EMC considerations for all frequencies of interest. We concentrate at present only on the low-frequency fields generated by the rail current in a solid mature, for which there is negligible arcing. CONCEPTUAL ISSUES AND OVERVIEW

Electromagnetic field measurements near a railgun

“…3, where the peak amplitude is 160 U. This equation applied to the parallel plate rails gives L = 0.52 pH [6]. The railgun inductance gradient, L', is taken as, L divided by I, or 0.52 pWm.…”

Physics and Dynamics of Current Sheets in Pulsed Plasma Thrusters

“…Because of the solenoidal nature of magnetic fields, the lines of force must wrap around the current sheet, resulting in a field with reversed direction in front of the current sheet. By solving the full boundary value problem with the conductors configured as illustrated, it can be shown that the magnetic field in front of (upstream side) the current sheet is much smaller than inside (downstream side) the current loop [29]. Nevertheless, there will be a region of finite width inside the current sheet where the magnetic field is reversed; this is interesting because it implies that the plasma near the leading edge of the current sheet will be accelerated opposite to the sheet velocity vector, v. The magnetic field reverses at some point inside the current sheeteventually reaching its peak value at the back edge of the current sheet.…”

Gas-Fed Pulsed Plasma Thrusters: Fundamentals, Characteristics and Scaling Laws

“…Using a conformal mapping technique to determine the electromagnetic fields in a parallel rail launcher, ref. [73] provides the necessary tabular data to determine the actual value of the inductance-per-unit-length for a given electrode aspect ratio. The graph in Fig.…”