The importance of three dimensions in the study of solid armature transition in railguns

Abstract

It is shown experimentally that the main processes first causing breakdown of the sliding solid contact (SSC) carrying large currents are not two-dimensional processes of the velocity skin-effect type, but pinch instabilities developing in the contact interface. Electromagnetic and electrothermal explosive ejection of ionized material of low-mass pinch waists ignite parasite-shunting arcs behind and ahead of the armature under acceleration. Thus one has to speak of transition into arcing mode not of the SSC alone, but of the launch process as a whole.It has been noted that during the transition into the arcing mode the solid armature begins to behave as a hybrid armature of pulling type. At this operation mode, two stable arcs appear, due to pinch cords moving forward along the contact surfaces, which are fixed to both contact surfaces of the armature near their leading edges. Consequently, the armature main body is accelerated under the action of internal tensile forces originating in its leading part where the I × B forces are applied.

On the role of induced fields in railguns

“…Figure 4 gives the model results for the two cases. The positive dI/dt inductance term produces a uniform, linear reduction in the rail-to rail electric field along the length of the rails in agreement with Equations (3) and (4). Localized flux creation, on the other hand, allows the high breech voltage to exist all along the rails.…”

“…The inset shows the assumed orientation of the ring between the rails. A similar approach is taken by Namias [4] to show that the fields at the armature are an equal mix of self-field and rail fields. Again, a 3 millisecond time duration was used to match typical railgun firing durations.…”

“…There are no fitting parameters for the theoretical plot; we used the measured value for Lx and the values of I and dI/dt needed for the equation were obtained from a polynomial fit through the measured time dependence of I. The agreement is remarkable, clearly verifying the validity of Equations (3) and(4).…”

“…Another source of rail damage is the transition from molten armature interface conduction to a higher temperature plasma brush interface which characterized by generalized damage on the rail surfaces. Drobyshevski et al [4] associate this transition with the merging of the pair of current tracks into a single downbore track because the increased tendency for pinch instability at higher currents.…”

Magnetic Diffusion in Railguns: Measurements Using CMR-Based Sensors

“…It leads to local heating of conducting materials, such as rails and armatures. The question of whether and to what extend this effect is responsible for the limited performance of solid armatures is still the subject of research [4], [5].…”