Measurements of the spatial distributions of current in a rail gun arc armature

The BRL 1 metre long, 12.7 mm square bore rail gun, powered by a 20 kJ inductive source, which utilizes arc drive to accelerate a 2.5 gram projectile, was used as a test bed for the study of dynamic properties of driven arcs. The measured signatures of four B-dot loops, time of arc arrival at four light pipe-PIN diode detectors and the instantaneous total current are used in an analysis of the size of the arc armature. A current density distribution in an arc armature was found whose calculated effect on inductive B-dot loops matches that of our actual measurements. A computer performed a Biot-Savart law integration over the arc volume of an arbitrary, parametrically defined, one dimensional current distribution and then calculated the time derivative of the resulting field to predict the induced voltage at each of the field measuring loops. The comparison between the predicted induced voltages and the observed signal was taken as the criterion for acceptance of a given current density distribution. A directed search routine which minimized the difference between the predicted induced voltage and the observed response was used to vary the current distribution parameters. The best fits to the data were obtained with a current distribution having an initial width at half maximum of 1.4 cm at 170 kA. This width increased inversely with the decreasing instantaneous total current. Our computed current density distributions compare favorably with predictions by a first principles model of a plasma arc armature and data obtained for light emission by the arc.