IEEE TRANSACTIONS ON MAGNETICS, VOL. zyxwvutsrqp 29, NO. 1, JANUARY 1993 A Triggering Mechanism for Secondary zyx Arcs' Hugh A. Calvin, Steve P. Virostek, and Jeffrey J. Anderson MARINE DIVISION, Sunnyvale, CA 94088 WESTINGHOUSE ELECTRIC CORPORATION zyxw . zyxw Abstrucr - A local zyxwvut change in the zyxwvuts magnetic field may be responsible for inducing secondary arcs at rail joints. We have developed zyxwvutsrq a model, consistent with the experimental data, that suggests a correlation between the length of the armature and the likelihood of developing secondary arcs. Introduction Secondary arcs affect railgun performance by diverting current from the primary arc (the armature), thereby reducing the accelerating force on the projectile. Different mechanisms have been proposed to account for the development of secondary arcs, such as bore wall ablation [l], temperature gradients within the armature [2], high acceleration interchange (flute) instabilities [3], and ablation-induced preferential current flow toward the rear edge of the armature [4]. A change in the rail geometry (cross-section) may occur at the rail joints, or at the power supply feed tab attachment points as illustrated in Figure 1. This change in geometry . zyxwv 8, v 'r-. i,kiiMuzzle ' I; P i 1. Rail joints and power supply feeds 757 results in a change in 'the current path leading to a disturbance in the local magnetic field add a consequent change in the armature current density. The result is a local variation in the J x B force. We do not claim that this mechanism is the only cause of secondary arc formation, but combined with other mechanisms, it may induce secondaries prematurely. In one configuration, the railgun has joints spaced at 4 meter intervals. Secondary arcs are spawned at the 4-m and 12-mjoints as illustrated in Figure 2, but no secondary arcs were spawned at the 8-m and 16-m joints. We believe this is significant. We are not able to measure the local variation in the J x B force directly, but we have correlated variations in the breech voltage as the armature passes through the perturbed region with secondary arc formation. Experimental Diagnostics The railgun performance is monitored with 8 armature B- dot probes spaced at approximately 0.5-m intervals on each of the 4-m long segments. Each segment has a voltage monitor, with additional voltage monitors at the muzzle and breech, for a total of six voltage monitors, and there are 23 rail B-dot probes on each of the 4-m long segments. The power supply current is measured by 10 air core B-dot sensors, one located in each 2 MJ power supply module inductor, and by two current transformers, one in each leg of the buswork feeding the railgun. ' The armature B-dot probes allow us to establish the trajectory and velocity of the plasma armature. The rail B- dot probes are useful in this regard as well; in addition, they allow us to establish the armature current density distribution. The voltage monitor array allows us to determine the railgun average L' and R' [5]. Combinations of wave forms from all these monitors, coupled with suitable electrical models, may be used to establish the *This work was supported by the Strategic Defense Initiative Office and administered by the Defense Nuclear Agency. 0018-9464/93$03.00 zyxwvutsr 0 1993 IEEE