The Use of High-Speed Video as an In-Bore Diagnostic for Electromagnetic Launchers

One of the difficulties associated with electromagnetic launch is the lack of diagnostic tools capable of surviving the in-bore environment. In railguns, the principal diagnostics are current and voltage measurements, in addition to magnetic field (B-dot) sensors. X-rays and high-speed video are often taken of projectiles after exit, but imaging the in-bore performance of armatures has been limited. In those cases where high-speed cameras have been used to look down the bore of the launcher, the short depth of field ultimately limits their usefulness. High-speed video has been used to look at solid armatures from the side, though this has been limited to either stationary or hybrid solid/plasma armatures. In most cases, laboratory railguns are surrounded by high-strength laminated steel containments that preclude the kind of visual access required for imaging. The University of Texas at Austin and the Institute for Advanced Technology have performed experiments using a modified containment system that allows real-time imaging of the armature as it is accelerated down the launcher. Numerous tests were performed using aluminum armatures on copper and copper-alloy rails at currents up to 150 kA and observed velocities up to around 300 m/s. Grayscale imaging of the armature was performed using a Phantom imaging camera and a 10-ms high-intensity flash lamp, which provided frame rates in excess of 40 000 pps and integration times down to 2 mu s. Successful imaging of the armature-rail interface was observed for a variety of test conditions including solid-state and transitioned armature-rail contacts. In addition to providing direct evidence of overall armature behavior, video was used to independently measure projectile velocity, which is usually inferred from relatively coarse B-dot measurements.