In-Flight Armature Diagnostics

A feasibility demonstration is reported for a method of determining instantaneous temperature and velocity of an armature in flight. Instantaneous diagnostics such as this could be critical for achieving further improvements in railgun operation. Such activity has the potential to enable design enhancements by providing information on the state of the armature and its relationship to the rail as it proceeds down the bore. The method exploits the temperature dependence of fluorescence from a phosphor coating applied to the armature. The demonstration used both a very small-scale portable railgun and a small-scale benchtop railgun. For these tests, the output of a pulsed ultraviolet (UV) laser is delivered by optical fiber through an access port drilled into the insulator between the rails. As the armature passes, the UV light illuminates a small area of phosphor on the armature. The phosphor fluoresces and decays at a rate dependent on the temperature of the phosphor. A second optical fiber in close proximity collects the fluorescence and conveys it to a detector and associated data acquisition system. Temperature is determined from a measurement of the decay time. To provide for velocity measurement on the small-scale railgun, light from a red diode laser, delivered by fiber probe inserted into the bore, produced distinctive reflections at the leading and trailing edges of the armature as it passed. Also, two grooves cut into the armature produced fiducial pulses that enabled velocity measurement