Plasma armature characterization measurements performed at the University of Texas

The Center for Electromechanics at The University of Texas at Austin (CEM-UT) and Austin Research Associates (ARA), in a cooperative effort, have performed a series of high performance railgun experiments in order to measure plasma armature characteristics. The tests were performed at CEM-UT using a 1 m long, 12.7-mm square bore railgun. Twenty experiments were performed. In each experiment plasma pressure, arc voltage, breech voltage, arc spectrum, gun current, and magnetic probe (B-dot) signals were recorded. Glass filled polycarbonate insulators and molybdenum rails were used in all experiments. Gun currents ranged from 150 to 450 kA. Experiments were performed primarily at atmospheric pressure. The projectile in most experiments was a polycarbonate cube approximately 2.5 g in mass. A few experiments were performed in vacuum with a free running arc. In all cases, the arc was initiated by electrically exploding an aluminum foil fuse. Experimental measurements made with a piezoelectric pressure transducer indicates that pressure profile is time correlated with the armature B-dot profile. The measured parabolic pressure profile is consistent with that expected by theoretical models. Peak pressures recorded were between 30 MegaPascals (MPa) (4,300 psi) and 200 MPa (30,000 psi) and correlated well to the pressure determined by dividing the Lorentz force (calculated from the gun current when the projectile passes the transducer) by the bore cross sectional area. Also of interest is that in-bore intensity measurements made with a PIN diode indicate light emission time is comparable to the pressure profile duration. Analysis of the emitted spectrum shows continuum radiation with strong absorption lines that are associated with neutral molybdenum and neutral aluminum. Plasma resistivity in the transducer region was calculated from the gun current, muzzle volts, bore area, and the armature length as determined by the B-dot probes. The resistivity calculated appears to be consistent with previously published articles indicating that plasma resistivity is inversely proportional to the fourth root of the plasma's pressure. Due to the quick turn around afforded by the modified railgun breech, the same experimental parameters were repeated on several tests producing results that typically correlated better than 5%.