Structure of plasma armature of a railgun

The use of streak photography to observe the visible radiation from plasma armatures in a railgun with a transparent body enables a number of insights into the plasma behaviour to be obtained. When the projectile exits from the railgun a rarefaction wave is generated an the front of the plasma. The streak photographs enable the time the wave takes to travel to the back of the armature to be measured, and hence the sound speed in the armature plasma to be found. From this speed it is computed that the plasma temperature is 55000K. Because the method gives the time of first arrival of the wave, it gives the temperature of the plasma where it is hottest, namely along the center of the bore. Knowing the temperature enables the armature mass to be calculated. It is 2mg in the 1cm square bore railgun used. Knowing the temperature also enables electrical resistivity of the plasma to be calculated. Using the gun parameters and gun current, a theoretical value for muzzle voltage is found, and this yields the surprisingly low value of 30V when the measured value is 150V. It is shown that the missing 120V can be accounted for by the effect of "constriction resistance" in the "sub arcs" which electrically connect the gun rails to the armature plasma. Finally, the streak photographs indicate that there is a flow of material forward in the armature and backwards along the insulating walls of the bore and that some of this is continuously escaping from the rear of the armature.