Resistance Changes Caused by Vaporization Waves in Exploding Wires Ddc

The marked Increase In electrical resistance occurring at temperatures above melting can be explained by assuming a vaporizaf on, expansion wave proceeding radially inward from the wire surface. This wave reduces the conducting cross section of the wire, thereby increasing the resistance. For wires well matched to the circuit the resistance rise occurs when the condenser voltage is practically zero and can be neglected; oud for thin wires that explode rapidly on the linear portion of the current curve, condenser voltage can be taken as constant. Under these assumptions the circuit equation can be integrated and explicit expressions obtained for current, power and deposited energy. The required velocity of the wave, deduced from data on copper wires, is about 250 meters per second, an order of magnitude smaller than sound speed. A theoretical fluid dynamical model of an expansion wave involving a phase change from liquid to wet vapor is investigated; this analysis shows the expected velocity of small amplitude waves to be very close to the experimental value cited above. The theory predicts both the onset of ■'.he wrave and the wave speed as a function of deposited energy up to the critical temperature. Beyond critical temperature, where the liquid to vapor expansion does not apply, the wave speed should be only a function of deposited specific energy. The experimental data for copper wires under a variety of conditions are found to correlate to a single curve; thus, the supposed resistance anomaly in high temperature copper wires is explained. TABI£ OF COWTEHTS Page 1. HfTROIXETION 7 2. THEORY 8 3. EXPERIMEWTAL 1U h. EXPANSION WAVES IN A TWO-PHASL SYSTEM 17 5. SUMMARY 23 6. REFERENCES 2k