Experimentation on Magnetically Driven Shock Waves

This experiment uses a relatively simple physical phenomenon to produce shock waves of an ionized gas. If two wires carry currents in opposite directions, they repel one another, due to the mutually coupled magnetic field. In this experiment, the "two wires" consisted of a stream of ionized gas and a fixed conductor. The stream of ionized gas was produced in an evacuated tube by discharging a bank of capacitors; it then was accelerated down the tube by the "push" exerted by the magnetic field. The magnetic field, created by the current flow in the circuit, can be determined from measurements of this current. Three independent techniques for the measurement of current agree within 25 per cent of one another. The velocity of the shock wave was measured, using two photo-multiplier tubes to sense the emitted light of the moving plasma at two positions along the shock tube. The velocity was around 10 cm/¿sec, or 300 times the speed of sound. To minimize the inherent complexity of the experiment, the pressure in the tube was held constant. The magnitude of the pressure was derived both by measiring the gas conductivity and by using a calibrated thermocouple vacuum gauge. The gas used was air at a pressure of 0.7 mm of mercury.