A Study of the Action of a Strong Transverse Magnetic Field on a Supersonic Current of Ionized Argon

The action of a transverse magnetic field on a supersonic-flow plasma has been studied in a shock tube constructed for the purpose and comprising a combustion chamber followed by a steel test tube. At the end of the tube there is a duct of insulating material containing electrodes of different types arranged diametrically opposite each other in radial cross-section. The magnetic field, parallel to the plane of the electrodes, is produced by a pair of Helmholtz coils placed against the duct. It is possible to obtain induction fields of up to 3 teslas for a duration of 100 μs. Shocks with a Mach number between 12 and 15 are produced by the rupture of a copper membrane separating the combustion chamber from the test tube, which is filled with argon at a pressure of 1 mmHg. The passage of these shocks through the test tube causes bursts of ionized argon with a duration of about 80 μs and a temperature exceeding 12 000°K. Trials have been carried out using (1) continuous and (2) segmented electrodes and a magnetic field of longitudinal range both (1) shorter and (2) longer than the length of the electrodes. On open circuit in case (1), particularly strong feedback currents develop inside the plasma which shunts the two ends of the electrodes. A simple calculation gives the shape of the voltage produced at the electrode terminals, and this shape is very different from the square pulse which might be expected. If the electrodes are connnected by a very low resistance and the coupling factor is high (∼18), both the voltage and current can change sign during the burst. The conservation of energy equation shows that the kinetic energy of the plasma may then be transformed into electrical energy and stored in the self-inductance of the equivalent circuit, or vice-versa. The time resolution of the luminescent phenomenon which accompanies displacement between the electrodes seems to indicate reflection of the ionizing shock wave and considerable variations in the flow rate. In case (2), open-circuit flow indicates that the anomalies observed above disappear completely; the feedback currents are suppressed and the terminal voltage again assumes the square pulse shape which represents the maximum voltage theoretically available. With closed-circuit tests there is no change in the current sign except when the current density and the coupling factor are simultaneously very high. (author)