Experimental Determination of the Current Distribution in a Simulated MHD Generator

The presence of the applied magnetic field in Faraday-type MHD generators with segmented electrodes makes the current distribution in the vicinity of the electrodes non-uniform. This non-uniformity has been analysed by numerous authors. In the present work, an iterative solution is obtained for the equations describing the current distribution in MHD generators with segmented electrodes. The calculated data are compared with the results of experimental measurements. The measurements were performed in a potassium seeded argon plasma at 2000K gas temperature, 1.1 atm. gas pressure, and 130 m/sec flow velocity, with a maximum magnetic field intensity of 10 kG. Since at these plasma parameters the current intensity produced by the induced (v x B) e.m.f. is too low, an additional electric field has been applied to each electrode pair. The current density was determined from the intensity of the potassium resonance lines. The intensity of these lines is a measure of the electron temperature which in turn is a measure of the local current density. The light emission in the direction of the applied magnetic field was measured in the region between two electrode pairs at a sufficiently high spatial resolution. The direction of the applied electrostatic field was either parallel or anti-parallel to that of the induced (v x B) e.m.f. The experimental data are qualitatively in good correspondence with the calculated values. However, the centrally-symmetric current distribution assumed in the calculations has not been found in the experiments. Since this discrepancy was found to be independent of the current direction; it cannot be related to electrode effects (cathode drop, etc.). At those electrodes where the current concentrates at the upstream electrode edge, the correspondence between the calculated and the measured data has always been satisfactory. At those electrodes where the current concentrates at the downstream electrode edge the discrepancy between the analytical and the experimental results was found to be independent of the polarity of the electrode. The potential distribution along the insulator wall between two electrodes on the same side of the duct has also been measured by means of electrostatic probes. The measurements taken on the side where the current concentrates at the upstream electrode edges are qualitatively in good agreement with the calculated data. The deviation from the theoretical values detected at the opposite wall are consistent with the results of the optical measurements. No theoretical explanation has been found for these deviations. They might be due to the effect of the plasma motion upon the current distribution. (author)