Performance of a Faraday Rare Gas Alkali MHD Generator
The performance of the IPP rare gas alkali MHD generator was theoretically and experimentally investigated. The open-circuit Faraday voltage, the short-circuit Hall voltage, and the load characteristic were measured at different segmentation ratios s/h and various magnetic field strengths. Furthermore, the current streamline pattern in the generator duct was determined from the equipotential lines measured by means of a movable probe. From these measurements it could be concluded that the generator is practically free of any leakage currents and internal plasma short-circuits. This conclusion was further supported by the reasonably good agreement between theory and experiment regarding the current voltage characteristic of the generator. In the calculation allowance was made for the influence of ionization instabilities, segmentation effects, and voltage drops at the electrode, but mechanisms producing leakage currents or internal plasma short-circuits were not considered. N rare gas alkali MHD generators the electron density and hence the electrical conductivity attain equilibrium in accordance with the electron temperature elevation. This "nonequilibrium ionization" in conjunction with the fact that the electron temperature elevation in the generator may show strong local variations and occurs in a medium flowing at high velocity through the generator duct gives rise to a number of complications which are not so pronounced in the case of an equilibrium generator or not even present there. The most important ones are: elongation of the current paths due to relaxation and convection effects particularly pronounced at the generator entrance, inhomogeneities in the current density distribution produced by the electrode configuration, axial leakage currents and ionization instabilities. All of these phenomena impair the performance of the generator. It is therefore necessary to investigate them, and if possible, lessen their influence. The performance of "nonequilibri um" MHD generators has already been investigated in a variety of test rigs. It was possible to achieve high power densities and electron temperature elevations in short-time MHD generator systems1'2 with operating times of up to a few milliseconds. The test rigs working in continuous operation3'4 involved technical problems which prevented the theoretically predicted power output from being attained. Better, but still not quite the ideal values of the power density and electron temperature elevation were obtained in the "blow-down facilities."5"7 Whereas this deviation is accounted for in Ref. 5 by assuming external leakages strictly associated with the duct technology, it is ascribed in Refs. 6 and 7 to the presence of highly conducting layers along the electrode walls which impair the generator performance. This paper reports investigations on a generator working in continuous operation. These were concerned not only with the measurement of the Faraday voltage across the duct, the Hall voltage between the electrodes, and the current flowing through one electrode pair, but also with the local distributions of the various plasma parameters, viz., the potential and current density distributions, the velocity profile, and the electron and gas