Occurrence and behavior of current spokes in MPD arcs

A pulsed MPD arc has been operated to experimentally determine the occurrence of the rotating spoke mode of operation and to investigate voltage characteristics and spoke frequency of the MPD arc. It was found that the single rotating current spoke occurs at intermediate current levels above a critical magnetic field strength. A comparison of the spoke frequency measurements with the actuator-disk theory of Fay and Cochran shows that the actuatordisk model gives a satisfactory prediction of the behavior of the spoke frequency until a limiting frequency is reached after which the spoke approaches a constant rotational speed which is the same for all gases. The variation in the electrode voltage was found to obey the correlation of Patrick and Schneiderman when the arc was operated in the diffuse mode at lowpower levels. In the spoke mode voltages several times this prediction were measured and can be explained in terms of the EMF generated by the rotating spoke. Several criteria are suggested which predict the observed spoke/no-spoke boundaries.

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