Observations of large nonequilibrium conductivity in a closed-cycle MPD channel with applied electric and magnetic fields

Experiments have been performed it the International Research and Devlopment (IRD) Mk II helium-cesium MPD facility with simultaneously applied electric and magnetic fields. By increasing the total current density in the plasma from 2 mA/cm
²
to 440 mA/cm
²
the plasma impedance decreased by a factor of 100 owing to a nonequilibrium increase of the electron temperature and number density. Nonideal behavior in the form of a much reduced Hall potential was always present and is attributed mainly to nonuniformities in the plasma conductivity, probably as a result of inadequate mixing of the seed material with the buffer gas. Other mechanisms known to contribute to the lowering of the generator performance include wall and plasma-to-ground leakage and finite segmentation. Further evidence for the large nonequilibrium decrease of plasma resistance with current was the amplification and harmonic generation of small voltage fluctuations.