Current transition in cathode boundary layers of coal-fired MHD generators

Abstract

Electrothermal instabilities occurring in the near-electrode regions can cause breakdown in the slag layer as well as in the adjacent plasma electrostatic sheath and can lead to arcing. The breakdown process can occur either from the slag layer or from the plasma sheath, depending on the electrode temperature and the slag electrical conductivity. Stability analysis has been carried out for the onset of instability at the cathode, using a perturbation approach both in the slag layer and in the adjoining plasma boundary layer. The critical current densities and the critical voltage drops for breakdown of both the slag layer and the plasma sheath have been calculated and the region of instability initiation has been identified. The initiation of instability has been found to be a function of electrode, slag and plasma parameters. It was also found that the constricted discharge can be limited to occur either in the slag layer or in the plasma sheath alone or it may occur in both layers.