Formation Mechanism of the So-Called Corona Figure

A dense radial array of chromium microneedles is formed on a field-emission cathode interacting with a negative corona discharge in ambient Cr(CO)6 of the order of 10-2 torr. The article discusses physical and chemical processes which underlie the formation of this so-called corona figure, based on extensive electron microscope observations. The overall shape of a corona figure is a function of the substrate geometry, indicating that field electrons emitted from the substrate play a vital role in nucleating the figure. A negative electric field curves the growth direction of the needles that compose the figure, and a high magnetic fileld applied externally well aligns them along magnetic-field lines. These facts support strongly the hypothesis that the figure is formed as the electron avalanches, originating at the substrate, propagate outward. The technical term "field-emission corona" is proposed for the present corona discharge, because the discharge is initiated and sustained by field-emitted electrons.