Presence and statistics of a discontinuous potential distribution in the plume of an electric propulsion hollow cathode

Abstract

The extraction voltage of an electric propulsion hollow cathode is mainly deposited in its plume region, and is usually believed to be proportional to the plasma oscillation amplitude. However, this was not the case in some recent results. To understand the reason for this, this study has measured the potential distribution and oscillation amplitude distribution in the plume of a hollow cathode, and checked whether the correlation between the two was affected by changing keeper materials and keeper biases. We found a discontinuous potential rise (‘step region’) in the middle of the plume, the voltage of which occupied over 40% of the total discharge voltage. The step region was sensitive to exterior electron emission, and could shift its location in a ∼18 mm range according to an increase/decrease of oscillation amplitude. However, its voltage remained almost unchanged and, as a result, the total extraction voltage remained constant regardless of amplitude changes. Statistics indicated that the step region was related to electron–electron non-equilibrium, with splitting and recombination between electron energy probability function multi-components, each at different flow states and trapping states. It was suspected that shockwaves and streaming instabilities were involved. Because the step region was accompanied by stronger oscillations and higher ion energies, inclusion of this plasma structure should be necessary to promote cathode test accuracy.