Current–voltage response of anodic plasma contactors with external ionization

A spherical, steady-state model that considers the combined influence of plasma emission and external ionization of neutral gas on the operation of electron-collecting contactors is presented. Ionization by both Maxwellian and streaming electrons is investigated. A core (or quasineutral cloud) and a no-core mode are considered and the transition conditions from one to another are derived. The collected current is determined in terms of the main plasma and contactor parameters. The current-voltage response reproduces the ignited regime observed in experiments. The transition to the ignited regime and the transition to the core mode are not equivalent. It is shown that ionization by the electron beam is the main driver of ignition. The Langmuir law for the double layer causes a self-regulation of the core that affects the collection of current and leads to a multiplicity of solutions. Different neutral gas profiles are analyzed. It is uncertain whether ignition is reached when the neutral gas is provided by the contactor exclusively. The appearance of nonmonotonic potential profiles for low plasma emissions is noted.