• Plasma Physics
• Hollow Cathode
• Electric Thrusters

Hollow cathodes have been used to sustain the discharge Hall Effect Thrusters and ion thrusters for years and the performance requirements were met, even if not all underlying physical phenomena were understood. Later on, the magnetic fluid dynamics of the cathode were studied, but the external region of the cathode, namely the plasma plume, has not been considered of primary interest, since the internal regions are the main drivers in determining the cathode performance. As such, the focus of theoretical and experimental works have always been on the active part, the emitter, to decrease the energy and temperature required to generate the current, and on the orifice-keeper region. More detailed studies of the plasma in the cathode plume have been presented lately, the increasing interest in understanding the plasma behavior depends mostly on the presence of high keeper erosion that limits the cathode life, with detrimental effects on the missions duration. This thesis is devoted to understanding the plasma behavior in the plume of hollow cathodes and to the formulation of a model able to obtain the plasma parameters in it. A theoretical model of the plasma is presented; the model considers all of the plasma species as fluids (ions, electrons, and neutrals), and is approximated in a 1D spherical reference frame, where all variables depend only on the radial coordinate. All the approximations are justified, and the most important physical phenomena highlighted. The most relevant data in literature are collected to compare the model simulations with the experiments; such comparison shows how classical plasma fluid modeling cannot detect some instabilities, so a parameter concerning the plasma resistance is introduced to fit the data; this parameter is present also in other works and seems the best way to reproduce the plasma behavior with a fast, simple code. Further developments of the work done in this thesis are suggested, both from a theoretical and an experimental perspective.