Hollow Cathodes for Space Electric Propulsion

Thermionic hollow cathodes are currently used as sources of electrons in a variety of space applications, in particular for the gas ionization and the beam neutralization in Hall effect and ion thrusters. Complex physical mechanisms take place in a hollow cathode and, given the small size (a few millimeters in internal diameter) and the high operating temperatures (well above 1000 K), the plasma diagnostics inside this device is difficult. As such, numerical tools are definitely needed to guide the design of hollow cathodes before their manufacturing and testing, since multiple geometrical parameters influence the cathode performance and lifetime. This dissertation presents a reduced-order, numerical model to assess the performance of orificed hollow cathodes. A time-independent, volume-averaged model is developed to determine plasma properties, wall temperatures, and cathode lifetime without requiring experimental data as input. A system of particle and energy balance equations is numerically solved without invoking a Saha-type equilibrium under the hypothesis of an ionization process which takes into account the direct electron impact and the stepwise mechanism. The heat transfer and the temperature gradients along the cathode are evaluated with the aid of a dedicated thermal model, which is coupled to the plasma model and accounts for temperature-dependent material properties. Furthermore, an estimation of the emitter lifetime is provided on the basis of the evaporation rate at the computed operating temperatures. The obtained results capture most of the characteristic features of hollow cathodes as compared with the available experimental data. In addition, the model gives insight into the most important power deposition processes allowing for the evaluation of the prevailing mechanisms involved in the emitter heating. The effect of the geometry on both plasma parameters and cathode performance is discussed to suggest design guidelines for the development of state-of-the-art hollow cathodes. The theoretical study paved the way to the development and testing of several hollow cathodes, each intended for a specific power class of thrusters. The respective geometry was defined with the aid of the numerical model, in accordance with the thruster unit specifications in terms of discharge current, mass flow rate, and lifetime. LaB6-based cathodes were successfully developed for Hall effect thrusters with discharge power ranging from 400 W to 20 kW, whereas both LaB6 and BaO-based cathodes were operated with a 100 W-class Hall thruster. Experimental campaigns were carried out in both stand-alone and coupled configurations. The comparisons between experimental results and model predictions are presented offering a sound theoretical framework to drive the design of future hollow cathodes.