Hollow cathodes for hall thrusters: Modelling and scaling trends

Summary form only given. In the context of electric space propulsion and in particular in Hall Thrusters (HTs), hollow cathodes are critical components supplying electron current to both sustain a plasma discharge and neutralize the expelled ion plume. HTs already in development are requiring always greater currents, sometimes as large as 100 A, to generate larger thrusts. Thus it is important to fully understand the behaviour of these cathodes in order to design more powerful ones.Most hollow cathodes are built up in the same way: an electron current is extracted from a refractory tube whose internal side is covered with a thermionic emissive material. An extraction potential of typically a few tens of volts is applied at one end to draw the current out. At the other end, propellant gas is fed at a constant flow rate so as to sustain a high density plasma inside and to allow for high current densities without being limited by space charge saturation. Since emission current density largely depends on the temperature of the material, it is critical to properly couple thermal aspects of the cathode to heat fluxes resulting from plasma bombardment. Several works have already partly covered these aspects, separetely1 and coupled2. Here, we developed a fluid quasi-neutral model of the internal region of the cathode accounting for electrons, single charge ions and neutrals. A complete description of the transport of ions and neutrals is included while electron current is modelled using a drift diffusion approximation. An energy equation for each species is included. A detailed description of the emission process is also used which allows us to accurately describe emission current density in both space charge limited and thermionic emission limited regimes. Plasma heat fluxes serve then as inputs to a simple thermal model describing the cathode´s heat dissipation. This modelling of the cathode enables us to fully describe a region that is not easily accessed by experimen- s. A short description of the model and of the general working principles of cathodes will be presented. Then, starting from an existing cathode and varying some design parameters (operating point, geometry, and thermal insulation), some trends in key parameters of the cathode such as extraction potential and maximum temperature of the insert will be analysed.