• Cathode
• Plasma Thruster
• Space Propulsion

Magnetoplasmadynamic (MPD) thrusters have demonstrated performance and power handling capabilities which make them attractive for use on thrust-intensive and high energy missions, such as Earth to Mars transfer of a cargo vehicle or a crewed Mars mission. However, the performance and lifetime demonstrated so far with gaseous propellants are at best marginal for these missions. Thermionic cathodes have been long identified as the run-time limiting components in both steady-state and pulsed devices making them a priority in the development of future high power MPD thrusters.

In this dissertation, an experimental and theoretical investigation of high-current hollow cathodes performance is carried out to provide guidelines for scaling and increasing MPDT lifetime. It is found that, whether the cathode technology, quasi-steady application results in erosion rates not compatible with any real mission duration. As a consequence, a theoretical model is derived for the performance evaluation of steady-state thermionic cathodes and verified by comparison with experimental data available in the literature. The model includes the effect of non-equilibrium multi-step ionization and allows for estimation of the plasma penetration length. It is shown that the cathode operative temperature can be effectively reduced by using low-work function refractory ceramics or by seeding the gaseous propellant with alkali metal vapours. However, since the seed neutral number densities required for an effective covering of the surface are in the order of the propellant density or higher, an emitting layer can be maintained only at very high seed mass flow rates. It is thus concluded that the use of non-diffusing low-work function insert is preferred.

The physical insights obtained from this study can aid in defining design criteria and general guidelines for high-current hollow cathode design and performance evaluation.