Mechanisms of cathode erosion in plasma thrusters

Cathode mass loss in quasi-steady and steady state plasma thrusters can occur through four mechanisms: ejection of molten droplets, chemical attack, evaporation and sputtering. Models for the rate processes are presented and examples illustrating coupling between rate steps are discussed. More detailed examples of processes important in thoriated tungsten cathodes are then presented, including evaporation, oxidation and sputtering of tungsten and evaporation of thorium and its effect on the work function.; A technique involving radioactive tracers produced by bombardment with a high energy ion beam was used to measure the erosion rate. A new tungsten activation scheme was developed to avoid target damage found in earlier activations. More accurate analysis procedures were implemented for time-integrated measurements and a time-resolved technique was developed. Methods which correct for diffusion of the tracer were developed so the technique could be applied in high temperature environments. A complete error analysis was performed and used to optimize the method.; The nonstationary electron emission processes in quasi-steady cathode operation are associated with very destructive erosion processes. Erosion measurements and study of the surface damage structures in a quasi-steady thruster revealed the dominant mechanisms. Vapor production and the ejection of small droplets in isolated emission sites dominate for low current densities. At a certain threshold current density the overlapping temperature fields of adjacent emission sites cause melting on a larger scale and the mass loss by droplet ejection increases. Confinement of the vapor jets by the azimuthal magnetic field and relatively slow gas diffusion limit the net evaporative mass loss, but even at currents below the threshold the erosion rate is too high for practical application of these devices.; Stationary thermionic emission is a less destructive process than cold cathode emission. Erosion and temperature measurements were combined with a characterization of the surface structure and chemical state in a steady-state arc to identify the important mass loss processes. Droplet ejection may contribute to the mass loss if the temperature exceeds the melting point. Chemical attack can dominate if there are even very small quantities of oxidizing impurities in the propellant. If there is no surface melting and the propellant does not contain reactive impurities, the mass loss is dominated by diffusion-limited evaporation.