| The research presented in this dissertation aims to scale orificed hollow cathodes for low-current electric propulsion applications. To realize the same effectiveness as 1-kW class electric propulsion systems, sub-300-W class thrusters require cathode power consumption of a few Watts at most. A number of 3.2-mm outer diameter cathodes were fabricated from designs based on scaling existing 6.4-mm diameter cathodes to low-current. A parametric experimental investigation examined the role of cathode geometry and materials in determining the power consumption. A thermographic investigation concluded that conduction was the primary heat loss mechanism from the cathode tip. Both the internal and external plasma environments were mapped, and the results were used both to study the electron emission processes and to validate the predictions of a hollow cathode model. In order to quickly evaluate the merits of a particular design, a hollow cathode model was developed based on previous works. The model predicted the plasma properties in the insert and orifice regions of the hollow cathode. From these properties, power consumption was estimated, and the factors limiting the cathode efficiency were examined. The results of the experimental and theoretical investigations were used to design a second-generation cathode which consumed twenty percent less power than its predecessors. The second-generation cathode operated in spot-mode consuming as little as 8-W or requiring a flow rate of only 0.6-sccm. |
