Regenerable Field Emission Cathodes
Plasma thrusters operate by expelling a high-velocity stream of positively charged ions to create reactive thrust. However, an isolated spacecraft that continually emits positive ions will very quickly develop a large imbalance in electrical charge due to an excess of negative electrons on-board. Plasma thrusters therefore require a device known as a “cathode” that is capable of expelling electrons into space at the same rate that the thruster emits ions.
Typical cathodes operate by using a small amount of propellant gas to create a plasma discharge – which exhausts the excess electrons into space. Since this gas does not effectively create any thrust it is effectively a waste of valuable propellant. Overall thruster performance could be drastically improved by a technology capable of emitting electrons without using any gas. Technology known as a field-emission cathode can create beams of electrons without any external gas feed or electrical heater. Field emission cathodes rely on quantum mechanical emission from very sharp (10-nm-radius) electrode tips. Because of the fragility of the nanometer-sized tips, the structures are susceptible to damage that blunts the tip and destroys the device functionality. This type of failure in space would render the propulsion system useless and leave the spacecraft stranded.
At Michigan Tech we have developed and patented a field-emission cathode for use with plasma thrusters has the ability to be re-generated when the emitter tip becomes damaged. The cathode is made by using a strong electric field to shape a small volume of liquid metal (such as indium or gallium). The metal is pulled into a nano-sharp tip by the electric field and then the resulting structure is frozen into permanence by removing electrical heat. The nano-tip then acts as an effective electron source. The tip still experiences erosion and blunting as do conventional field-emitters, however the sharpening process takes only seconds and can be repeated an unlimited number of times, continually refreshing the tip to like-new condition and rendering the lifetime unlimited.