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Completed TRL 3 (started at 3, targeting 4)
High power electric propulsion can revolutionize human exploration architectures by significantly reducing propellant mass. Advancements in thruster design, such as magnetic shielding, have addressed traditional life-limiting mechanisms, emphasizing the need to address other failure modes. As electric propulsion (EP) power levels increase, the demands on the discharge current produced by hollow cathodes also increase. Hollow cathodes are used in gridded ion and Hall thrusters to produce the electrons that ionize the propellant gas and for charge neutralization of the beam. Little work has been done to investigate the optimal operating conditions of high-current hollow cathodes or to study the effects on lifetime and reliability of higher emission current variants of the state of the art (SAO) technology needed to support high power electric propulsion systems. In order to support the development of High Power EP applications, cathode advancement must keep pace with the rest of the thruster, particularly in the area lifetime and reliability. Barium-oxide (BaO) based hollow cathodes have demonstrated the required lifetime and reliability in providing the 10's of amps for electric propulsions systems below 15 kW in power. The substantial increase in cathode discharge current required to support the thrusters that will be utilized on the 300 kW class exploration vehicles has unknown implications on cathode lifetime and will require evolution of the current state of the art BaO cathode designs. The goal of this research and development effort is to understand the critical parameters that allow us to construct long-life hollow cathodes for high power EP applications, with an emphasis on the health and condition of the barium-oxide emitter element.
In order to support the needs of higher power electric propulsion systems of the future, hollow cathode technology will need to scale accordingly, delivering 100-250 amps of discharge current. Not only will these cathodes be required to output higher currents, they will also need to provide >10,000 hours of reliable lifetime. Optimal operating characteristics for BaO emitters have not been established for high-current hollow cathodes capable of long life. Limited work has been done in this area and building this knowledge base will enable the necessary development of reliable, long-life cathodes. Near term (5 to 10 years), this work will help to reduce the risk of high power EP for exploration missions by determining operating regimes of high-current hollow cathodes. In the 10 to 20 year range, this work will help enable high-power EP at the 50-100 kW string level in support of 300 kW class vehicles for Mars exploration applications.
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