The objective of NEMESIS (Novel Electride Material for Enhanced electrical propulSion solutIonS) is to explore employing the calcium aluminate (mayenite Ca12Al14O33) based electride (C12A7:e-) as material for the full span of ceramic based electron emitter technologies. Such devices are required to operate most of the currently used high-Isp EP thrusters on spacecrafts independent of the type and size of thruster as well as of the mission scenario ranging from LEO to deep-space missions. Thus, the objective addresses a truly transversal issue of importance. The materials properties of the electride C12A7:e- are extremely advantageous for application in ceramic-based neutralizers or cathodes. In particular, the work function of the material and its chemical inertness should allow the realization of ceramics-based neutralizers that exhibit a higher reliability and better performance than their currently employed counterparts based on BaO or LaB6 ceramic material and may be operated at much lower temperatures.
The lower the work function of the emitter material, the less thermal energy is required for achieving electron emission off the material suitable for neutralizer applications. C12A7:e- exhibits a low work function, and can operate at working temperatures half of the typical operating temperatures of BaO and LaB6 based neutralizers of 800 and 1500 °C, respectively.
Failure of the neutralizer system implies failure of the EP system. Current strategy for overcoming this risk of failure is:
– designing neutralizers with sufficient thermal margins (leading to strong machining constraints to keep an outer envelope as small as possible);
– performing extensive lifetime tests (with strong schedule and cost impacts on the final product);working with enough redundancy, i.e., installing two neutralizer systems where one is considered as a backup.
– working with enough redundancy, i.e., installing two neutralizer systems where one is considered as a backup.
A common reason for the failure of currently employed ceramic electron emitters is thermal stress of the material and this is caused by the high temperatures up to 1000 °C required for operation in case of the most common ceramics.
The power for the heating of the cathode (in all concepts other than the heaterless hollow cathode) to its operating temperature impacts also the overall power budget of the EP system, decreasing the thrust to power ratio.
Lower operating temperatures are then equivalent to an increase of efficiency of the EP system. Furthermore, the low operating temperature of C12A7:e- based cathodes is compatible with the use of conventional heaters allowing for larger electron emitting surface areas due to the low radiative heat losses. That way, the space charge density restrictions that typically limit the current between neutralizer and thruster plume (acting as anode) can also be overcome.
Furthermore, chemical poisoning of the ceramic’s active surface, which degrades the electron emission properties, is another common cause of neutralizer failure. For example, BaO and LaB6 emitter materials are prone to degrade when run in an Iodine environment, i.e., their work function value will increase due to Iodine-induced surface reactions. Thus, hollow cathodes based on the conventionally used ceramics may only be operated with a few high-purity propellants, such as Xenon or other noble gases, and they are not compatible with alternative propellants such as Iodine or with concepts such as air-breathing thrusters. In contrast, C12A7:e- as emitter material is reported to be compatible with Iodine. The good chemical inertness of C12A7:e- allows one to overcome the propellant restrictions.
Novel C12A7:e- based neutralizer technology should be more reliable and more efficient with a wider range of applications than corresponding technology employing conventional ceramic emitter materials. Thus, C12A7:e- neutralizer technology has the potential to really disrupt the EP market.
The main objective of NEMESIS is to reach TRL4 for C12A7:e- technology for neutralizers, starting from the current lower TRL levels and to analyse in detail future applications.
The additional objective of NEMESIS, beyond reaching TRL4 for C12A7:e- technology for neutralizers, is also to help generating awareness and to accelerate this new disruptive technology understanding and adoption by the European space industry and preserve and improve Europe’s competitiveness. To reach this goal, it is planned to have achieved the following objectives by the end of NEMESIS:
– To demonstrate and measure improvements in EP devices’ performance using the C12A7:e- novel thermionic emitter material.
– To validate solutions designed against the material degradation modelled at scientific Labs.
– To generate and spread solid know-how on this new disruptive technology in the European industry and institutions via dissemination and communication actions.
– To mature and demonstrate four C12A7:e- based electron emitter concepts at minimum up to TRL4 qualification.
– To drive an Innovation Management Board with the target to identify and launch new projects to widen the scope of this project towards higher TRLs/new future applications in the space industry, and also to identify other domains of applicability to industries out of the space market.
