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Novel Electride Material for Enhanced electrical propulSIon Solutions

Periodic Reporting for period 1 - NEMESIS (Novel Electride Material for Enhanced electrical propulSIon Solutions)

Reporting period: 2019-10-01 to 2020-09-30

Electric propulsion (EP) is an increasingly adopted technology for spacecraft propulsion in station keeping, orbit raising, or primary propulsion applications. A common need for most of the EP thruster technologies is to operate a device for electron emission either for plasma generation or for charge neutralization of impellent positive ions. Permanent research on new materials and architectures is mandatory to optimize efficiency of such devices in the extreme operation conditions and limited resources in space.

Improving the performance of on board EP devices from NEMESIS project will improve competitiveness and strength of the European space industry, with associated employment and scientific level improvements. On the other side, it will accelerate the availability of simpler and cheaper platforms for small satellites, and even to pave the way for enabling brand new exploration, scientific and commercial missions not yet available due to actual constraints of traditional thermionic emitters technology.

Other potential impacts for society will be driven by the faster and wider deployment of satellites and constellations, that will bring many different applications like environmental surveillance, or safety and security applications based on Earth Observation. Tele-medicine and similar tele-assistance applications for isolated population will also be enabled by large deployment of telecom satellites.

The overall objective of the NEMESIS project is to demonstrate performance improvements of EP devices based on a novel thermionic emitter material, namely C12A7:e- electride, vs present state of the art with traditional emitters such as LaB6 or BaO. For this purpose, NEMESIS will be developing electride-based cathode technology, which is compatible with all kinds of electric propulsion (EP) systems requiring neutralization or electron emission.

Four types of ceramic-based neutralizer device concepts will be addressed and developed to TRL4.

Chemical inertness, low work function, much lower operational temperatures, and lifetime/duration are some of the characteristics of the C12A7:e- electride material that will drive the performance improvements.
Most of the actions planned for test and environment preparation for starting C12A7:e- cathode tests and LaB6 comparisons have been completed, and some have already started. Requirements definition, as well as set-ups manufacturing and finetuning have taken place.

Continuous improvements on processes and techniques for C12A7 ceramic synthesis and its transformation into C12A7:e- electride have been investigated during this first period of NEMESIS project. A synthesis and transformation method leading to high electron concentration electride has been achieved, with resulting sample resistances in the low milli ohm range.

Many findings of C12A7:e- behaviour and interaction have been shared amongst the Consortium members for consideration to the design of C12A7:e- devices and test set-up equipment. Three technical notes on different topics have been produced, periodically updated, and shared with the team members. Issues have also been detected in extreme conditions of use, and actions have been anticipated to guarantee the durability and reliability of C12A7:e--based systems.

Website, Data management Plan, and IPR protection and Exploitation Plan are available and applied. The periodic updates were performed according to plan.

Since September, everything is in place to resume activities on the NACES cathode after 6 months of closure of UPM labs and premises due to COVID-19 crisis mobility restrictions. The test chamber and environment are ready to start the NACES performance and endurance tests in a few weeks.

The C12A7:e- compatibility study with alternative propellants is completed. Initial tests show none or only little degradation in presence of alternative propellant candidates.

A hollow cathode has been designed for LaB6 and C12A7:e- operation with alternative propellants. The test chamber and environment are ready to use. Current tests are carried out with the hollow cathode equipped with a LaB6 emitter. The C12A7:e- hollow cylinders mechanization is ongoing for the next test campaign of the hollow cathode.

C12A7:e- characterization tests for 5A and 1A class cathode tests have been completed, and studies of C12A7:e- and LaB6 comparison regarding work function, emission, and 5 A-class cathode performance were also completed. A 1 A class cathode, designed for LaB6 vs C12A7:e- comparison tests and procurement, has already been ordered.

The iPott test-chamber adaption to the requirements of the HET was made according to the provided HET characteristics. Subsequently, the HET thruster was tested and characterized with iodine in the adapted test chamber.

The ULP technical requirements are ready, based on commercial off-the shelf products, with special focus on the electron beam current and the heater power. In line with the schedule, the test plan and environment were defined and are becoming ready to start testing the ULP cathode with C12A7:e- as emitter. The test procedures were defined and software was developed to allow autonomous data acquisition and script-based tests for ULP performance measurements.
C12A7:e- properties are superior to those of conventional ceramics currently employed in EP neutralizer technology. 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. Apart from less required thermal energy, C12A7:e- lower work function enables devices operating at working temperatures half of the typical operating temperatures of BaO and LaB6 based neutralizers. Additionally, lower heating power is essential to decrease the system power-to-thrust ratio.

For the use of C12A7:e- as electron source in EP applications, where the neutralization of the ion plume is crucial for operation, the reliability and lifetime needs to be increased versus the presently used thermionic emitters. If the electride-based neutralizer can be operated at significantly lower temperatures, the lifetime of the cathode and the whole subsystem could be strongly increased towards the intended long-term operation (up to 4 years and more for scientific missions).

In addition, higher electron emission density at certain operating conditions would facilitate the miniaturization of the cathodes, which will turn into significant saves in the devices mass and volume.

Within the NEMESIS project, LaB6 and C12A7:e- comparison has shown an order of magnitude higher emission current density at 900 ºC for a 8 mm diameter disc sample: 0,01 mA vs 0,2 mA.

Thermionic emission from novel electride material C12A7:e- deposited thin films has been documented by the very first time worldwide. C12A7:e- deposited thin film (250 nm) on graphite has shown thermionic emission. Up to now, no report was found on getting a C12A7:e- deposition really operational by emitting current in vacuum. Thus, opening the door to many possible additional applications.

A 5 A-class cathode was able to generate an electron current when equipped with a C12A7:e- emitter. The cathode was fired in self-heated regime down to 0,1 A, while for the same cathode equipped with an LaB6 emitter, the minimum required current for selfsustained operation is 2 A, demonstrating lower energy needs by C12A7:e- electride to furnish electrons.
ATD test setup
FOTEC test setup
ATD test setup