NEW CONFERENCES

EPIC Workshop 2022

New cathode technology for space electric propulsion: NEMESIS achievements

A. Post‡, J.F. Plaza∗, and J. Toledo†

Advanced Thermal Devices, Alcorcón, Madrid, 28925, Spain

Abstract:

The NEMESIS (Novel Electride Material for Enhanced electrical propulSion solutIonS) project is a transversal H2020 project strategically aiming at developing electride-based cathode new technology which is compatible with all kinds of space electric propulsion (EP) systems requiring neutralization or electron emission.

It is an EU’s Horizon 2020 Research and Innovation program, funded project under Grant Agreement number 870506, with the objective 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.

The project challenge is to fully transfer the theoretically ideal materials properties of C12A7:e- to neutralizer and any other kind or necessary electron emitting devices, in order to fully make use of its potential for application to achieve the best performance and reliability and to become a disruptive force in the cost-driven satellite market.

This work will present and discuss the project achievements in terms of material synthesis, the lessons learnt for the adequate design of C12A7:e- based cathodes, and the performance analysis of the 8 different cathode designs that are being developed under the NEMESIS project.

At the same time that the project did overcome the material synthesis huge complexity by producing high purity single phase C12A7 ceramic oxide and the transformed C12A7:e- electride material with a high electron concentration, the characterization of many physical-chemical parameters of this thermionic material has allowed for deriving quite a few lessons on how to best approach the use of this material in electron emitter devices.

The anticipated advantages of this new cathode technology are being confirmed in terms of lower operational temperatures, lower power consumptions, and compatibility with more propellants, including iodine, versus the existing cathode technologies based on traditional thermionic materials like LaB6 or BaO.

Several performance analysis will be described in this work, comparing the same electron emitting devices when based on C12A7:e- or based on other alternative thermionic materials.

Novel approach to EP based on NH3 as propellant and on-board energy generation

J.F. Plaza∗, J. Toledo†, and A. Post‡

Advanced Thermal Devices, Alcorcón, Madrid, 28925, Spain

Abstract:    

To overcome the storage and transportation issues of Hydrogen, NH3 is increasingly being investigated to extend Green Hydrogen use. NH3 characteristics like its high energy density and low temperature and/or pressure needs for storage (10 bar at 20ºC liquid ammonia), make it very valuable for simplified unexpensive storage and transportation of H2. And these characteristics makes it also especially suitable for on-board spacecrafts EP purposes. First, 1Kg of NH3 occupies 1.6 litter at 10 bar, which represent one of the best propellant mass/volume ratio (1Kg/1.6 lit at 10 bar). The system does not require any heating or conditioning element but the usual gas flow control of any EP system. On the other hand, 1 Kg NH3 contains 176 gr of H2 that means 5,88 KWh energy or 3.5 KWh electric energy using a common fuel cell (considering 60% efficiency of the fuel cell).

Novel electride material C12A7:e- plays a key role as outstanding catalyst in this huge improvement of NH3 synthesis and dissociation processes, as reported by researches in Japan, Europe and USA.

ATD has over 5 years of experience in the C12A7:e- material research and in the development of EP devices based on it as thermionic emitter, and has been carrying out several research initiatives on NH3 generation and dissociation processes with C12A7:e- as catalyst. Based on this, we have concluded that a new NH3 based EP could be a very valuable innovation for the European space industry.

The approach is based on the of use NH3 as propellant including partial dissociation into Nitrogen and Hydrogen. The Nitrogen and NH3 not dissociated are used as propellant for the thruster, and the Hydrogen extracted (99,99% purity) is used to get stored in a fuel cell for electric energy generation.

NH3 dissociation can be performed through a specific C12A7:e-Ru doped cathode (plasma method) working as cathode for neutralizers or thrusters (with cathode), or through a small tube reactor using C12A7:e-Ru doped as catalyst, working separately from any neutralizer or thruster. Benefitting from the experience got in the H2020 NEMESIS project, this presentation will outline this new approach to EP providing propellant and electric energy through NH3.

8th SPC, Estoril 9-13.05.2022

Neutralizer design with flat C12A7:e- insert

Malina Reitemeyer, Peter J. Klar

Justus-Liebig-Universität Giessen, Heinrich-Buff-Ring 16, 35392 Giessen, Germany

Accepted for 8th Space Propulsion Conference, Estoril 9-13 Mayo 2022

Abstract:

Neutralizers are still a frequent failure source in electric propulsion systems [1]. One reason are the high temperatures required to operate state of the art insert materials (1300 K for BaO and 1400 K for LaB6). C12A7:e- is a crystalline anionic material, which offers completely new perspectives to existing cathodes. It has a low work function compared to state-of-the-art materials [2], which allows lower operation temperatures. Thanks to this, thermal shielding could be reduced, weight gained. Improvements in reliability and lifetime for the overall system are therefore expected. Furthermore, first results indicate a good compatibility with alternative propellants. Instead of expensive xenon, future systems may work with iodine. However, C12A7:e- has to major drawbacks: First it has a low thermal conductivity together with a low melting point. This may cause overheating inside the hollow cylinder since heat cannot be conveyed [3]. A low wall thickness would therefore be desirable. This interferes with the second point, a difficult machinability of the material.

As a solution, this work proposes a flat cathode design for C12A7:e- neutralizers. The device uses a 2 mm thick, 1 inch diameter disk as insert. This geometry is considerably easier to manufacture and also allows innovative concepts, like C12A7:e- thin films or single crystals to be tested as insert material. The neutralizer is equipped with a flat heating element underneath to support ignition. A schematic view of the first design version is shown in figure 1. In the proposed paper, an overview of the test campaigns will be given. Initial tests were performed with an insert of the state-of-the-art material LaB6, which could be successfully ignited. This demonstrates the design concept. Afterwards, electride material with a medium electron concentration was tested with the neutralizer. The C12A7:e- insert showed good performance and extraction currents up to 1 A were reached. The ignition could be successfully reproduced. An image of operation is shown in figure 2. Different keeper orifice sizes and their influence on the operation were tested. To address persisting problems like heater degradation and high propellant consumption, an improved design will be tested in the coming weeks and the results of which will be reported.

A low power heaterless plasma discharge (HPD) cathode for electric propulsion applications

L. Conde 1, D. Ovejero 1, R. Marey 1, J.L. Domenech-Garret 1, J. González 2 and J.M. Donoso 1.

1 Departamento de Física Aplicada. Escuela Técnica Superior de Ingeniería Aeronáutica y del Espacio. Universidad Politécnica de Madrid, 28040 Madrid. Spain.

2DIFFER Dutch Institute for Fundamental Energy Research. De Zaale 20, 5612 AJ Eindoven. The Netherlands.

Abstract:

An essential component of plasma thrusters are electron sources (cathodes) mainly employed for the ionization of the propellant gas and the neutralization of ion flow exhaust [1,2]. Reliable hollow cathodes (HC) have been used successfully in different missions and their basic design has not changed in the last 50 years [2]. Nowadays, low electric power HCs and, also heaterless (HHC) concepts intended for small (< 500 kg) and mini (100 − 500 kg) satellites are presently under development [1,2].

The implementation of HCs and HHCs in these compact spacecrafts faces important challenges, such as the high operating temperatures required for electron thermionic emission, the reduced gas flows and the low electric power available.

In the present study the small sized heaterless discharge cathode (HPD) of Figs. 1 and 2 that operates close to room temperature is introduced. The Fig. 3 shows the typical current-voltage (IV) characteristic curve of this cathode/plasma source that works with neutral gas (Argon and Xenon). A low power (P < 30 W) voltage-controlled electric discharge (V!” ∼ 0.8 − 1.4 kV and I!” ≤ 10 mA) produce the gas partial ionization. Heating is not needed since secondary electrons are mainly produced by ion impact instead of by thermionic emission. The plasma plume observed in Fig. 1 is produced by the collisional excitation of non-ionized neutral gas that leaks together with emitted electrons through the holes of the hollow anode (see Fig. 2).

These features significantly reduce its operational requirements, allowing for smaller sizes, the use of simple materials, and simplified associated electronics. Characterization of its plasma emission performance will be analyzed on the basis of the IV curves obtained at our UPM longterm testing facility. The physical parameters of produced plasmas (electron temperature T# and density n#) obtained with Langmuir probes will be discussed. Finally, considerations on design improvements in the perspective of miniaturization of the HPD plasma source will be introduced.

Performance analysis of several C12A7:e- based cathode devices with different design architectures and configurations

J. Toledo, J.F. Plaza, A. Post et alter.

Advanced Thermal Device, S.L.

Accepted for 8th Space Propulsion Conference, Estoril 9-13 Mayo 2022

Abstract:

C12A7:e- electride material is increasingly known for its adequate characteristics as thermionic emitter: low work function, lower operational temperature than alternative materials, stability at room temperature and chemical inertness. The material properties, together with the need to progress in the development of efficient electron emitter devices operating at low temperatures for all kind of Electric Propulsion technologies, is extending the research and development initiatives using this novel material as electron emitter in neutralizers and thruster devices.

An intensive research has been performed by ATD (Advanced Thermal Devices) on this C12A7:e- electride material over the last 6 years. With the experience acquired on the key design parameters to get the best performance as thermionic emitter, several cathode designs have been developed, with different architectures and configurations, all of them using C12A7:e- as electron emitter (see Figure 1). In this work will be presented different cathode devices and the more relevant performance indicators achieved by each of them, either as neutralizers and/or as EP thrusters. In addition, a comparison between C12A7:e- electride and LaB6 materials as emitters have also been performed, for some of these cathode devices, and the results will be also discussed in this work (see Figure 2).

Several cathode designs are examined in terms of performance and emission stability and control, and performance tests are carried out for different types of gases and mass flows working with plasma. Special attention is given to performance ratios like anode current to spent power ratio (Ianode/Pcathode in mA/W), keeper current losses (Ikeeper/Icathode in %), and operational temperature. Also, current stability over time will be analyzed, including stability during ignition process starting from room temperature. In addition, all the cathode designs incorporate the required components to prevent the instabilities that are usually reported in all literature papers related to C12A7:e- based cathodes.

37th IEPC, Boston 19-23.06.2022

Heaterless plasma discharge (HPD) cathode for electric propulsion applications

L. Conde, D. Ovejero, R. Marey, J.L. Domenech-Garret and J. M. Donoso

Departamento de Física Aplicada. Escuela Técnica Superior de Ingeniería Aeronáutica y del Espacio.

Universidad Politécnica de Madrid, 28040 Madrid. Spain.

J. Gonzalez

DIFFER Dutch Institute for Fundamental Energy Research.

De Zaale 20, 5612 AJ Eindoven. The Netherlands.

Accepted for 37th IEPC, Boston 19-23 June 2022

Abstract:

An essential part of plasma thrusters are the sources of electrons that are employed, mainly for the ionization of the propellant neutral gas and the neutralization of the ion flow exhaust that imparts momentum. The efficiency of these two processes is essential as they compromises the performance of the propellant [1–3].

In addition to thermionic emitters, such as low-work function materials heated to high temperatures or dispenser

cathodes, the reliable hollow cathodes are typicaly used as an integral part of the propulsion system [1–3]. The latter area mature technology and supply electrons produced in the partial ionization of neutral gas stream. Conventional hollow cathodes (HC) are heated [1, 4] and also exist low temperature heaterless (HHC) concepts [5, 6]. In both cases, the thermionic emission of electrons plays a fundamental role in their operation.

The HCs have been used successfully in different missions and their basic design has not changed in the last 50 years.

Its operating principle is the partial ionization of a gas flow by means of an low pressure electrical discharge, triggered

by the electrons emitted by a low work function material (insert) heated up to its thermionic emission [1, 4]. HHCs also typically use an electrical discharge that do not require a heating system, so they generally require of higher gas flows in the initial stages of the electric discharge. Once HHCs have reached their steady state of operation, the ohmic heating of electrodes produced by the discharge current facilitates the thermionic emission of electrons that contribute to its operation [5, 6].

Plasma thrusters for small (<500 kg) and mini (100 􀀀 500 kg) satellites require of small sized cathodes capable to

operate with a low electrical power. Stable emission currents below 1A and low gas flows necessary to operate them

are needed, since this last parameter compromises the specific impulse of the propulsive system. The high operating

temperatures required for thermionic emission of electrons present significant challenges to implement conventional HC and HHC designs on the compact satellites mentioned above.

In present study we will introduce a new small sized heaterless discharge cathode (HPD) where the fundamental

process is the production of electrons by ionic impact, instead of thermionic emission at elevated temperatures. This

significantly reduces its operating requirements, allows for smaller sizes, and simplifies the design of its power processing unit (PPU) electronics.

Design and Operation of a Hollow Cathode with a C12A7:e- Insert in Comparison with a LaB6 Insert

Daniel Zschätzsch*, Malina Reitemeyer † and Peter J. Klar ‡

Justus-Liebig-Universität, Giessen, Hessen, 35392, Germany

Accepted for 37th IEPC, Boston 19-23 June 2022

Abstract:

The electride C12A7:e- gains more and more attention in the EP community. It is of particular interest for hollow cathode neutralizers, since the material imposes an alternative to the currently used insert materials and offers various advantages. The insert is the key component of every neutralizer based on thermionic emission; consequently, an improved insert material has a large impact on the overall performance in terms of operation temperature and current extraction. Anticipated for key advantages of the electride are its low work function and low operation temperature.

Although the knowledge about C12A7:e- material and its properties increases, a reliable application of the material in a hollow cathode configuration is still pending. A major hurdle is the low thermal conductivity of the electride yielding a danger of overheating and melting the material in the operating device resulting in its failure.

The design of a hollow cathode neutralizer based on LaB6 reported in [1] was adopted for the use with C12A7:e- and

manufactured. The cathode follows a non-orificed design, which helps to achieve a large active plasma region

along the length of the insert and equally distributes the heat load. Nevertheless, the orifice plate and the keeper orifice are exchangeable to vary the plasma parameters inside the insert, if necessary. The built-in insert is interchangeable; accordingly, the same cathode can be operated with an LaB6 and an electride insert. With inserts of similar size, a direct comparison between the two materials and their operational behavior is possible.

The tests of the hollow cathode are accompanied by thermal simulations with COMSOL to mitigate a destruction of

the electride insert by overheating. Upper limits for the input power are calculated. The base material for the electride insert is manufactured by ATD in Spain and is of very high purity and possesses a high electron concentration in the order of 9×1020 cm3.

Following the approach described above, the investigations will lead to a comparison of the insert materials LaB6

and C12A7:e- in an application-related environment. The operational parameters will be evaluated and key aspects of the design, ignition, and operation phase will be presented. Additionally, the results of the thermal modeling will be shown and compared with the measured parameters obtained during the tests.

The hollow cathode was successfully operated with an LaB6 insert and xenon as propellant. The collected data

during the tests were used to verify and refine the thermal model. Figure 1 shows the hollow cathode during the first hours of operation with an LaB6 insert. Results of the thermal model are shown in Figure 2. The measured temperatures agree very well with the calculated ones; therefore, the temperatures inside the insert can be predicted, which are very difficult to measure directly. With this setup, the operation of the electride insert is currently constrained to about 60W of input power before reaching its melting point.

C12A7:e- neutralizer operation with alternative propellants

Malina Reitemeyer*, Daniel Zschätzsch† and Peter J. Klar‡

Institute of Experimental Physics I, Justus-Liebig-University Giessen, 35392 Giessen, Germany

Accepted for 37th IEPC, Boston 19-23 June 2022

Abstract:

A neutralizer’s common failure source is its heating element. Reducing the working temperature and thus the overall thermal load is therefore highly desired. The novel C12A7:e- electride material allows cooler working points and is therefore a promising candidate for future development [1]. Another problem to address is the replacement of the commonly used but expensive propellant xenon. For this, compatibility of the exiting hardware has to be studied and adaptions implemented.

Within this work we want to elaborate the compatibility of two different neutralizer designs operating with C12A7:e- as insert material with the alternative propellants krypton and iodine. First, a short discussion of the insert material’s compatibility itself will be presented. Electride samples were exposed to various alternative propellants and analyzed with Raman Spectroscopy, XPS and surface imaging. An iodine surface layer was found as part of these studies, even,though the bulk seems to be unaltered. Krypton did not affect the C12A7:e- electride.

To verify these results, tests in real use-case are planned. For this the different neutralizer designs will be tested with the alternative propellants iodine and krypton. The performance and the degradation of the insert material will be analyzed.

As preparatory work, these designs were already successfully operated with xenon as propellant. All designs show stable operation in different current regimes (Ampere and Milliampere).

Excellent performance of a C12A7:e- (electride) cold cathode based on charge coupling techniques

J.F. Plaza, J. Toledo, A. Post.

Accepted for 37th IEPC, Boston 19-23 June 2022

Abstract:

C12A7:e- is a well-known electride compound derived from the calcium aluminate ceramic oxide material C12A7 and characterized by a low work function of 2.4 eV. Despite this low work function, which allows the C12A7:e- to start thermionic emission at relatively low temperatures, its current density levels are lower than expected, being in the order of nA/cm2 at 600 ºC and µA/cm2 at 900 ºC. When aiming to increase the emission levels by increasing its temperature, the most common issue described in literature about C12A7:e- based cathodes is the emission instabilities, which in some cases derive into sputtering or even melting of the electride material. A new technique based on charge coupling showed a remarkable performance, even at low temperatures of around 300ºC, up to ten times better than other typical emitter material such as LaB6 in terms of current emission and power consumption.

An intensive research has been performed by ATD (Advanced Thermal Devices) in the C12A7:e- material, including several experiments to analyze the low thermionic current density levels and to discover its root cause and nature. As presented in Figure 1, our results determined that the presence of a dielectric layer at the material surface acts as a barrier for thermionic emission. Once demonstrated that this limitation in the thermionic emission is caused by this effect, we have developed alternative solutions to overcome this issue through different ways. One of them is the design of optimized cathodes able to operate with C12A7:e- material as electron emitter. The second approach is the production of high-quality C12A7:e- material for the reduction of this effect.

This work will describe the evidence found on such surface effect, within the H2020 funded project NEMESIS, identifying the root cause of the emission barrier effect responsible for the low emission density levels. In addition, this work will discuss the results achieved in emission increase through different techniques to overcome the surface barrier effect, and the performance improvements reached by using charge coupling techniques in the operation of a C12A7:e- based neutralizer cathode.