PROJECT PROGRESS

1^st quarter 2023

TRL4 achieved and technological understanding reached to proceed to qualify higher levels

Several cathode architectures have reached TRL 4 with different propellants. Solutions found for all major technology issues popping up during project execution. Research and development of new configurations of C12A7:e- based cathodes is ongoing and will continue beyond the NEMESIS project thanks to the Collaboration Agreements signed with some European space industry companies.

Operating a C12A7:e- hollow cathode with Iodine

Successful iodine ignition got reaching a relatively stable operation in which we were able to maintain for over 40 minutes the discharge above 250 mA in an Exotrail’s Development cathode. However, and due to cathode situation at the end of these plasma characterizations, it is difficult to conclude whether the emission came from the C12A7:2e- insert or from other residues on the parts constituting the cathode (following the plasma sputtering).

An Exotrail’s 50W class Hall thruster was fired in JLU facilities for more than 16 hours (cumulated) with full iodine injected in the anode and full xenon injected in the cathode. Plasma characterizations were performed in order to compare the discharge between xenon and iodine injected in the anode.

ULP endurance test operating with Argon

Over 1000 hours endurance tests performed at FOTEC in three runs, one of them reaching an uninterrupted average anode collected current of 25.38 mA (85% of cathode current) for approximately 400 h with less than 3 W cathode effective power (240 V, 30 mA, 40% duty cycle), which represents a performance ratio of 8.5 mA/W at an Argon flow rate of 0.25 mbar l/s.

C12A7 and LaB6 characterization coupled with 50 W and 150 W HET

C12A7 and LaB6 cathodes were ignited with a 50 W or 150 W anode block, with stable and better results obtained with C12A7 compared to LaB6. Moreover, we also obtained a better electric field efficiency with C12A7 compared to LaB6.

Discharge power characterization and cycling were performed on a 50W and 150W THD with a heaterless C12A7 cathode. Successful heated and heaterless ignitions were performed with stable discharges for the C12A7 cathode. Plasma discharge was stable with a C12A7 insert (left chronograph) and unstable with a LaB6 insert of a same design (right chronograph). Heater Power, Discharge power and Total injected xenon mass flow are represented from the top to the bottom. .

Over 10 different C12A7:e- cathode architectures tested

Reaching performance ratios of up to 10 mA/W, and losses at keeper lower than 5% (I_anode/I_cathode > 95%). Some of them operated with powers below 1 W and many of them in heater less configurations reaching operating temperatures < 200 ºC. General common characteristics of heater-less discharge cathodes using C12A7:e- is a very low total power consumption with only a small fraction of it devoted to keep the plasma and the rest serving for current extraction at anodes of some tenths of mA, which overall bring a state of the art performance ratio between 4 and up to 10 mA/W.

4^th quarter 2022

C12A7:e- cathode operated with ammonia

A 50 h test has been performed with NH 3 to verify compatibility of the electride with ammonia, which can become an abundant unexpensive fuel. Over 30 mA current extracted at anode with less than 6 W and negligible current losses under 10 sccm NH₃ flow. Working temperatures measured bellow 200 ºC.

ULP endurance test environment preparation

Igniter and pulsed mode electronic boxes, together with version v22 prototype, moved to a bigger chamber for endurance tests preparation.

NACES 300h endurance test with Krypton

The NACES cathode operated over 300 hours on a single insert with krypton as propellant. The anode to cathode current ratio continuously decreased over the course of the test. The reason was the significant amount of insert sputtering erosion during the test. After cleaning, the cathode can be operated again with a significantly lower anode voltage and the overall performance improved notably. However, the wear of the electride insert itself had a negative effect over the course of the test. Visual inspection of the insert after the test campaign reveals a significant recess up to 0.7 mm deep (with 2 mm insert thickness) in the insert. Nevertheless, Raman spectroscopy of the insert confirms a good sample quality, also after 300 hours of operation.

Up to 0.4 A extraction current with planar thermionic cathode

The thermionic plasma cathode V2, developed by JLU, was operated with an extraction current of up to 0.4 A at 38 V anode potential. In total, stable extraction was achieved over two hours at 0.2 A anode current.

Phase decomposition of C12A7:2e- electride insert at elevated temperatures

Research at JLU found a C12A7 phase decomposition at elevated temperatures. These may occur during operation of a C12A7:2e- insert within a hollow cathode. The C12A7 decomposes into other calcium aluminate phases and impedes thermionic emission properties of the material. A decoloration of the insert material can be seen. Such foreign phases were successfully detected with Raman spectroscopy.

Successful mitigation of melting or phase decomposition in hollow cathode

Researchers at JLU further improved their hollow cathode operation and design parameters to mitigate C12A7 phase decomposition in the insert (as previously reported). Several tests with operation durations over 20 hours were performed. Subsequent Raman analysis of the insert confirmed the crystalline integrity of the sample. No foreign CA-phases were found.

C12A7:e- electride depositions on flexible unbreakable substrates

Yttria doping to reduce C12A7:e- electride fragility was unsuccessful, moving then to research on depositions of electride material on flexible unbreakable substrates. Good results achieved with depositions on flexible carbon fiber substrates, since the current emission levels are comparable to bulk samples, while fragility is significantly reduced. Patent has already been presented for those processes and techniques providing successful results.

3^rd quarter 2022

Performance improvements in C12A7:e- emitter with charge coupling technique (ATD patent)

Long duration tests performed at JLU with pulsed mode operation showed a significant improvement in performance and stability, and slightly better anode to cathode current ratio: 22 mA / 29 mA vs. 20 mA / 29 mA.

C12A7:- cathode coupled with EXOTRAIL 150 W class thruster

C12A7:e- emitter designed by EXOTRAIL for compatibility with 150 W class thrusters was successfully operated in firing mode, and a first I/V characterization was registered for one and a half hours with stable self heated operation.

2^nd quarter 2022

Krypton long run

The NACES cathode was operated for over 100 hours with the alternative propellant krypton. The plasma discharge could be maintained over the whole time span. Multiple reignitions were required but all successful. Performance Mappings were taken at the beginning and the end of the test. Material characterization of the insert prior and subsequent to the test showed a depletion of electron density due to sputtering of the material.

First iodine operation of NACES cathode

The NACES cathode was successfully operated with iodine as propellant at the iodine test facility at Justus Liebig University of Giessen. After an initial pre-heating to avoid iodine condensation, the device was ignited and operated for 2 hours 45 minutes (test ended nominally) with iodine. The discharge could be maintained rather stably, but no current extraction towards the anode was possible. After operation, the insert showed no other degradation, but the device specific plasma degradation also observed for noble gases.

5 hour operation of JLU disk insert cathode

The design operating with a disk shaped electride insert developed at Justus Liebig University of Giessen was operated for over 5 hors with xenon. Only negligible insert melting was observed.

14 hour operation of electride hollow cathode

The JLU hollow cathode was operated for over 14 hours in total with a C12A7:2e- electride insert. The cathode was operated with 1.4 A – 1.6 A of plasma heating with 130 W additional external heating. 0.2 A of current could be extracted from the device. After an initial stable phase of 10 hours, the required keeper voltage increased, probably due to sputtering issues.

Heaterless hollow cathode development by EXOTRAIL

A heaterless cathode was designed and successfully ignited in diode mode for a plasma characterization in self-heated mode. Preliminary results revealed good electro-plasma properties with successful and fast ignitions (few seconds). However, the thermal design has to be improved to prevent any melting areas during this stressful ignition.

C12A7:e- cathode design by EXOTRAIL for compatibility with 150 W class HET

In parallel, other emitters were designed to be compatible with a 150 W class Hall thruster in xenon. With larger emitting area, the diode mode characterization revealed the possibility to extract a high electron current without reaching the melting point of the C12A7. During the design iterations, the emitter lifetime was a point of interest regarding our previous experience and the thermal behavior of this material.

C12A7:e- insert surface analysis after operation with iodine

Following unsuccessful tests, some surface analyses were performed in order to investigate the iodine compatibility of the emitter. These analyses revealed undesired elements at its surface which could be the reason of the previous non-conclusive tests.

Performance tests of two new cathode design architectures

Two new hollow cathode design architectures have been designed, developed, and tested by ATD, showing  currents from the plasma extracted at the anode between 20 and 60 mA with performance ratios of 4 to 5 mA/W.

Presentations at EPIC Workshops, SPC and IEPC

The work of the NEMESIS consortium was presented in 11 contributions in total at the EPIC Workshop 2022 in Cologne, at the 8^th Space Propulsion Conference 2022 in Estoril, and at the 37^th International Electric Propulsion Conference 2022 in Boston . More details can be found in section “Publications”

120 hour operation with Argon

The NACES cathode was successfully operated with argon as propellant at ATD, and the Raman tests performed after the 150 hour operation in the C12A7:e- surface was showing little differences with respect to C12A7:e- surface prior to the long run test.

The cathode was providing above 40 mA stable current with 5,8 W total power consumption, thus operating above 7 mA/W performance ratio.

1^st quarter 2022

Charge coupling technique

A solution for performance instabilities of C12A7:e- thermionic emission was designed based on charge coupling techniques and successfully tested with good results and a total absence of sparks and instabilities. Solution technique IP rights have been protected via patent. Electronic circuitry implemented in a box like previously done for the igniter and protection power supply box. First 30 hours of successful continuous operation with this charge coupling solution starting from room temperature and operating bellow 150 ºC.

C12A7:e- cathode coupled with EXOTRAIL 50 W class thruster

After different machining and post-processing with the C12A7 provided by ATD, emitters have been integrated and tested in different Exotrail’s cathodes. A C12A7 cathode, designed to operate with 50 W class Hall thrusters, was ignited with a heating power close to 35 W and electrically characterized in diode mode while operating in self-heated mode. Compared to LaB6, an decrease of 13 W of heater power has been observed, with a same cathode design and with better electrical parameters during operating time.

A C12A7 cathode was also integrated and successfully coupled with a 50 W class thruster. Despite a large range of discharge power (from 35 W to 73 W), the C12A7 emitter was stable without any sign of degradation. This cathode and the Hall thruster have been operated with xenon for few hours in self-heated mode and several successful ignitions have been performed.

Following xenon characterizations and thanks to the compatibility between C12A7 and iodine, a thruster head was tested with this alternative propellant. A 50 W anode block and its C12A7 cathode was integrated in a JLU (Justus Liebig University) vacuum chamber with a specific setup designed to fit the JLU facilities and the Exotrail’s thruster head

Preliminary results show that, after successful ignitions of the thruster head with xenon, the contact of iodine with the C12A7 emitter dramatically reduced the emission capabilities of the C12A7. It was neither possible to ignite the thruster head with iodine injection, nor with xenon after the iodine exposure.

4th. Quarter 2021

The NACES cathode was successfully tested in the JUMBO vacuum chamber at JLU Gießen, which possesses a pumping speed of 9900 l/s plus cryo pumping possibility. This allows testing of the neutralizer in an almost space like environment. As expected, the ignition required higher gas flows, but could be achieved reliably. Performance mappings were taken.

Besides the initial operation with argon, the NACES cathode was be operated with the noble gases krypton and xenon. Performance mappings were taken in will be presented at the next international conference.

Performance tests of NACES cathode was also successfully tested at FOTEC premises, with the data mappings shown hereafter.

The JLU hollow cathode was successfully operated over more than three hours with an electride insert and xenon as propellant .





C12A7:e- synthetized by ATD were machined and integrated in an Exotrail cathode design, in order to be coupled with a 50W-class Hall thruster. Two miniature C12A7:e- cathodes with heater were characterized in diode mode, in an operating range from 0.07A to 0.45A, and compared to the LaB6 emitter. Heating power required to start the cathode were 35 W and 48 W for C12A7:e- and LaB6, respectively.

Some new additional improvements have been undertaken in the NACES prototype at ATD, and have driven to the results in performance ratio I_anode/P_cathode (mA/W) shown in the attached figure for different gases and gas flows for 10 V anode voltage.

Leveraging on the know-how developed within the project, several new cathode designs and architectures are being developed. One of them is the hollow cathode developed at ATD whose prototype and performance results are attached hereafter.

3rd. Quarter 2021

NACES cathode is achieving very good result as heater-less neutralizer working at low temperatures, as can be seen in the performance rations shown in the following table.

Anode current is measured even in absence of anode power supply (V_anode=0), when the anode is grounded and the only external power supply is the one provided by the cathode power supply.

Another remarkable ratio is the I_anode/I_cathode (%), which is reaching values above 60-70%, meaning that most of all the injected cathode current is delivered at anode, with minimal current losses at keeper.

With some additional design variants we have been able to further improve this ratios till reaching performance ratios above 6 mA/W for anode current vs cathode power, and keeper current below 2% of the cathode current. The prototype and its plume can be seen in the following pictures, and the characterization and performance curves are also shown hereafter.

After successful manufacturing of hollow cylinders from high quality electride material, they were tested in JLU Giessen hollow cathode design. The plume can be seen left of the black keeper.

First tests of the NACES cathode were performed at JLU. The easy ignition procedure from room temperature of the could be reproduced and current was extracted. Different argon gas flows in the range from 20 sccm to 50 sccm were tested.

2nd. Quarter 2021

The first tests of the improved cathode prototype have shown very promising results. The
performances of a 20 W cathode power are displayed in the figures.

Prototypes at ATD for delivery to UPM, JLU Giessen, and FOTEC.

Several characterization tests have been performed on 8 different C12A7:e- production batches, each one with a different electron concentration, with 5
characterization techniques: Electron Spin Resonance, Raman, Hall Effect, Thermionic emission, and Kelvin Probe Force Microscopy.

1st. Quarter 2021

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First prototype tests at ATD have shown an outstanding performance
of the C12A7:e- material when operating under ionized gases. Cathode
design improvements are being introduced and new prototype versions.

Testing the same cathode design to compare LaB6 material under Xe and Ar gas flows, showed a higher performance of C12A7:e- material.

The data acquisition and control software for UPM test activities has been checked and validated in a realistic electron emission experiment using a DC heated tungsten hairpin.

First tests performed with the EXOTRAIL 1A lab prototype comparing LaB6 and C12A7:e- materials.

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4th. Quarter 2020

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Thermal camera measurements performed by Exotrail revealed thermal gradients of up to 50 ºC across the emitting surface of C12A7:e- .

Finished the build up of the test chamber Dory at JLU Giessen, which is now fully operational for neutralizer testing, starting by the Hollow Cathode with LaB6 insert.

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September 2020
Completion of the vacuum chamber, test environment, and control and data adquisition SW at UPM.

FOTEC has documented the first thermionic emission got from a C12A7:e– thin film deposition via sputtering technique.

August 2020
Thermionic emission comparison tests performed at Exotrail for LaB6 and C12A7:e– emitting surfaces of 6 mm diameter disc.

Set-up improvements developped by Fotec.

Successful ignition and operation of neutralizer with LaB6, and thermal models of electride and LaB6 developed at JLU.

July 2020
New samples batches of C12A7:e- with high electron density produced at ATD