field emission cathodes for an electrodynamic tether

Trans. JSASS Aerospace Tech. JapanVol. 16, No. 1, pp. 63-68, 2018DOI: 10.2322/tastj.16.63

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Copyright© 2018 by the Japan Society for Aeronautical and Space Sciences and ISTS. All rights reserved.

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Field Emission Cathodes for an Electrodynamic Tether Experiment on the H-II

Transfer Vehicle

By Yasushi OHKAWA,1) Teppei OKUMURA,1) Yuuta HORIKAWA,2) Yoshiyuki MIURA,3) Satomi KAWAMOTO,1) and Koichi INOUE3)

1) Research and Development Directorate, JAXA, Chofu, Japan

2) Research and Development Directorate, JAXA, Tsukuba, Japan 3) Space Technology Directorate I, JAXA, Tsukuba, Japan

(Received June 22nd, 2017)

A field emission cathode (FEC) using a carbon nanotube was developed for an electrodynamic tether experiment on H-II transfer vehicle 6 (HTV-6). The mission is called the Kounotori Integrated Tether Experiment (KITE). Development of the FEC began at the beginning of 2013 and was completed in the spring of 2016. KITE began in January 2017 and the FEC worked well, with various types of on-orbit data being obtained despite unsuccessful tether deployment. All eight cathode units operated without any critical trouble throughout the experiment period. The total operation time reached 50 hours and the maximum emission current was approximately 5.8 mA, and thus exceeded expectations based on ground experiments. The electrical current loop via an ambient space plasma without the tether was probably formed due to the collection of electrons on the anodic parts of the HTV’s solar cells.

Key Words: Active Debris Removal, Electrodynamic Tether, Field Emission Cathode, Carbon Nanotube, HTV-6

Nomenclature

Jet : emitter current (electron current emitted from emitter material), mA

Jg : gate current (electron current through gate electrode), mA

Jes : emission current (electron current extracted from cathode unit), mA

Vg : gate voltage, V H : electrical potential of HTV with

reference to plasma space potential, V 1. Introduction The growing amount of space debris on orbit around the Earth has become a severe problem. Removing large debris objects from crowded low Earth orbit (LEO) is considered one of the effective ways to prevent the ongoing growth of the space debris population.1,2) In order to pursue active debris removal (ADR) activities in the near future, low-cost ADR systems should be developed.

An electrodynamic tether (EDT) is a prospective candidate for the deorbit propulsion of ADR and can contribute to simplifying the overall system due to its following advantages: 1) The EDT requires no propellant because thrust is generated by electromagnetic interaction between the tether current and the geomagnetic field; 2) high electrical power is not necessary as self-induced electromotive force can drive the tether current; and 3) other unique virtues of EDTs for debris removal are no thrust vector control or strict restrictions required on the clamping point and force applied to debris due

to low thrust levels. One essential component of the EDT is an electron emission device for supplying electrical current through the tether. This device should be simple in order to realize low-cost ADR systems. A carbon-nanotube-based field emission cathode (FEC) has been developed by the Japan Aerospace Exploration Agency (JAXA) to satisfy this requirement. The FEC is suitable for this purpose because of its potential simplicity and low-power consumption. As a first step toward realizing low-cost ADR, JAXA planned and conducted an on-orbit experiment of the EDT on H-II transfer vehicle 6 (HTV-6). This plan is called the Kounotori Integrated Tether Experiment (KITE). Figure 1

Fig. 1. Expected image of KITE on orbit.

Trans. JSASS Aerospace Tech. Japan Vol. 16, No. 1 (2018)

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shows the expected image of KITE on orbit. The FEC was used in KITE as a key device of the EDT system. HTV-6 was launched in December 2016 and KITE began in January 2017 after the HTV left the International Space Station (ISS). Although the tether could not be deployed, the FEC operated without any critical trouble throughout the mission. This paper describes the details of the FEC developed for KITE, and gives quick reviews of the on-orbit operation results. 2. Overview of KITE3,4) The primary objectives of KITE are to obtain data on the fundamental characteristics of the EDT components and increase technology readiness levels to design and develop an EDT system for ADR. Devices and instruments for KITE are installed on HTV-6, a Japanese cargo transfer spacecraft. For the KITE mission, a bare tether (720 m in length) is planned to be deployed from the HTV toward the zenith, and electrical current of approximately 10 mA will flow due to electron emission from the FEC on the HTV to an ambient space plasma. Figure 2 shows the main KITE components installed on the HTV. These include the tether, reels for housing and braking, end mass, release mechanism of the end mass, camera for observing tether dynamics, FEC module, potential monitor for measuring the absolute electrical potential of the HTV (LP-POM),5) magnetic sensor (MAGS), and the data handling unit and power control unit (DHU/PCU). These components were located at various places in the HTV to meet their own functional requirements. The most crucial component of the EDT system is the tether.6) The tether has a total length of about 720 m, and is housed in the end mass in its initial condition. The tether consists of thin aluminum and stainless-steel wires, and features a mesh structure to prevent it from being severed by the impacts of small-sized debris.

The motion of the end mass was planned to be observed by rendezvous sensors (RVSs) of the HTV. The RVSs are not KITE components but HTV instruments, which are originally

used for approaching the ISS. 3. Field Emission Cathode (FEC) 7-9) 3.1. Fundamentals The electron emission device is an essential component of the EDT system, in addition to the tether. JAXA has researched and developed FECs as promising candidates. Figure 3 shows a schematic drawing of the FEC. Electrons are extracted by simply applying an electric field on the surface of the emitter material; consequently, the system can be simplified compared with other cathode devices, such as a hollow cathode and an electronic gun.

The electron emitter is made of a carbon nanotube (CNT) because its nano-scale structure enhances the local electric field, so as to require low gate voltage. Tolerance to atomic oxygen (AO) is one of the critical issues when the CNT-FEC is used in LEO. The results of AO irradiation tests suggested that the CNT-FEC is durable in typical ADR operation when the AO flux does not directly impinge the emitter surface.

As shown in Fig. 3, part of the electrons emitted from the emitter flows to a gate electrode and forms the gate current. When the anode potential (or plasma potential in space) is low, the gate current is increased by the electron backflow attributable to the space charge limit. Plasma electrons also flow into the gate electrode and amplify the gate current in on-orbit operations. Management of the gate current is also a key point for effective FEC operation on orbit.

3.2. FEC module The CNT-FEC for KITE was developed based on a laboratory model researched by JAXA. Figure 4 shows the upper view of the flight model (called FECH). This model consists of eight cathode units, each of which can emit electrons at up to 2.2 mA. When the anode voltage (see Fig. 3) is high enough to ignore the space charge limit, about 90% of the emitter current is extracted from the cathode units and then reaches the anode. The footprint of FECH is approximately 240 200 mm.

FECH is mounted on the FEC module as shown in Fig. 5. The electron emitter surface of FECH was set parallel to the orbital motion axis to prevent the direct impact of AO flow. FECH was initially covered by a protective cover (called FECG), which is used to protect FECH against flakes, contamination and AO impingement. The FECG opens just before the start of electron emission operation. A power

Fig. 2. Main components of KITE.

Fig. 3. Schematic of the FEC.


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control unit (called FECC) is also mounted on the module. The FEC module is located on the thrust module of the HTV as shown in Fig. 2. 3.3. Electrical circuit and algorithm Figure 6 shows a schematic of the electrical circuit including FECH and FECC. The emitter electrodes of FECH are connected to the cable from the tether via three parallel switches and resistors. These components are necessary to reduce the inrush current, which flows through the tether and the circuit on connecting because the potential difference between the tether and the HTV body becomes approximately

100 V in maximum due to the electromotive force of the tether. In this circuit, three resistors (R2 > R1 > R0) are installed in parallel, and three switches (SW2, SW1, and SW0) are turned on in sequence in order that the potential difference decreases in stepwise fashion and then the inrush current becomes low. Two literatures 10,11) on on-orbit EDT demonstration were referred to design the circuit.

A single DC-DC high voltage (HV) converter drives one cathode unit so that the gate voltage of each unit is independently controlled and any malfunction of a single HV or cathode unit does not lead to a complete loss of function. A maximum gate voltage of 1000 V is to be supplied by the HV converters. FECC possesses several automatic operation modes such as current-voltage (I-V) measurement, tether current and voltage measurement, autonomous consecutive operation, and full manual operation. In the actual on-orbit operation, only the I-V measurement and autonomous consecutive operation modes were used due to unsuccessful tether deployment. In the autonomous consecutive operation mode, a simplified control algorithm is adopted for automatic adjustments in gate voltage to maintain constant gate current at all times. This algorithm is effective in EDT application where the potential difference between the plasma as a virtual anode and the FEC fluctuates. The cathode units (CH1-CH8) to be operated can be manually selected in all operation modes. 4. Development of the FEC module Fabrication and testing of the engineering model of the FEC components began in February 2013. The flight unit fabrication began in October 2014, and the final electron emission characteristics were obtained at the final functional test (see Fig. 7) conducted in May 2016 after the qualification process. Final assembly and coordination were conducted, and then the module was installed on the HTV in July 2016.

5. Quick Reviews of On-Orbit FEC Operation 5.1. Overview of KITE results HTV-6 was launched in December 2016 and KITE began in January 2017 after HTV-6 left the ISS. The altitude of the HTV orbit during the KITE mission was around 370 km.

Fig. 4. Flight model of FECH.

Fig. 5. Flight model of the FEC module.

Fig. 6. Electrical circuit in the FECC and FECH.

Fig. 7. Setup of the FEC module for final functional test.


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Table 1 summarizes the events in KITE. After the initial checkout of the KITE instruments, the tether deployment process for ejection of the end mass was pursued, but release of the end mass was not detected. Despite various efforts and attempts to recover this malfunction during the KITE mission period, the end mass could not be released. Although tether deployment was unsuccessful, the FEC operated well without any critical trouble throughout the mission period. The KITE mission was terminated just before HTV-6 began re-entry maneuver.

5.2. Overview of FEC operation results The FEC operation was started by opening the FECG on Day 3 and ended at the final I-V characteristics measurement on Day 8. All eight cathode units operated well from Day 3 to Day 8 without any critical failure, such as a short-circuit between the electrodes. Table 2 summarizes the FEC operation. The accumulated operation time, that is, the period during autonomous operations were conducted accompanying electron emission, reached 50 hours in the total exposure time of 130 hours. These periods are effective for discussing the lifetime of the FEC on orbit because the orbit altitude is quite low (370 km) compared with the expected orbit where ADR will be conducted in the future, and thus this experiment is regarded as an acceleration test on the degradation of FEC performance in AO and the plasma environment.

The total maximum emitter current and emission current

were approximately 10.2 and 5.8 mA, respectively. Note that “emitter current” indicates the electron current emitted from the CNT emitter; “emission current” means the electron current extracted to outside the cathode units. The emission current is calculated by subtracting the gate current from the emitter current. 5.3. Example of FEC operation Figure 8 shows an example of the trend in total emitter and gate current of the eight cathode units. The figure shows the trends in electron emission in five successive autonomous operations. The intermissions between the operations are attributable to the downlinking of the data. The gate current was kept almost constant and the emitter current widely varied in magnitude. These results show that the gate voltage of each cathode unit was adjusted to maintain constant gate current as intended and described in 3.3, and the emitter current (i.e. emission current) varied in accordance with external environmental factors. The most important external factor regarding the FEC operation on-orbit is the electrical potential difference between the ambient space plasma as a virtual anode and the emitter of the cathode units. The emitter lines are connected to the HTV body as shown in Fig. 6 so that the emitter potential is equivalent to the HTV potential. Figure 9 shows the trend in HTV potential with reference to the ambient plasma in the same time range shown in Fig. 8. The HTV potential was measured by two potential monitoring sensors installed on the LP-POM. The trend in HTV potential largely depends on the voltage generation by the HTV’s solar cells, that is, the solar irradiation conditions. By comparing Figs. 8 and 9, one can see that the emitter current is larger when the HTV potential is lower. This tendency is reasonable because the space charge limit is mitigated when the potential difference between the FEC and the virtual anode becomes larger.12) Another point to be discussed in Fig. 8 is the relation of magnitude between the emitter and gate current. The emitter current is typically larger than the gate current, but this

Table 2. Summary of FEC operation.

Accumulated operation time 50 h

Accumulated exposure time 130 h

Number of I-V measurement operation 84

Number of autonomous consecutive operation 22

Maximum emitter current (sum of 8 cathode units) 10.2 mA

Maximum emission current (sum of 8 cathode units) 5.8 mA

Table 1. Overview of KITE events.

Day Event

Day 1Checkout of KITE components.

Tether deployment attempt. Resulted in unsuccessful.

Day 2 Discussion on tether deployment malfunction.

Day 3Opening of FECG.

Measurement of I-V characteristics of FEC.

Day 4 Successive FEC operation at low current level.

Day 5Tether deployment re-attempt. Unsuccessful.

Measurement of I-V characteristics of FEC.

Day 6Tether deployment re-attempt. Unsuccessful.

Successive FEC operation at high current level.

Day 7Successive FEC operation at three different current levels.

Successive FEC operation at three different HTV Yaw attitude .

Day 8Measurement of I-V characteristics of FEC.

Shutdown of KITE components.

Fig. 8. Trend in electron emission characteristics on Day 4. Five successive autonomous operations were conducted on this day.

Fig. 9. Trend in HTV potential with reference to ambient plasma in same time range as in Fig. 8.


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relation is reversed several times when the HTV potential shown in Fig. 9 is close to the plasma potential. This condition means that the inflow of electrons from the plasma to the gate electrodes is larger than the outflow of electrons from the FEC to plasma at such times. If the tether was deployed as planned, on the other hand, we infer that the emitter current was always larger than the gate current in the autonomous operations. This is because the negative HTV potential enough to mitigate the space charge limit could be kept by the electromotive force of the tether. 5.4. Electron emission characteristics The performance of the FEC can be characterized from various aspects, and one of the essential indicators is the electron emission efficiency against the anode (or plasma on-orbit) potential. Note that “electron emission efficiency” indicates the ratio of emission current to emitter current. This indicator is important for estimating the tether current in the EDT system under various potential conditions. Figure 10 shows the electron emission efficiency plotted against the plasma-to-HTV potential difference. This plot is based on the results of autonomous operation on Day 6. Note that the data when the emitter current was less than 1 A were excluded in order to avoid the inaccurate efficiency calculation. The results of the final ground test are also shown in this figure. The plate anode located 200 mm away from FECH was used for current collection in the ground test. The on-orbit data in Fig. 10 shows that emission efficiency depends on the plasma-to-HTV potential difference as expected, and that the characteristics are better than those obtained by the ground test using the plate anode. For example, the emission current at the potential difference of about 40 V reached 0.60 on orbit, as compared to approximately 0.43 in the ground test. The reason for the difference is not clear at present, and further investigation is needed in considering the local sheath geometry, properties of the ambient plasma, orbital velocity, and unexpected phenomena in the ground test.

5.5. Current loop formation via plasma One of the essential questions about the FEC operation described here is whether the electrons were really emitted to

the ambient plasma in this experiment, in which a bare tether did not exist. In the EDT fundamentals, the tether generates the potential difference between the tether ends, and thus the electron current loop via the electron emitter, ambient plasma and bare tether is formed in a steady state. Although further discussion is necessary, we infer that the voltage generated by the HTV’s solar cells plays a role of the tether voltage, and that the exposed anodic potential area of the solar cells collects electrons from the ambient plasma. Figure 11 illustrates the conceptual image of the current loop formation via space plasma in this experiment. Supporting evidence of the inference above is the HTV’s potential behavior in accordance with electron emission from the FEC. Figure 12 shows the trends in electrical potential of the HTV and in emission current from the FEC at the I-V measurement operations on Day 3. Although the trend in HTV potential is complicated due to various external factors, there is a correlation between the HTV potential and emission current. The negative potential of the HTV was mitigated by the electron emission from the FEC. This relation indicates that the potential balance between the HTV and the space plasma in the nominal condition was violated by the FEC operation, thus establishing a new potential equilibrium that includes the effect of electron emission from the FEC. From another viewpoint, the FEC can be applied to control or mitigate spacecraft charging for preventing malfunction caused by unintended discharges.

6. Conclusion This paper described the fundamentals, components, development, and results of on-orbit operation of the field

Fig. 10. Comparison of electron emission efficiency between on-orbit and ground experiments. Emission efficiency is plotted against potential difference between the FEC and plasma (or anode in the ground test).

Fig. 11. Conceptual image of electrical current loop formation via space plasma without a tether.

Fig. 12. Trend in HTV potential in accordance with FEC operation.


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emission cathode developed for an electrodynamic tether experiment on HTV-6. Although the tether could not be deployed, the field emission cathode operated without any critical trouble throughout the experiment period.

All eight cathode units worked well and the total operation time reached 50 hours. The maximum emission current extracted to the ambient plasma was approximately 5.8 mA. The electron emission characteristic against the potential difference to the ambient plasma was better than expected based on the ground experiments. Although further discussion is necessary, we infer that the current loop between the HTV and space plasma without the tether was formed by the collection of electrons on the anodic parts of the HTV’s solar cells. Acknowledgments The authors appreciate the generous support from JAXA-HTV members regarding KITE development and operation. The authors also wish to thank the KITE members for their significant efforts in pursuing the project. This work was partly supported by JSPS KAKENHI Grant Number JP16H04595.

References

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10) Stone, N. H.: Electrodynamic Characteristics of the Tethered Satellite System during the TSS-1R Mission, AIAA Space Programs and Technologies Conference, Huntsville, USA, AIAA Paper 1996-4472, 1996.

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field emission cathodes for an electrodynamic tether

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