Trans. JSASS Aerospace Tech. Japan Vol. 17, No.5, pp. 589-595, 2019 DOI: 10.2322/tastj.17.589

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1,000-hours Demonstration of a 6-kW-class Hall Thruster

for All-Electric Propulsion Satellite

By Ikkoh FUNAKI,1),2) Shinatora CHO,3) Tadahiko SANO,2) Tsutomu FUKATSU,2) Yosuke TASHIRO,4) Taizo SHIIKI,4) Yoichiro NAKAMURA,4) Hiroki WATANABE,5) Kenichi KUBOTA,3) Yoshiki MATSUNAGA,3) and Kenji FUCHIGAMI6)

1)Institute of Space and Astronautical Science, JAXA, Sagamihara, Japan

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

4)IHI Aerospace, Tomioka, Japan 5)Department of Aeronautics and Astronautics, Tokyo Metropolitan University, Hino, Japan

6)IHI, Yokohama, Japan

(Received September 1st, 2018)

A 1,000-hours preliminary life test of a 6-kW-class xenon Hall thruster for Engineering Test Satellite-9 (ETS-9) was conducted by using a bread board model thruster. Thrust degraded by approximately 2% before and after the test, which is mainly attributed to erosion and contamination of discharge channel made of boron nitride. The thruster was disassembled at an accumulated operational time of 652 hours to find 2.3-mm erosion at the channel’s downstream surfaces. From 652 to 1,012 hours, low erosion rates about 10 μm/khr were found for the channel and the cathode; exception was found in the case of the insert, whose erosion rate was relatively high as 60 μm/khr. Thruster’s estimated lifetime extrapolated from the test is more than 10,000 hours, which is satisfactory for all-electric propulsion satellites.

Key Words: Hall Thruster, Engineering Test Satellite 9 (ETS-9), Life Test

Nomenclature

B : magnetic field, T D : effective thruster diameter, m E : electric field, V/m F : thrust, N g : gravitational acceleration, m/s2 Id : discharge current, A Ik : keeper current, A

Isp : specific impulse, s jθ : azimuthal current density, A/m2 �̇� : mass flow rate, kg/s Pw : total input power, W Vd : discharge voltage, V Vcg : cathode-to-ground voltage, V Vk : keeper voltage, V 𝜂 : thrust efficiency, %

1. Introduction Hall thruster is an annular-shaped plasma accelerator for satellite propulsion.1-3) The role of Hall thruster in the past was auxiliary propulsion to compensate disturbing forces on a satellite and to keep the satellite’s operational orbit such as geostationary orbit. Recently, the thrust level of Hall thruster was drastically increased to behave as a main propulsion

system to cruise medium (3 ton) to large (6 ton) satellites from their launch orbits to final orbits. A satellite in which all or most of maneuvers are conducted by electric propulsion (EP) is called as an all-electric propulsion (all-EP) satellite, and it is spreading among the world due to its high payload transfer efficiency.4-10) Until now, Hall thrusters up to 305 mN were flight qualified and they were used for various all-EP satellites and also there are many future plans to use them.11-18) To satisfy the growing demand for high thrust and high specific impulse (Isp) Hall thrusters, a new 6-kW class Hall thruster with a thrust level of 390-mN is pursuit by Japan Aerospace Exploration Agency (JAXA).19,20) Figure 1 shows laboratory operation of a breadboard model thruster that is being developed for its flight experiment onboard Japanese Engineering Test Satellite-9 (ETS-9).21-23)

Fig. 1. Hall thruster operation in vacuum chamber.

© 2019 The Japan Society for Aeronautical and Space Sciences

Plasma inAnnularChannel

Cathode

Xe IonBeam

=

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To maintain high performance during long-term operation is the most important and critical part in designing a Hall thruster. Let the authors firstly explain Hall thruster and its important processes to be dealt with in this paper. As is plotted in Figs.1 and 2, Hall thruster produces a plasma in an annular ceramic channel, and the plasma is emitted as a high velocity jet that will produce a repulsive force to propel a satellite. A hollow cathode is set in the center of the thruster to provide electrons to the channel; the electrons are necessary to produce ions as a result of collisions with xenon neutrals. In the downstream region of the channel in Fig.2(b), ion acceleration happens, and high velocity ions are neutralized by the electrons, which are also provided from the cathode. Considering that some of the high-velocity xenon ions produced in Hall thruster can impinge on thruster’s surfaces and facility’s walls, erosion by ion bombardment (Fig.2(b)) and contamination by eroded materials (shown later in Fig.5) are the main factors that could lead the Hall thruster to a failure mode. Among these concerns, channel erosion was considered as the inevitable life limiting factor. The life test of BPT-4000 Hall thruster however changed this situation. The life test of BPT-4000 exhibited that channel erosion rate was decreasing to reach an almost zero-erosion rate at around 5,600-hours.13) After the BPT-4000 life test, the erosion of channel is considered to be controllable and it is not an inevitable life limiting factor if a Hall thruster is properly designed. One extreme idea is to establish an erosion-less channel design, and this concept was provided as “magnetic shielding” (MS) design.24-26) Zero or nearly-zero erosion channel is a clear and ideal solution when a long-term interplanetary mission is concerned. However, if correctly managed, zero-erosion thruster is not necessarily required in the case of all-EP propulsion satellite, where a moderate erosion thruster can

complete a mission that is not so long as interplanetary missions. In contrast to channel’s erosion, cathode wear seems unavoidable and hence needs more careful considerations. For the cathode, in addition to the wears of insert and orifice by cathode plasma in Fig.2(c), the wears of keeper and its sounding materials, such as the pole cover in Fig.2(a), were identified and their wear mechanisms were extensively studied in particular for MS thrusters.27-30) Smooth coupling between a thruster’s plasma and a cathode plasma is the key for a low erosion cathode, and high electron emission capability is required in this case. The objective of this study is to demonstrate the low erosion thruster design of a 6-kW Hall thruster in a limited 1,000-hours test paying attention to the erosions of channel, cathode, keeper and its surrounding. 2. Experimental Setup In the following, the design and setup of the 6-kW Hall thruster is shown along with its erosion and contamination concerns. A Hall thruster is usually designed to produce and accelerate ions in the form of a well-collimated ion beam. To enhance thruster’s efficiency and to lower power deposition onto the channel wall, the thruster employed “plasma lens” type magnetic field topology31-33) as shown in Fig.2(a) in the channel. In this topology, a highly skewed upstream magnetic field along with mostly straight B-field at the channel exit was arranged to make a focused ion beam with suppressing ion impingement on the wall. For this purpose, channel configuration of the 6-kW Hall thruster’s was optimized in the previous study.19,20) In the ionization region in Fig.2(b), electrons are gyrating and drifting in an EXB field, which are caused by a radial magnetic field (B-field) and an electric field (E-field) between an anode ring in the upstream of the channel

Fig. 2. Schematics of Hall thruster and its wear processes.

Electrons

Xe gas

HeaterP.S.

Keeper P.S.

Anode P.S.

Outer CoilP.S.

Outer Coils MagneticCircuit

Anode

Keeper (Graphite)

CeramicWall(Channel)

Xe+ Ions

Inner Coils

CathodeCommon

InnerCoil P.S.

Xe gas

Thruster Body

Cover Wall(Graphite)

Heater

Tube(Ta)

Cathode

Id

Electrons

Channel

IonizationRegion

AccelerationRegion

++

++

-+

Initial ProfileEroded Profile

Ion Flux

Channel (BN)

Channel (BN)

Eroded Region

+-

LaB6 Insert Orifice (W)

- +-

-

-

-- -

-

-

-

+

++-

Xe gas

Xe gas

Channel Plasma and Erosion Processes

Cathode Plasma and Erosion Processes

BE

jθ

(a)

(b)

(c)Hall Thruster Head and Electric Connections

Vd

Vk

+

+-

--

-

-

-

v

v

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and the hollow cathode. The energized electrons in these fields contribute to plasma production. Then, just downstream the ionization zone, an acceleration region exists. As shown in Fig.2(b), ions in both the ionization zone and the acceleration zone will impinge on to the channel walls to erode the channel surface, and it is expected that the downstream surface of the channel will be eroded out by ion impingement (the lower closed-up figure in Fig.2(b)). Since the channel material is protecting magnetic poles, it should remain during thruster’s operation. Except the region covered by the channel, graphite was used to protect the poles from ion bombardment. All these materials must remain through thruster’s service life. For the cathode, as shown in Fig.2(c), the cathode tube, made of Tantalum, had an insert in the shape of a cylinder that was placed inside the tube and pushed against the orifice, made of Tungsten. The insert was made of a low work function material, Lanthanum hexaboride (LaB6), featuring high electron emission capability and good resistance to contamination.34,35) The cathode tube was surrounded by a carbon heater and heat shield combination that raised the insert temperature to electron emitting temperatures above 1,400 °C to initiate a discharge. Outside these tube elements, a keeper electrode was placed to start-up and maintain a cathode discharge. To initial a main discharge, a bias voltage was applied between the anode ring in the upstream of the channel and the hollow cathode. Then, the keeper power supply was switched off and a steady state was achieved after the thruster reached a thermal equilibrium. Typically, thermal equilibrium of the thruster is obtained after two-hours of continuous operation. As for the cathode, configuration for low orifice erosion was surveyed by some stand-alone tests by keeping high current emission with a high insert temperature of about 1,600 °C. The preliminary life test was conducted in a vacuum chamber at JAXA. The set-up in Fig.3 shows both xenon gas and electricity were provided from outside the vacuum through feedthroughs. Major equipments, that is to say, flow controllers for the thruster and for the cathode, an anode discharge power supply, coil power supplies, and a thrust measurement system, were depicted. In addition to these, several power supplies are required for heating the cathode, and for the keeper to initiate a cathode discharge. Also, voltages and currents were measured by a data recorder while their transient profiles were monitored by an oscilloscope.

Fig. 3. Experimental setup. Prior to the test, the vacuum chamber was evaluated by

cryogenic pumps to reach a base pressure of 2 × 10() torr and thruster operation was conducted at a pressure < 2 × 10(+ torr (corrected for xenon). Shroud panels inside the vacuum chamber were arranged to keep them in room temperatures during thruster operation and also to prevent direct ion bombardment to the pumps from the thruster. During the test, thruster was mounted on a thrust stand, and performance measurement was conducted by approximately 100-hours interval. Specific impulse (Isp) and thrust efficiency (𝜂) are obtained by 𝐼𝑠𝑝 = 𝐹 (�̇�𝑔)⁄ and 𝜂 = 𝐹6 (2�̇�𝑃8)⁄ , where F is thrust, �̇� is total mass flow rate, and 𝑃8 is total input power. Also, at 605 hours and 1,012 hours, the vacuum chamber was returned to atmospheric pressures. At these timings, the thruster was disassembled to measure the wearing status of the channel, the cathode orifice and keeper, and the cathode insert. 3. Results and Discussion 3.1. Operation of Hall thruster The operational parameters for 1,000-hours testing are listed on Table 1. The test was conducted at a discharge voltage of 300 V and discharge currents of 19.6-19.8 A, which corresponded to discharge powers of a bit below 6 kW. Additional power for magnetic coils (29 to 43 W) was also spent during steady operation, and the total power was set at 6 kW±50 W by adjusting the mass flow rate. During operation, both the anode and the cathode common were floated and only the thruster’s body was grounded. To see the cathode-to-thruster coupling status, Vcg was monitored as shown in Fig.3 and it was defined as a voltage between the ground and the cathode common. On Table 1, Vcg was about -6.6 V and the small magnitude in Vcg indicated that electron emission capability from the hollow cathode was high and that electron transport from the cathode to the anode was easy. Another floated voltage is that for the keeper, and it is a parameter to judge if stable or unstable cathode discharge is realized.36-41) Rule of thumb in the case of standalone operation is a spot mode or a similar stable mode is obtained for Vk < 10 V, and 20 V or higher Vk are accompanied by voltage oscillations meaning an unstable, plume mode operation. The spot mode is visually observed as a ball or “spot” of plasma just downstream of the cathode orifice with little visual glow from the downstream plasma, whereas the plume mode typically shows a widely diverging diffuse plasma extending from the cathode.1,36,37) The experiment showed a keeper voltage of 8.5 V without noticeable oscillation; this fact along with the small Vcg in magnitude suggests efficient coupling between the cathode plasma and the main discharge as a result of a stable mode similar to spot mode. The words “similar to spot mode” is used here because operational modes of Hall thruster to cathode plasma coupling are not fully understood although several important works were conducted in the past,42-46) and it is not sure if the low coupling voltage was a result of spot mode. Since the low coupling voltage causes voltage utilization efficiency (the effective beam voltage to discharge voltage ratio) to increase, total thruster efficiency was as high as 62.9%. It is expected that the center position cathode configuration also contributes to obtain this high coupling efficiency.42,46)

Cryo PumpstoMechanicalPumps

Vacuum Chamber

Cooled BeamDamper (Ti)

Cooled Shroud (Ti)

Pendulum-typeThrust Stand

XeTank

MFCs

Thruster Head

+–

Cathode

Anode

Xe Feed Lines

Power Lines

Beam Plasma

CoilPS

+–

AnodePS(Vd)

Id

Vcg

CathodeCommon

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3.2. Operational history during 1,000-hours test Histories of the life test and major operational parameters were plotted in Fig.4. Operational events were summarized in Fig.4(a). Thrust measurements along with calibration were conducted without breaking vacuum condition at 138-hours, 156-hours, 246-hours and so on at the timing of solid lines in Fig.4(a). In the calibration process, a shift of zero thrust point was evaluated just after stopping thruster's operation, and then, the thrust stand pendulum was calibrated with known weights. At 605-hours and 1,012-hours, after the thrust measurements, vacuum was broken, and the chamber was returned to atmospheric pressures either to measure geometry or to check the thruster outlook. From Fig.4(b), it is found that thrust slowly decreased from 393 mN to reach a constant value of about 384 mN at around 500 to 600 hours. The same trend was found in Fig.4(c), in which Isp and thrust efficiency were plotted, and they decreased from 1,940 s and 62.9% to reach the constant values of about 1,900s and 60%. Discharge current profile (Id in Fig.4(d)) was not so smooth but data scattered at around 19.7 A, and the current slightly increased after 600-hours. The Id reached the maximum value of 19.9 A at 672.8 hours, then it was slightly decreasing. More apparent fluctuations of data were found in the case of Id oscillation in Fig.4(e). Id oscillation is defined as the half of peak-to-peak oscillations. Smaller Id oscillation below 20% is the target of this thruster, but temporal violation of this criteria was found at 258 hours and 626 hours. After 670 hours, Id oscillation was

found to exceed 20% so the coil current was increased by 4% to suppress oscillations. In the same figure, there are transient drifts for example at 759 and 894 hours (indicated as small dotted circles in Fig.4(e)) and so on that correspond to temporal thrust cut off and restart for calibrating the thrust stand. Small dots in the figures indicate transient values and open circles denote equilibrium values. Along the test, Vk and Vcg in Fig.4(f) were almost constant, therefore stable cathode operation with high electron emission was confirmed.

Table 1. Operational parameters of Hall thruster for 1,000-hours test.

*Includes transient values when stopping and restarting the thruster.

Parameters Values (0 hr)

Values during test

Values (1012 hr)

Power to thruster, W 5,978 6,000±50 5,978 Discharge voltage, V 300 300 300 Anode mass flow rate, mg/s 18.8 18.7 to 19.0 18.8 Cathode mass flow rate, mg/s 1.87 1.87 to 1.90 1.87 Discharge current, A 19.7 19.6 to 19.9* 19.7 Thrust, mN 392.8 382.9 to 392.8 382.9 Isp, s 1,946 1,905 to 1,946 1,897 Thrust efficiency, % 62.9 59.8 to 62.9 59.8 Discharge power, W

5,939 5,914 to 6,007*

5,970

Coil power, W 39 29 to 43* 43 Keeper voltage, V 8.5 7.0 to 9.4* 8.1 Cathode-to-ground voltage, V -6.6 -6.4 to -5.0* -6.3

Fig. 4. Operation history; (a) event, (b) thrust, (c) Isp and thrust efficiency, (d) discharge current (Id), (e) Id oscillation, (f) keeper voltage (Vk) and cathode-to-ground voltage (Vcg).

360365370375380385390395400

0 200 400 600 800 1000 1200

Thru

st [m

N]

Accumulated Operational Time [hrs](b)

160016501700175018001850190019502000

50

55

60

65

70

0 200 400 600 800 1000 1200

Isp

[s]

Thrust Efficiency [%]

Accumulated Operational Time [hrs](c)

0 200 400 600 800 1000 1200Accumulated Operational Time [hrs]

Performance Check Points (quick stop)

ErosionProfileMeasurements(chamber returned to air& thrusterwasdisasembled)

605.

8 hr

s

(a)

1,01

2.3

hrs

1012

.3 h

rs

Coil Current (arb)changed at 673 hrs

18

18.5

19

19.5

20

0 200 400 600 800 1000 1200

Id [A

]

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0

5

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20

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Id O

cilla

tion

[%]

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-10

-5

0

5

10

0 200 400 600 800 1000 1200

Vk

and

Vcg

[V]

Accumulated Operational Time [hrs](f)

Vk

Vcg

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3.3. Wear characteristics Figure 5 summarizes the profiles of eroded and contaminated

surfaces of the Hall thruster. Measured channel surface geometry at 0-, 605- and 1,012-hours was plotted in Fig.5(a). Channel erosion at the beginning (between 0 and 605 hours) was as large as 2.3 mm near the channel corner, but afterwards, the erosion rate drastically decreased, and only slight change was found between 605 hours and 1,012 hours. This means that the channel surface geometry was gradually converging into a surface. To explain this channel erosion history, the surface status is schematically plotted in Fig.5(c) and a photo after 1,012 hours test is provided in Fig.5(d). The eroding chamfer region of the channel at 1,012 hours showed a white-gray surface, but in contrast, the upstream surface in the straight part of the channel was contaminated by black materials. The black surface was observed as a result of deposition of sputtered beam target (shown in Fig.3), and deposition was superior to erosion by ion bombardment there. Vice versa, white channel surface was considered to be continuously eroded by ions, and as a result, erosion was overwhelming against contamination. The white-gray surface would correspond to a transition between the two, and hence the downstream channel surface erosion rate was very low. Channel surface was almost unchanged after 605 hours, and it is expected that the channel erosion almost stopped when zero-erosion since situation similar to magnetic shielding was established. It is expected thrust efficiency and thrust converged to equilibrium values after 605 hours because the status of channel surface erosion and contamination didn’t change significantly but was almost the same as far as discharge parameters was concerned.

As was discussed, the major parameters of the Hall thruster gradually converged to values at around 500 to 600 hours, but parameters related to cathode discharge did not change so much from the beginning of the test to 1,012 hours. This suggests

cathode discharge and cathode to thruster coupling did not change and no drastic change happened as far as the cathode geometry and contamination were concerned. As explained in Fig.5(b), the thickness of the keeper electrode did not change but the keeper end plate was slightly contaminated by the beam target material, Titanium. Contamination was more clearly seen on the keeper orifice and the radius of the keeper orifice shrank due to contamination. By a witness plate measurement, Titanium’s deposition rate was 8 μm/khr. It is expected that contamination by Titanium affected the erosion rate of the keeper end plate. It should be noted that backsputtering by the beam target material is specific to ground testing, but it would not happen in space. Zero erosion rate of the keeper end plate in the ground testing hence could underestimate the erosion rate of the keeper end plate. Careful evaluation is required to estimate the erosion rate of the keeper end plate in space.

Lastly, wear of the cathode tube is discussed. The erosion rate of the orifice was found to be very low; only 5 μm change was found from the beginning to 605 hours; in contrast, from 605 to 1,012 hours, no appreciable erosion was found. Since the original orifice diameter was 2 mm, the 5-μm change will not affect orifice’s function to keep the inner pressure inside the cathode tube constant. From these results, it can be said that low erosion operation of the cathode was successfully demonstrated, and stable and unchanged cathode performance throughout the experiment was confirmed. A potential issue was however found for the LaB6 insert. The insert surface was seen to be recessed at a rate of 60 μm/khr due to evaporation. Based on the evaporation rate,47) the temperature of the insert is estimated to be 1,560 °C or higher. If a 1-mm thick insert or a thicker insert is used and the evaporation rate doesn’t change, 20-khr operation is possible, and hence, the life is limited but enough for all-electric propulsion satellite that requires accumulated operation of 10 khr in total.

Fig. 5. Summary of erosion and contamination profiles of the Hall thruster.

Anode

KeeperEnd Plate(almost nochange)LaB6

~ –60 µm/khr

Cathode

Channel

(b) Cathode Erosions

Outer Channel Wall

Inner Channel Wall

(c) Thruster Body Contaminations

0 hr605 hr

1012 hr

Ti BacksputterfromBeam Target+8 µm/khr

–2.3 mm/khr

KeeperOrifice+5 µm/khr

Pole Piece Cover

Black Surface(Ti Contaminated)

Contaminatedby BN and Ti

BN Backsputterfrom Channel

No appreciablechange

Pole Piece CoverContaminatedby Ti

White Surface(BN is eroding)

Orifice–5 µm/khr

(a) Channel Erosions

(d) Thruster Outlook after 1,012-hours Operation

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4. Conclusion The 1,000-hours preliminary life test revealed low-erosion and stable operation of the 6-kW Hall thruster. The erosion rate of the channel was high only at the beginning of the test and the erosion rate of the cathode materials stayed around 10 μm/khr or less from the measurements at 0, 602, and 1,012 hours even when subtracting titanium back sputtering effect. The largest erosion rate was 60 μm/khr for the cathode insert, so the insert material will restrict the lifetime of the thruster during the operation of mass flow rates of 18 mg/s for the anode and 1.9 mg/s for the cathode at a discharge voltage of 300 V. The change of performance form 0 hr to 1,012 hr was only 2.3% in thrust, so it is concluded that low-erosion and low-degradation features of the new Hall thruster was demonstrated. For a flight demonstration of the thruster onboard ETS-9, next steps are to conduct full-duration demonstration of thruster’s lifetime and to characterize how throttling operation effects the lifetime. Acknowledgments This study is supported by the Space Technology One Directorate and the Research and Development Directorate in Japan Aerospace Exploration Agency. The authors appreciate contributions from staffs at Japan Aerospace Exploration Agency (Dr. Daisuke Goto, Dr. Takahiro Yabe, Dr. Kiyoshi Kinefuchi, Dr. Yasuyoshi Hisamoto), staffs at IHI Aerospace (Dr. Shigeyasu Iihara, Mr. Yuya Hirano Mr. Daisuke Fukushima, Mr. Suisei Yamagishi, and Ms. Yukiko Yamaura), and staffs at IHI (Dr. Kazuo Uematsu and Mr. Gen Ito).

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