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Accelerated erosion of keeper electrode during coupling discharge between Hall thruster and hollow cathode
Vacuum 172 (2020) 109040
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Accelerated erosion of keeper electrode during coupling discharge between Hall thruster and hollow cathode Tianhang Meng a, Caixia Qiao b, c, Yanan Wang b, c, Zhongxi Ning a, *, Daren Yu a a
School of Energy Science and Engineering, Harbin Institute of Technology, China Shanghai Institute of Space Propulsion, China c Shanghai Engineering Research Center of Space Engine, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Hall thruster Hollow cathode Keeper Ion sputtering Lifetime
In this paper, keeper erosion of hollow cathode was found to be significantly accelerated due to the presence of divergent plume from Hall thruster. The results retrieved from multiple Retarding Potential Analyzers (RPAs) located at different positions near the exit of an HET-80 Hall thruster show that, aside from the 50–100eV ions produced near cathode exit due to ionization and associated instabilities, there exist 200–300eV incoming ion fluxes from thruster exit due to plume divergence, which will increase the equivalent sputtering yield of keeper material by 2.5–6.7 times and significantly shorten keeper lifetime. The result revealed a potential conflict be tween performance and lifetime when optimizing cathode location onboard Hall thrusters.
1. Introduction Hall thruster is a plasma propulsion device that features simple structures but complicated physics. Inside the thruster, two plasmas: the plasma in acceleration channel and the one produced by the hollow cathode, are coupled by a layer of closed-drifting E � B electrons and strong parasitic electric field [1]. As a result, cathode parameters, including location and orientation, impact overall discharge behavior non-negligibly [2–5]. Despite lack of knowledge of the physics underlying thruster-cathode coupling, a few optimization directions have appeared on the applica tion side. Jameson [6] noticed that the coupling voltage is the most responsive parameter when adjusting cathode location, and could all most account for the changes in total efficiency. Sommerville [7,8] successfully reduced the coupling voltage by extending magnetic sepa ratrix beyond the cathode location. Walker [9,10] discovered that the coupling voltage, as well as the oscillations in the near field, are closely correlated with the electrons captured by the flanking magnetic field. Meng [11,12] further quantitatively linked the configuration of the magnetically-captured electrons with the coupling voltage and plume divergence angle and proposed the cathode axis being aligned with local magnetic field [13]. Plume focusing [14] and ignition [15] were also improved. Guided by these findings, cathode location and orientation were
optimized on an HET-80 Hall thruster [16]. The keeper orifice was embedded into the same plane with the surface of outer magnetic pole and cathode axis aligned with local magnetic field, tilting 30� from thruster axis. These tunings improved thruster efficiency, especially discharge stability. Despite gains in performance, however, unexpectedly fast erosion was observed on the keeper plate, which was quite similar with that observed in earlier tests on SPT-100 [17,18]. The sputtering rate was then estimated to be risky for fulfilling the mission request. In search for the reason, diagnostics were conducted on the ion environment that surrounded the cathode during coupling discharge. The adopted thruster and diagnostics will be introduced in Section 2. Results will be in Section 3, based on which the influence of thruster plume on keeper sputtering will be discussed in Section 4. Several suggestions regarding design and tests will be given in the Conclusion section. 2. Experiment setup The experiment was conducted in the Ф4m � 8 m vacuum facility (VF-6) in Shanghai Institute of Space Propulsion, with eight Ф1m cryogenic pumps providing an ultimate vacuum of 1 � 10 5Pa. The adopted thruster was a HET-80 Hall thruster with an externally-located BaO hollow cathode [16]. The discharge voltage was 310 V and the peak magnetic field strength on channel centerline was 160Gs. During the
* Corresponding author. E-mail address: [email protected] (Z. Ning). https://doi.org/10.1016/j.vacuum.2019.109040 Received 1 September 2019; Received in revised form 25 October 2019; Accepted 25 October 2019 Available online 30 October 2019 0042-207X/© 2019 Elsevier Ltd. All rights reserved.
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Vacuum 172 (2020) 109040
Fig. 1. Illustration of shielding grids in RPA.
Fig. 3. Xeþ-Mo sputtering yield.
Fig. 2. Locations and orientations of RPAs.
discharge, 4.12 mg/s Xenon was fed through the anode and 0.31 mg/s through the cathode maintaining 3.4 A current, meanwhile the vacuum backpressure was 1.4 � 10 3Pa. Ion Energy Distribution Functions (IEDF) was retrieved by Retarding Potential Analyzer (RPA). The RPA had a Ф8mm opening and 30 mm deep tunnel, which was intersected by 4 layer of 500 mesh Molybdenum grids, as shown in Fig. 1. From exterior to interior are the floating grid, the electron-shielding grid that was biased 24 V, the ion-retarding grid that was swept from 0 V to 300 V and the 24 V grid that shields counter-flowing secondary electrons due to ion impingement on the collector. The collector was connect to the negative electrode of cathode, which was kept floating and taken as the reference potential. Because sputtering yield is relevant to both incident energy εi and incident angle θi , 6 RPAs were installed on different locations near thruster exit and given different orientations to distinguish the origins of the incoming fluxes, as shown in Fig. 2. The three RPAs on the cathode side are:
Fig. 4. Xeþ-Ta sputtering yield.
Because RPA-b was 30 mm separated away from the cathode axis, its results were merely an approximation of that on keeper exit. To reduce the difference, RPA-b was placed in the azimuthally downstream of E � B drift, so that the electrons within its view angle would partly preserve the information from keeper exit, therefore the IEDFs would also be similar. RPA-c,d,e and RPA-b,a,f were mirrored with respect to the vertical symmetric intersection of thruster. By comparing the IEDFs from RPA–c,d,e and RPA–b,a,f, the origin of a specific part on the IEDF could be identified: cathode or thruster plume. Direct measurement of differential sputtering yield was not available due to lack of apparatus, instead Stopping and Range of Ions in Matter (SRIM) [19] Software based on Monte Carlo methods was used to simulate sputtering yields Yi under different incident energies εi and incident angles θi . The investigated ðεi ; θi Þ combination covers 5–600eV spaced by 50eV and 0–89.9� spaced by 10� . In each ðεi ; θi Þ combination 6 � 104 Xenon particles were released to get statistics. The results are shown in Fig. 3 (Xeþ-Mo) and Fig. 4 (Xeþ-Ta). Comparison was made between several (Xeþ-Mo) data points in simulated results of (50–300eV, 0� ) incidence and a few collected experiment values [20–23]. As shown in Fig. 5, simulation agreed well with experiment values on lower ion energies (<80eV), but differed greatly on higher ion energies (100–300eV), in latter case the simulation could underestimate by 60%. Therefore, following discussion on
a) perpendicular to cathode axis and pointing towards the thruster axis, measuring IEDFs of tangentially incident ions towards keeper orifice plate, its view angles covered 7.36 mm downstream of nearest channel exit, which corresponds to 68� ~75� plume divergence half angle; b) parallel to cathode axis and pointing towards thruster, measuring IEDFs of normally incident ions towards keeper orifice plate; f) parallel to thruster axis and pointing towards infinitely remote downstream, measuring IEDFs of back-flowing ions from down stream plume. 2
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Vacuum 172 (2020) 109040
4. Discussion 4.1. Extent of erosion acceleration Lifetime prediction encourages quantitative check on the influence of ion anisotropy on sputtering. The main problem is expanding 1-D IEDF(εi ) into 2-D IEDF(et, en), in which et represents tangential inci dent energy and en normal incident energy. Fortunately, RPA-a and RPAb were perpendicular to each other, so they have approximately given IEDF(et) and IEDF(en), based on which the IEDF(et, en) can be recon structed on certain terms. Assuming that IEDF(et, en) can be projected tangentially and nor mally with respect to a surface into to l peaks { et1,et2,et3,… …,etl} and m peaks {en1,en2,en3,… …,enm}. To reconstruct IEDF(et, en) from the two sets is to find matrix Al�m such that: � Al�m ⋅1m�1 ¼ ft;l�1 ft ¼ IEDFT ðet Þ; known � (2) A’m�l ⋅1l�1 ¼ fn;m�1 fn ¼ IEDFT ðen Þ; known
Fig. 5. Simulated vs. experiment sputtering yields.
energetic ions’ contribution should be regarded as a conservative esti mation, the contribution in reality can be greater.
Eq. (2) can be rewritten into: Al�m ⋅ 1m�1 ⋅11�l ⋅Al�m ¼ ft;l�1 ⋅f ’n;1�m
3. Results
(3)
which is:
The ion environment surrounding the cathode was asymmetric, ac cording to the IEDFs retrieved from different locations in Fig. 6. Two ion groups could be observed on the IEDF on RPA-a: a 5–100eV low energy group and a high energy 200–300eV group. The energy range of the low energy group coincided with that observed in cathode standalone discharge [24–26]. Its presence in IEDFs on RPA-b and RPA-c and absence on RPA-d indicate that this low energy group probably origi nated from the virtual cathode extending from the cathode exit [11,12]. Ionization and ion acoustic modes [27,28] are currently believed to be producing and heating these ions. Comparison between RPA-a, RPA-d and the rest ones show that high energy group was detectable only when facing the thruster exit, indi cating that the group was mainly from thruster exit due to plume divergence. Note that RPA-a and RPA-d were located in 68–75� view angle from thruster axis, the 250–270eV energy peak of the group was difficult to explain, since the space potential on exit of a Hall thruster is usually less than 200 V (except for the magnetic shielding types). Judging from the continuum distribution of the IEDF, we suspect that the breathing mode [29] and the resultant broadening of ionization zone [30] might be one of the causes. Another facilitating factor might be nonlinear acceleration from thruster exit to cathode location due to ion transit instability [31], similar with that reported in Ref. [32]. However, this awaits verification. The unexpectedly detected high energy group means that in the coupling discharge, the observed keeper erosion was more correlated with anisotropy rather than density of incident ions. Using the IEDF on RPA-a, the density of high energy group is estimated by: rffiffiffiffi � Ih εl nh ¼ nl (1) � 3 � 1015 m3 Il εh
Al�m ⋅ 1m�l ⋅Al�m ¼ ftn;l�m
(4)
Note that the left side is highly nonlinear for iteration methods to converge. Also because the rank l of the matrix is less than number of unknowns, there can be no solution. However, we found that when l ¼ 1, m�2 or l�2, m ¼ 1, there is a fixed solution. In other words, when IEDF (et) and IEDF(en) form a combination of ‘one peak plus two peaks’, IEDF (et, en) can be approximately reconstructed by IEDF(et, en) �IEDF(et) ⋅ IEDFT (en). Here we introduced an underlying assumption that the broadening of each peak obeys normal distribution (IEDF from RPA-b), so that the effective information in a long set of IEDF can be shrank into a few numbers. Validity of this assumption relies heavily on the linearity of the physics effects behind each peak. Nonlinearity might distort normal distribution from the first place, in which case l or m value needs to be increased. Fig. 7 shows the reconstructed IEDF(et,en) using IEDF(et) and IEDF (en) from RPA-a and RPA-b in Fig. 6. The tangential high energy group can be seen on the up-left corner. To exclude the impingement of thruster plume and investigate solely into the influence of cathode standalone discharge, another IEDF(et,en) is reconstructed using only IEDF(en) from RPA-b, which is shown in Fig. 8. The tangential incident energy is limited under 50eV and total energy below 100eV. Because the transparency coefficient of RPA was unreliable, the actual ion flux density was unknown. Therefore, we can only give an estimation on sputtering yield, instead of sputtering rate. The sputtering yield Yi is weight-averaged over IEDF(et, en): Xk Yi ðεi ; θi Þf ðεi ; θi Þ i¼0 Yi ¼ Xk f ðεi ; θi Þ i¼0
Here n is density (nl � 1 � 1016 =m3 ), I peak value on IEDF, ε peak en ergy, while subscript l and h denote low and high energy, respectively. Although the high energy group was one magnitude lower in density than cathode plume, it contributed significantly different keeper erosion from cathode standalone discharge. In such cases, cathode location and orientation can be more important than its operation parameters on determining cathode lifetime.
εi ¼ et þ en
(5)
� � et θi ¼ tan 1 en in which k represents the number of data points that participated the averaging, in this paper k ¼ 4.62 � 106, εi total energy of incident ions, θi incident angle. Table 1 compares the equivalent sputtering yield with and without tangential high energy group. Obviously, thruster plume accelerated the sputtering by several times. There are two main reasons. Firstly, the higher energy group can transition the sputtering from single knock-ons 3
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Vacuum 172 (2020) 109040
Fig. 6. IEDFs on all locations (RPA-a is nearest to cathode).
to linear collision cascades (LCC), or even to thermal spikes [33]. Sec ondly, the incident angle of the high energy group happened to be within the optimum angle (60–80� ), of which the sputtering yield is usually magnitudes higher than that of normal incidence. Given the erosion-acceleration coefficient in Table 1, the keeper
lifetime in coupling discharge might be only 14.9% (Mo) to 40% (Ta) of that in standalone discharge. In this case, the impingement of thruster plume will be the limiting factor of cathode lifetime, instead of the en ergetic ions [24–26] produced by cathode itself.
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Fig. 9. IEDFs retrieved on RPA-a and RPA-b on a 5-kW magnetic-focusing-type Hall thruster.
observations with a centrally-mounted cathode. The result also re sembles some phenomena observed on SPT-100 [17,18]. With the popularization of magnetic shielding configurations and resultant out ward relocation of peak magnetic field, similar accelerated erosion might become a trending problem. One costly but fundamental resolution is to reduce plume diver gence. To illustrate effect of plume focusing, additional experiment was conducted in Harbin Institute of Technology, where some of the coauthors are affiliated to. The adopted thruster was a 5-kW permanent magnet Hall thruster. Its discharge voltage was 300 V and discharge current 15 A. Its peak magnetic field was 2 mm downstream the pole surface. After ignition, it was tuned into magnetic focusing mode. The cathode was hidden 4 mm axially behind the plane of pole sur face, with 10 mm radial distance towards the edge of outer magnetic pole. The cathode axis was parallel with the magnetic field at cathode orifice, and also approximately parallel with thruster axis. Only two RPAs were used. RPA-a was installed perpendicular to the thruster axis. Its location was 33 mm radially away from edge of outer magnetic pole, and 60� azimuthally away from cathode location. Its collecting orifice was 4 mm axially downstream the pole surface, so its function was to measure the radial IEDFs from thruster plume. Another RPA-b was located beside the cathode, hiding 16 mm axially behind the pole surface, with a 15 mm radial distance toward the outer magnetic pole. Its axis was also parallel with thruster axis. Its function was to measure IEDFs inside the aurora surrounding the outer magnetic poles. Fig. 9 shows the retrieved IEDFs. As can be seen, IEDF on RPA-a showed one-peak distribution instead of two-peaks in Fig. 6. Although a few details still needed investigation (e. g. the energetic tail extended to ~400eV, which is even higher than discharge voltage), it can be inferred that without the higher energy peak, the cathode lifetime can be much longer.
Fig. 7. Reconstructed IEDF(et,en) using IEDF(et) and IEDF(en) from RPA-a and RPA-b in Fig. 6 considering ion anisotropy.
Fig. 8. Reconstructed IEDF(et, en) using only IEDF(en) from RPA-b in Fig. 6, assuming isotropy. Table 1 Equivalent sputtering yield of keeper material.
Molybdenum Tantalum
Equivalent sputtering yield (with high energy group)
Equivalent sputtering yield (without high energy group)
Acceleration coefficient
0.4139 0.4136
0.0616 0.1654
6.719156 2.500605
4.3. Implications for cathodes Modifying cathode location is also expected to ease the problem, however the location choices are usually limited. Ref. [6–13] have shrank the alternative locations for installing cathode to a relatively small area near the outer magnetic pole, of which the magnetic field strength is greater than a critical value so that cathode discharge is more stabilized. Meanwhile the keeper electrode should be separated at a distance from outer magnetic pole to avoid sparking. These consider ations can limit the options to less than 1 cm2, which is too small. Dis obeying these principles might lead to enhanced discharge instability [36] and some performance loss. Increasing cathode mass flow rate, which is usually effective in
4.2. Effect of plume focusing Despite scarceness of similar reports, we estimate that this is a widely spread problem, rather than an individual case. To the authors’ knowledge, the majority of Hall thrusters, whether under R&Ds or already in service, have quite divergent plumes. Actually, our result agrees with some previous original works on HERMeS Thruster [34,35], including numerical predictions for similar effect and experimental
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Fig. 10. Influence of cathode flow rate on IEDF on RPA-a.
stabilizing discharge and reducing energetic ions in cathode standalone discharge [37], also proved doubtful in easing the reported problem in coupling cases. Fig. 10 shows the influence of increasing cathode flow rate on IEDFs on RPA-a in the added experiment. The increased flow rate had little effect on the 200–300eV range, while the total efficiency was decreased due to higher propellant consumption. Although the 20–100eV range was reduced in density, the peak energy was actually continuously increased from 50eV to 70eV with the flow rate. The overall sputtering rate then increased, which was opposite of decreasing in most cathode standalone discharges. Therefore there was absolutely not any improvement in the end. As a result, ensuring the sputtering life of keeper through design should be the priority direction. Some existing methods include adopt ing sputter-resistant materials such as graphite, thickening the keeper or installing extra protective shield [35]. Consciousness of leaving margin for coupling effects is also necessary during design procedure, which is hardly present in current standalone discharge scenario. On the other hand, the duration of standalone life test should be mission duration multiplied by an acceleration coefficient, the latter of which should be determined experimentally given specific thruster type and cathode location. Taking the case in this paper for instance, the mission duration is 8000 h, then the cathode standalone life test should last for 53800 h (Mo) or 20000 h (Ta), instead of the original 8000 h. The logics behind the elongated test for all cathode components rather than keeper alone is related to strategy changes after keeper failure. Without the keeper, the thruster cannot re-ignite so it will have to remain in discharge ever since. At the same time, the thruster on the opposite side of the satellite will also have to remain in discharge so that the net thrust is canceled. Note that these are happening when inner components of cathode are exposed to thruster plume, so the endurance of cathode will deplete in a quicker way. For another instance, cathode lifetime was demonstrated at 23000 h in standalone test. Then in coupling discharge, the keeper orifice plate could only withstand 23000 � (0.14 þ 0.4)/2 ¼ 6210 h. The mission plan was 8000 h, so the left 1800 h had to be extended to 1800 � 9 ¼ 16200 h nonstop, supposing the thruster was designed to operate for 1 h every other 8 h. So the final duration was 6210 þ 16200 ¼ 22410 h, which was 2.8 times of original 8000 h. Considering the erosion accel eration of the inner parts in thruster plume, it is not impossible that the actual equivalent duration can be even more times of original plan.
5. Conclusion After optimizing cathode location onboard an HET-80 Hall thruster to improve efficiency and discharge stability, keeper erosion was observed to be faster than acceptable. To find out the reason, this paper utilized multiple RPAs to diagnose the ion environment that the cathode was immersed in during thruster-cathode coupling discharge and investigated the impact of high-energy ions on keeper sputtering. Retrieved IEDFs show that aside from an quasi-isotropic 50–100eV group that resembled that in cathode standalone discharge, there existed another 200–300eV group that was tangentially incident on keeper orifice plate. The latter group was identified as the thruster plume in large divergent angle, however its energy was higher than normal. The reason is currently unclear, yet suspected to be the breathing mode as well as ion transit instability. A coarse estimation shows that the existence of the high energy group will increase the equivalent sputtering yield of keeper material by 2.5–6.7 times, thus significantly shortening keeper lifetime. Therefore, cathode location and orientation are neglected, however sometimes determinant factors of cathode lifetime, and cathode lifetime was a neglected factor of thruster-cathode coupling. Some well-known research directions would help balance between performance and lifetime when determining cathode location, including decreasing plume divergence angle, enhancing the sputter-resistant ca pabilities of the keeper and modifying the topology of exterior magnetic field. Moreover, we suggest that the duration of cathode standalone life test should be the mission duration multiplied by an acceleration coef ficient, of which the value should be experimentally determined given specific thruster type, operation parameter and cathode location. Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, ‘Accelerated Erosion of keeper electrode during coupling discharge between Hall thruster and hollow cathode’, authored by Tianhang Meng, Caixia Qiao, Yanan Wang, Zhongxi Ning and Daren Yu. 6
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Acknowledgements
[17] C.E. Garner, J.E. Polk, L.C. Pless, K.D. Goodfellow, John R. Brophy, Performance evaluation and life testing of the SPT-100, Int. Electric Propul. Conf. (1993). Paper No. IEPC-93-091, https://erps.spacegrant.org/uploads/images/images/iepc_art icledownload_1988-2007/1993index/IEPC1993-091.pdf. [18] B.A. Arhipov, et al., The results of 7000-hour SPT-100 life testing, Int. Electric Propul. Conf (1995). Paper No. IEPC-95-039, http://www.erps.spacegrant.org/upl oads/images/images/iepc_articledownload_1988-2007/1995index/IEPC1995-39. pdf. [19] J.F. Ziegler, Stopping and Range of Ions in Matter (SRIM) Software Ver. http://www.srim.org, 2013. [20] R.P. Doerner, D.G. Whyte, D.M. Goebel, Sputtering yield measurements during low energy xenon plasma bombardment, J. Appl. Phys. 93 (2003) 5816–5823, https:// doi.org/10.1063/1.1566474. [21] R.D. Kolasinski, J.E. Polk, D. Goebel, L.K. Johnson, Sputtering yield measurements at glancing incidence using a quartz crystal microbalance, J. Vac. Sci. Technol. A 25 (2007) 236–245, https://doi.org/10.1116/1.2435375. [22] C.H. Weijsenfeld, A. Hoogendoorn, M. Koedam, Sputtering of polycrystalline metals by inert gas ions of low energy (100–1000 eV), Physica 27 (1961) 763–764, https://doi.org/10.1016/0031-8914(61)90093-3. [23] J.J. Blandino, D.G. Goodwin, C.E. Garner, Low energy sputter yields for diamond, carbon–carbon composite, and molybdenum subject to xenon ion bombardment, Diam. Relat. Mater. 9 (2000) 1992–2001, https://doi.org/10.1016/S0925-9635 (00)00350-2. [24] D.M. Goebel, K.K. Jameson, I. Katz, I.G. Mikellides, Potential fluctuations and energetic ion production in hollow cathode discharges, Phys. Plasmas 14 (2007) 103508, https://doi.org/10.1063/1.2784460. [25] Q. Yu, et al., The analysis of high amplitude of potential oscillations near the hollow cathode of ion thruster, Acta Astronaut. 134 (2017) 265–277, https://doi. org/10.1016/j.actaastro.2017.02.012. [26] C.C. Farnell, J.D. Williams, C.C. Farnell, Comparison of hollow cathode discharge plasma configurations, Plasma Sources Sci. Technol. 20 (2011), 025006, https:// doi.org/10.1088/0963-0252/20/2/025006. [27] B.A. Jorns, I.G. Mikellides, D.M. Goebel, Ion acoustic turbulence in a 100-A LaB6 hollow cathode, Phys. Rev. E 90 (2014), 063106, https://doi.org/10.1103/ PhysRevE.90.063106. [28] B.A. Jorns, C. Dodson, D.M. Goebel, R. Wirz, Propagation of ion acoustic wave energy in the plume of a high-current LaB6 hollow cathode, Phys. Rev. E 96 (2017), 023208, https://doi.org/10.1103/PhysRevE.96.023208. [29] E. Choueiri, Plasma oscillations in Hall thrusters, Phys. Plasmas 8 (2001) 1411–1426, https://doi.org/10.1063/1.1354644. [30] Z. Ning, H. Liu, D. Yu, Z. Zhou, Effects of ionization distribution on plasma beam focusing characteristics in Hall thrusters, Appl. Phys. Lett. 99 (2011) 221502, https://doi.org/10.1063/1.3665632. [31] S. Barral, K. Makowski, Z. Peradzynski, M. Dudeck, Transit-time instability in Hall thrusters, Phys. Plasmas 12 (2005), 073504, https://doi.org/10.1063/1.1947796. [32] Y. Lim, et al., Nonlinear ion dynamics in Hall thruster plasma source by ion transittime instability, Plasma Sources Sci. Technol. 26 (2017), 03LT01, https://doi.org/ 10.1088/1361-6595/aa58d9. [33] E. Bringa, Ion Tracks in Solids: Sputtering and Surface Modification, Ph.D Dissertation University of Virginia, 2000, p. 82~83. [34] A. Ortega, I.G. Mikellides, D.M. Goebel, Numerical simulations for life assessments of the BaO and LaB6 cathode options in the Hall effect rocket with magnetic shielding (HERMeS), 35th Int. Electric Propul. Conf. (2017). Paper No. IEPC-2017152, https://iepc2017.org/sites/default/files/speaker-papers/iepc-2017-152_final. pdf. [35] G. Williams, J.H. Gilland, H. Kamhawi, M. Choi, P.Y. Peterson, D. Herman, Wear trends of the HERMeS Thruster as a function of throttle point, 35th Int. Electric Propul. Conf. (2017). Paper No. IEPC-2017-207, https://iepc2017.org/sites/defa ult/files/speaker-papers/2017_iepc_rev6.pdf. [36] T. Meng, Z. Ning, D. Yu, Triggering of ionization oscillations in hollow cathode discharge by keeper electrode, Phys. Plasmas 26 (2019), 093510, https://doi.org/ 10.1063/1.5116830. [37] E. Chu, D.M. Goebel, R.E. Wirz, Reduction of energetic ion production in hollow cathodes by external gas injection, J. Propuls. Power 29 (2013) 1155–1163.
This work was funded by Science and Technology Commission of Shanghai Municipality under Grant. No. 17DZ2280800. References [1] A.I. Morozov, V.V. Savelyev, Fundamentals of stationary plasma thruster theory, in: B.B. Kadomtsev, V.D. Shafranov (Eds.), Rev. Plasma Phys., vol. 21, Springer Inc., Boston, 2000, p. 203. [2] D. Tilley, K. de Grys, R. Myers, Hall thruster-cathode coupling, 35th joint propulsion conf. & exhibit, Paper No. AIAA 1999-2865, https://doi.org/10.2514/ 6.1999-2865, 1999. [3] M.S. McDonald, A.D. Gallimore, Cathode position and orientation effects on cathode coupling in a 6-kw Hall thruster, 31st Int, in: Electric Propulsion Conf, 2019. Paper No. IEPC-09-113, http://erps.spacegrant.org/uploads/images/images /iepc_articledownload_1988-2007/2009index/IEPC-2009-113.pdf. [4] D.M. Goebel, K.K. Jameson, R.R. Hofer, Hall thruster cathode flow impact on coupling voltage and cathode life, J. Propuls. Power 28 (2012) 355–363, https:// doi.org/10.2514/1.B34275. [5] T. Miyasaka, et al., Influences of cathode position on operations and thrust performance of side by side Hall thruster system, Vacuum 167 (2019) 524–529, https://doi.org/10.1016/j.vacuum.2018.12.056. [6] K.K. Jameson, Investigations on Hollow Cathode Effects on Total Thruster Efficiency of a 6-kW Hall Thruster, PhD Thesis University of California, Los Angeles, 2009. [7] J.D. Sommerville, L.B. King, Hall-effect thruster–cathode coupling, part I: efficiency improvements from an extended outer pole, J. Propuls. Power 27 (2011) 744–753, https://doi.org/10.2514/1.50123. [8] J.D. Sommerville, L.B. King, Hall-effect thruster–cathode coupling, part II: ion beam and near-field plume, J. Propuls. Power 27 (2011) 754–767, https://doi.org/ 10.2514/1.50124. [9] J.A. Walker, J.D. Frieman, M.L.R. Walker, V. Khayms, D. King, P. Peterson, Electrical facility effects on Hall-effect-thruster cathode coupling: discharge oscillations and facility coupling, J. Propuls. Power 32 (2016) 844–855, https:// doi.org/10.2514/1.B35835. [10] J.A. Walker, S.J. Langendorf, M.L.R. Walker, V. Khayms, D. King, P. Peterson, Electrical facility effects on Hall current thrusters: electron termination pathway manipulation, J. Propuls. Power 32 (2016) 1365–1377, https://doi.org/10.2514/ 1.B35904. [11] T. Meng, Z. Ning, S. Eliseev, D. Yu, A. Kudryavtsev, Simulation of electron streamline distribution and coupling voltage in the coupling area of a Hall thruster, Plasma Sources Sci. Technol. 28 (2019), 035016, https://doi.org/10.1088/13616595/ab019b. [12] D. Yu, T. Meng, Z. Ning, H. Liu, Confinement effect of cylindrical-separatrix type magnetic field on the plume of magnetic focusing type Hall thruster, Plasma Sources Sci. Technol. 26 (2017), 04LT02, https://doi.org/10.1088/1361-6595/ aa6425. [13] T. Meng, Z. Ning, D. Yu, Influence of background magnetic field on hollow cathode discharge characteristics, 34th Int. Electric Propulsion Conf. (2015). Paper No. IEPC-2015-77, https://spacegrant.org/uploads/images/2015Presentations/IE PC-2015-77_ISTS-2015-b-77.pdf. [14] Y. Ding, et al., Effect of relative position between cathode and magnetic separatrix on the discharge characteristic of Hall thrusters, Vacuum 154 (2018) 167–173, https://doi.org/10.1016/j.vacuum.2018.05.005. [15] W. Li, et al., Study on the influence of change in axial position of cathode on the ignition process of a Hall thruster, Vacuum 167 (2019) 234–243, https://doi.org/ 10.1016/j.vacuum.2019.06.007. [16] X. Lu, X. Wang, H. Tang, Z. Zhang, X. Kang, Beam characteristics of HET-80 Hall thruster with 700Watts, J. Propuls. Technol. 39 (2018) 1426–1433 (In Chinese), http://mall.cnki.net/magazine/Article/TJJS201806028.htm.
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Accelerated erosion of keeper electrode during coupling discharge between Hall thruster and hollow cathode Tianhang Meng a, Caixia Qiao b, c, Yanan Wang b, c, Zhongxi Ning a, *, Daren Yu a a
School of Energy Science and Engineering, Harbin Institute of Technology, China Shanghai Institute of Space Propulsion, China c Shanghai Engineering Research Center of Space Engine, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Hall thruster Hollow cathode Keeper Ion sputtering Lifetime
In this paper, keeper erosion of hollow cathode was found to be significantly accelerated due to the presence of divergent plume from Hall thruster. The results retrieved from multiple Retarding Potential Analyzers (RPAs) located at different positions near the exit of an HET-80 Hall thruster show that, aside from the 50–100eV ions produced near cathode exit due to ionization and associated instabilities, there exist 200–300eV incoming ion fluxes from thruster exit due to plume divergence, which will increase the equivalent sputtering yield of keeper material by 2.5–6.7 times and significantly shorten keeper lifetime. The result revealed a potential conflict be tween performance and lifetime when optimizing cathode location onboard Hall thrusters.
1. Introduction Hall thruster is a plasma propulsion device that features simple structures but complicated physics. Inside the thruster, two plasmas: the plasma in acceleration channel and the one produced by the hollow cathode, are coupled by a layer of closed-drifting E � B electrons and strong parasitic electric field [1]. As a result, cathode parameters, including location and orientation, impact overall discharge behavior non-negligibly [2–5]. Despite lack of knowledge of the physics underlying thruster-cathode coupling, a few optimization directions have appeared on the applica tion side. Jameson [6] noticed that the coupling voltage is the most responsive parameter when adjusting cathode location, and could all most account for the changes in total efficiency. Sommerville [7,8] successfully reduced the coupling voltage by extending magnetic sepa ratrix beyond the cathode location. Walker [9,10] discovered that the coupling voltage, as well as the oscillations in the near field, are closely correlated with the electrons captured by the flanking magnetic field. Meng [11,12] further quantitatively linked the configuration of the magnetically-captured electrons with the coupling voltage and plume divergence angle and proposed the cathode axis being aligned with local magnetic field [13]. Plume focusing [14] and ignition [15] were also improved. Guided by these findings, cathode location and orientation were
optimized on an HET-80 Hall thruster [16]. The keeper orifice was embedded into the same plane with the surface of outer magnetic pole and cathode axis aligned with local magnetic field, tilting 30� from thruster axis. These tunings improved thruster efficiency, especially discharge stability. Despite gains in performance, however, unexpectedly fast erosion was observed on the keeper plate, which was quite similar with that observed in earlier tests on SPT-100 [17,18]. The sputtering rate was then estimated to be risky for fulfilling the mission request. In search for the reason, diagnostics were conducted on the ion environment that surrounded the cathode during coupling discharge. The adopted thruster and diagnostics will be introduced in Section 2. Results will be in Section 3, based on which the influence of thruster plume on keeper sputtering will be discussed in Section 4. Several suggestions regarding design and tests will be given in the Conclusion section. 2. Experiment setup The experiment was conducted in the Ф4m � 8 m vacuum facility (VF-6) in Shanghai Institute of Space Propulsion, with eight Ф1m cryogenic pumps providing an ultimate vacuum of 1 � 10 5Pa. The adopted thruster was a HET-80 Hall thruster with an externally-located BaO hollow cathode [16]. The discharge voltage was 310 V and the peak magnetic field strength on channel centerline was 160Gs. During the
* Corresponding author. E-mail address: [email protected] (Z. Ning). https://doi.org/10.1016/j.vacuum.2019.109040 Received 1 September 2019; Received in revised form 25 October 2019; Accepted 25 October 2019 Available online 30 October 2019 0042-207X/© 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Illustration of shielding grids in RPA.
Fig. 3. Xeþ-Mo sputtering yield.
Fig. 2. Locations and orientations of RPAs.
discharge, 4.12 mg/s Xenon was fed through the anode and 0.31 mg/s through the cathode maintaining 3.4 A current, meanwhile the vacuum backpressure was 1.4 � 10 3Pa. Ion Energy Distribution Functions (IEDF) was retrieved by Retarding Potential Analyzer (RPA). The RPA had a Ф8mm opening and 30 mm deep tunnel, which was intersected by 4 layer of 500 mesh Molybdenum grids, as shown in Fig. 1. From exterior to interior are the floating grid, the electron-shielding grid that was biased 24 V, the ion-retarding grid that was swept from 0 V to 300 V and the 24 V grid that shields counter-flowing secondary electrons due to ion impingement on the collector. The collector was connect to the negative electrode of cathode, which was kept floating and taken as the reference potential. Because sputtering yield is relevant to both incident energy εi and incident angle θi , 6 RPAs were installed on different locations near thruster exit and given different orientations to distinguish the origins of the incoming fluxes, as shown in Fig. 2. The three RPAs on the cathode side are:
Fig. 4. Xeþ-Ta sputtering yield.
Because RPA-b was 30 mm separated away from the cathode axis, its results were merely an approximation of that on keeper exit. To reduce the difference, RPA-b was placed in the azimuthally downstream of E � B drift, so that the electrons within its view angle would partly preserve the information from keeper exit, therefore the IEDFs would also be similar. RPA-c,d,e and RPA-b,a,f were mirrored with respect to the vertical symmetric intersection of thruster. By comparing the IEDFs from RPA–c,d,e and RPA–b,a,f, the origin of a specific part on the IEDF could be identified: cathode or thruster plume. Direct measurement of differential sputtering yield was not available due to lack of apparatus, instead Stopping and Range of Ions in Matter (SRIM) [19] Software based on Monte Carlo methods was used to simulate sputtering yields Yi under different incident energies εi and incident angles θi . The investigated ðεi ; θi Þ combination covers 5–600eV spaced by 50eV and 0–89.9� spaced by 10� . In each ðεi ; θi Þ combination 6 � 104 Xenon particles were released to get statistics. The results are shown in Fig. 3 (Xeþ-Mo) and Fig. 4 (Xeþ-Ta). Comparison was made between several (Xeþ-Mo) data points in simulated results of (50–300eV, 0� ) incidence and a few collected experiment values [20–23]. As shown in Fig. 5, simulation agreed well with experiment values on lower ion energies (<80eV), but differed greatly on higher ion energies (100–300eV), in latter case the simulation could underestimate by 60%. Therefore, following discussion on
a) perpendicular to cathode axis and pointing towards the thruster axis, measuring IEDFs of tangentially incident ions towards keeper orifice plate, its view angles covered 7.36 mm downstream of nearest channel exit, which corresponds to 68� ~75� plume divergence half angle; b) parallel to cathode axis and pointing towards thruster, measuring IEDFs of normally incident ions towards keeper orifice plate; f) parallel to thruster axis and pointing towards infinitely remote downstream, measuring IEDFs of back-flowing ions from down stream plume. 2
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4. Discussion 4.1. Extent of erosion acceleration Lifetime prediction encourages quantitative check on the influence of ion anisotropy on sputtering. The main problem is expanding 1-D IEDF(εi ) into 2-D IEDF(et, en), in which et represents tangential inci dent energy and en normal incident energy. Fortunately, RPA-a and RPAb were perpendicular to each other, so they have approximately given IEDF(et) and IEDF(en), based on which the IEDF(et, en) can be recon structed on certain terms. Assuming that IEDF(et, en) can be projected tangentially and nor mally with respect to a surface into to l peaks { et1,et2,et3,… …,etl} and m peaks {en1,en2,en3,… …,enm}. To reconstruct IEDF(et, en) from the two sets is to find matrix Al�m such that: � Al�m ⋅1m�1 ¼ ft;l�1 ft ¼ IEDFT ðet Þ; known � (2) A’m�l ⋅1l�1 ¼ fn;m�1 fn ¼ IEDFT ðen Þ; known
Fig. 5. Simulated vs. experiment sputtering yields.
energetic ions’ contribution should be regarded as a conservative esti mation, the contribution in reality can be greater.
Eq. (2) can be rewritten into: Al�m ⋅ 1m�1 ⋅11�l ⋅Al�m ¼ ft;l�1 ⋅f ’n;1�m
3. Results
(3)
which is:
The ion environment surrounding the cathode was asymmetric, ac cording to the IEDFs retrieved from different locations in Fig. 6. Two ion groups could be observed on the IEDF on RPA-a: a 5–100eV low energy group and a high energy 200–300eV group. The energy range of the low energy group coincided with that observed in cathode standalone discharge [24–26]. Its presence in IEDFs on RPA-b and RPA-c and absence on RPA-d indicate that this low energy group probably origi nated from the virtual cathode extending from the cathode exit [11,12]. Ionization and ion acoustic modes [27,28] are currently believed to be producing and heating these ions. Comparison between RPA-a, RPA-d and the rest ones show that high energy group was detectable only when facing the thruster exit, indi cating that the group was mainly from thruster exit due to plume divergence. Note that RPA-a and RPA-d were located in 68–75� view angle from thruster axis, the 250–270eV energy peak of the group was difficult to explain, since the space potential on exit of a Hall thruster is usually less than 200 V (except for the magnetic shielding types). Judging from the continuum distribution of the IEDF, we suspect that the breathing mode [29] and the resultant broadening of ionization zone [30] might be one of the causes. Another facilitating factor might be nonlinear acceleration from thruster exit to cathode location due to ion transit instability [31], similar with that reported in Ref. [32]. However, this awaits verification. The unexpectedly detected high energy group means that in the coupling discharge, the observed keeper erosion was more correlated with anisotropy rather than density of incident ions. Using the IEDF on RPA-a, the density of high energy group is estimated by: rffiffiffiffi � Ih εl nh ¼ nl (1) � 3 � 1015 m3 Il εh
Al�m ⋅ 1m�l ⋅Al�m ¼ ftn;l�m
(4)
Note that the left side is highly nonlinear for iteration methods to converge. Also because the rank l of the matrix is less than number of unknowns, there can be no solution. However, we found that when l ¼ 1, m�2 or l�2, m ¼ 1, there is a fixed solution. In other words, when IEDF (et) and IEDF(en) form a combination of ‘one peak plus two peaks’, IEDF (et, en) can be approximately reconstructed by IEDF(et, en) �IEDF(et) ⋅ IEDFT (en). Here we introduced an underlying assumption that the broadening of each peak obeys normal distribution (IEDF from RPA-b), so that the effective information in a long set of IEDF can be shrank into a few numbers. Validity of this assumption relies heavily on the linearity of the physics effects behind each peak. Nonlinearity might distort normal distribution from the first place, in which case l or m value needs to be increased. Fig. 7 shows the reconstructed IEDF(et,en) using IEDF(et) and IEDF (en) from RPA-a and RPA-b in Fig. 6. The tangential high energy group can be seen on the up-left corner. To exclude the impingement of thruster plume and investigate solely into the influence of cathode standalone discharge, another IEDF(et,en) is reconstructed using only IEDF(en) from RPA-b, which is shown in Fig. 8. The tangential incident energy is limited under 50eV and total energy below 100eV. Because the transparency coefficient of RPA was unreliable, the actual ion flux density was unknown. Therefore, we can only give an estimation on sputtering yield, instead of sputtering rate. The sputtering yield Yi is weight-averaged over IEDF(et, en): Xk Yi ðεi ; θi Þf ðεi ; θi Þ i¼0 Yi ¼ Xk f ðεi ; θi Þ i¼0
Here n is density (nl � 1 � 1016 =m3 ), I peak value on IEDF, ε peak en ergy, while subscript l and h denote low and high energy, respectively. Although the high energy group was one magnitude lower in density than cathode plume, it contributed significantly different keeper erosion from cathode standalone discharge. In such cases, cathode location and orientation can be more important than its operation parameters on determining cathode lifetime.
εi ¼ et þ en
(5)
� � et θi ¼ tan 1 en in which k represents the number of data points that participated the averaging, in this paper k ¼ 4.62 � 106, εi total energy of incident ions, θi incident angle. Table 1 compares the equivalent sputtering yield with and without tangential high energy group. Obviously, thruster plume accelerated the sputtering by several times. There are two main reasons. Firstly, the higher energy group can transition the sputtering from single knock-ons 3
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Fig. 6. IEDFs on all locations (RPA-a is nearest to cathode).
to linear collision cascades (LCC), or even to thermal spikes [33]. Sec ondly, the incident angle of the high energy group happened to be within the optimum angle (60–80� ), of which the sputtering yield is usually magnitudes higher than that of normal incidence. Given the erosion-acceleration coefficient in Table 1, the keeper
lifetime in coupling discharge might be only 14.9% (Mo) to 40% (Ta) of that in standalone discharge. In this case, the impingement of thruster plume will be the limiting factor of cathode lifetime, instead of the en ergetic ions [24–26] produced by cathode itself.
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Fig. 9. IEDFs retrieved on RPA-a and RPA-b on a 5-kW magnetic-focusing-type Hall thruster.
observations with a centrally-mounted cathode. The result also re sembles some phenomena observed on SPT-100 [17,18]. With the popularization of magnetic shielding configurations and resultant out ward relocation of peak magnetic field, similar accelerated erosion might become a trending problem. One costly but fundamental resolution is to reduce plume diver gence. To illustrate effect of plume focusing, additional experiment was conducted in Harbin Institute of Technology, where some of the coauthors are affiliated to. The adopted thruster was a 5-kW permanent magnet Hall thruster. Its discharge voltage was 300 V and discharge current 15 A. Its peak magnetic field was 2 mm downstream the pole surface. After ignition, it was tuned into magnetic focusing mode. The cathode was hidden 4 mm axially behind the plane of pole sur face, with 10 mm radial distance towards the edge of outer magnetic pole. The cathode axis was parallel with the magnetic field at cathode orifice, and also approximately parallel with thruster axis. Only two RPAs were used. RPA-a was installed perpendicular to the thruster axis. Its location was 33 mm radially away from edge of outer magnetic pole, and 60� azimuthally away from cathode location. Its collecting orifice was 4 mm axially downstream the pole surface, so its function was to measure the radial IEDFs from thruster plume. Another RPA-b was located beside the cathode, hiding 16 mm axially behind the pole surface, with a 15 mm radial distance toward the outer magnetic pole. Its axis was also parallel with thruster axis. Its function was to measure IEDFs inside the aurora surrounding the outer magnetic poles. Fig. 9 shows the retrieved IEDFs. As can be seen, IEDF on RPA-a showed one-peak distribution instead of two-peaks in Fig. 6. Although a few details still needed investigation (e. g. the energetic tail extended to ~400eV, which is even higher than discharge voltage), it can be inferred that without the higher energy peak, the cathode lifetime can be much longer.
Fig. 7. Reconstructed IEDF(et,en) using IEDF(et) and IEDF(en) from RPA-a and RPA-b in Fig. 6 considering ion anisotropy.
Fig. 8. Reconstructed IEDF(et, en) using only IEDF(en) from RPA-b in Fig. 6, assuming isotropy. Table 1 Equivalent sputtering yield of keeper material.
Molybdenum Tantalum
Equivalent sputtering yield (with high energy group)
Equivalent sputtering yield (without high energy group)
Acceleration coefficient
0.4139 0.4136
0.0616 0.1654
6.719156 2.500605
4.3. Implications for cathodes Modifying cathode location is also expected to ease the problem, however the location choices are usually limited. Ref. [6–13] have shrank the alternative locations for installing cathode to a relatively small area near the outer magnetic pole, of which the magnetic field strength is greater than a critical value so that cathode discharge is more stabilized. Meanwhile the keeper electrode should be separated at a distance from outer magnetic pole to avoid sparking. These consider ations can limit the options to less than 1 cm2, which is too small. Dis obeying these principles might lead to enhanced discharge instability [36] and some performance loss. Increasing cathode mass flow rate, which is usually effective in
4.2. Effect of plume focusing Despite scarceness of similar reports, we estimate that this is a widely spread problem, rather than an individual case. To the authors’ knowledge, the majority of Hall thrusters, whether under R&Ds or already in service, have quite divergent plumes. Actually, our result agrees with some previous original works on HERMeS Thruster [34,35], including numerical predictions for similar effect and experimental
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Fig. 10. Influence of cathode flow rate on IEDF on RPA-a.
stabilizing discharge and reducing energetic ions in cathode standalone discharge [37], also proved doubtful in easing the reported problem in coupling cases. Fig. 10 shows the influence of increasing cathode flow rate on IEDFs on RPA-a in the added experiment. The increased flow rate had little effect on the 200–300eV range, while the total efficiency was decreased due to higher propellant consumption. Although the 20–100eV range was reduced in density, the peak energy was actually continuously increased from 50eV to 70eV with the flow rate. The overall sputtering rate then increased, which was opposite of decreasing in most cathode standalone discharges. Therefore there was absolutely not any improvement in the end. As a result, ensuring the sputtering life of keeper through design should be the priority direction. Some existing methods include adopt ing sputter-resistant materials such as graphite, thickening the keeper or installing extra protective shield [35]. Consciousness of leaving margin for coupling effects is also necessary during design procedure, which is hardly present in current standalone discharge scenario. On the other hand, the duration of standalone life test should be mission duration multiplied by an acceleration coefficient, the latter of which should be determined experimentally given specific thruster type and cathode location. Taking the case in this paper for instance, the mission duration is 8000 h, then the cathode standalone life test should last for 53800 h (Mo) or 20000 h (Ta), instead of the original 8000 h. The logics behind the elongated test for all cathode components rather than keeper alone is related to strategy changes after keeper failure. Without the keeper, the thruster cannot re-ignite so it will have to remain in discharge ever since. At the same time, the thruster on the opposite side of the satellite will also have to remain in discharge so that the net thrust is canceled. Note that these are happening when inner components of cathode are exposed to thruster plume, so the endurance of cathode will deplete in a quicker way. For another instance, cathode lifetime was demonstrated at 23000 h in standalone test. Then in coupling discharge, the keeper orifice plate could only withstand 23000 � (0.14 þ 0.4)/2 ¼ 6210 h. The mission plan was 8000 h, so the left 1800 h had to be extended to 1800 � 9 ¼ 16200 h nonstop, supposing the thruster was designed to operate for 1 h every other 8 h. So the final duration was 6210 þ 16200 ¼ 22410 h, which was 2.8 times of original 8000 h. Considering the erosion accel eration of the inner parts in thruster plume, it is not impossible that the actual equivalent duration can be even more times of original plan.
5. Conclusion After optimizing cathode location onboard an HET-80 Hall thruster to improve efficiency and discharge stability, keeper erosion was observed to be faster than acceptable. To find out the reason, this paper utilized multiple RPAs to diagnose the ion environment that the cathode was immersed in during thruster-cathode coupling discharge and investigated the impact of high-energy ions on keeper sputtering. Retrieved IEDFs show that aside from an quasi-isotropic 50–100eV group that resembled that in cathode standalone discharge, there existed another 200–300eV group that was tangentially incident on keeper orifice plate. The latter group was identified as the thruster plume in large divergent angle, however its energy was higher than normal. The reason is currently unclear, yet suspected to be the breathing mode as well as ion transit instability. A coarse estimation shows that the existence of the high energy group will increase the equivalent sputtering yield of keeper material by 2.5–6.7 times, thus significantly shortening keeper lifetime. Therefore, cathode location and orientation are neglected, however sometimes determinant factors of cathode lifetime, and cathode lifetime was a neglected factor of thruster-cathode coupling. Some well-known research directions would help balance between performance and lifetime when determining cathode location, including decreasing plume divergence angle, enhancing the sputter-resistant ca pabilities of the keeper and modifying the topology of exterior magnetic field. Moreover, we suggest that the duration of cathode standalone life test should be the mission duration multiplied by an acceleration coef ficient, of which the value should be experimentally determined given specific thruster type, operation parameter and cathode location. Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, ‘Accelerated Erosion of keeper electrode during coupling discharge between Hall thruster and hollow cathode’, authored by Tianhang Meng, Caixia Qiao, Yanan Wang, Zhongxi Ning and Daren Yu. 6
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Acknowledgements
[17] C.E. Garner, J.E. Polk, L.C. Pless, K.D. Goodfellow, John R. Brophy, Performance evaluation and life testing of the SPT-100, Int. Electric Propul. Conf. (1993). Paper No. IEPC-93-091, https://erps.spacegrant.org/uploads/images/images/iepc_art icledownload_1988-2007/1993index/IEPC1993-091.pdf. [18] B.A. Arhipov, et al., The results of 7000-hour SPT-100 life testing, Int. Electric Propul. Conf (1995). Paper No. IEPC-95-039, http://www.erps.spacegrant.org/upl oads/images/images/iepc_articledownload_1988-2007/1995index/IEPC1995-39. pdf. [19] J.F. Ziegler, Stopping and Range of Ions in Matter (SRIM) Software Ver. http://www.srim.org, 2013. [20] R.P. Doerner, D.G. Whyte, D.M. Goebel, Sputtering yield measurements during low energy xenon plasma bombardment, J. Appl. Phys. 93 (2003) 5816–5823, https:// doi.org/10.1063/1.1566474. [21] R.D. Kolasinski, J.E. Polk, D. Goebel, L.K. Johnson, Sputtering yield measurements at glancing incidence using a quartz crystal microbalance, J. Vac. Sci. Technol. A 25 (2007) 236–245, https://doi.org/10.1116/1.2435375. [22] C.H. Weijsenfeld, A. Hoogendoorn, M. Koedam, Sputtering of polycrystalline metals by inert gas ions of low energy (100–1000 eV), Physica 27 (1961) 763–764, https://doi.org/10.1016/0031-8914(61)90093-3. [23] J.J. Blandino, D.G. Goodwin, C.E. Garner, Low energy sputter yields for diamond, carbon–carbon composite, and molybdenum subject to xenon ion bombardment, Diam. Relat. Mater. 9 (2000) 1992–2001, https://doi.org/10.1016/S0925-9635 (00)00350-2. [24] D.M. Goebel, K.K. Jameson, I. Katz, I.G. Mikellides, Potential fluctuations and energetic ion production in hollow cathode discharges, Phys. Plasmas 14 (2007) 103508, https://doi.org/10.1063/1.2784460. [25] Q. Yu, et al., The analysis of high amplitude of potential oscillations near the hollow cathode of ion thruster, Acta Astronaut. 134 (2017) 265–277, https://doi. org/10.1016/j.actaastro.2017.02.012. [26] C.C. Farnell, J.D. Williams, C.C. Farnell, Comparison of hollow cathode discharge plasma configurations, Plasma Sources Sci. Technol. 20 (2011), 025006, https:// doi.org/10.1088/0963-0252/20/2/025006. [27] B.A. Jorns, I.G. Mikellides, D.M. Goebel, Ion acoustic turbulence in a 100-A LaB6 hollow cathode, Phys. Rev. E 90 (2014), 063106, https://doi.org/10.1103/ PhysRevE.90.063106. [28] B.A. Jorns, C. Dodson, D.M. Goebel, R. Wirz, Propagation of ion acoustic wave energy in the plume of a high-current LaB6 hollow cathode, Phys. Rev. E 96 (2017), 023208, https://doi.org/10.1103/PhysRevE.96.023208. [29] E. Choueiri, Plasma oscillations in Hall thrusters, Phys. Plasmas 8 (2001) 1411–1426, https://doi.org/10.1063/1.1354644. [30] Z. Ning, H. Liu, D. Yu, Z. Zhou, Effects of ionization distribution on plasma beam focusing characteristics in Hall thrusters, Appl. Phys. Lett. 99 (2011) 221502, https://doi.org/10.1063/1.3665632. [31] S. Barral, K. Makowski, Z. Peradzynski, M. Dudeck, Transit-time instability in Hall thrusters, Phys. Plasmas 12 (2005), 073504, https://doi.org/10.1063/1.1947796. [32] Y. Lim, et al., Nonlinear ion dynamics in Hall thruster plasma source by ion transittime instability, Plasma Sources Sci. Technol. 26 (2017), 03LT01, https://doi.org/ 10.1088/1361-6595/aa58d9. [33] E. Bringa, Ion Tracks in Solids: Sputtering and Surface Modification, Ph.D Dissertation University of Virginia, 2000, p. 82~83. [34] A. Ortega, I.G. Mikellides, D.M. Goebel, Numerical simulations for life assessments of the BaO and LaB6 cathode options in the Hall effect rocket with magnetic shielding (HERMeS), 35th Int. Electric Propul. Conf. (2017). Paper No. IEPC-2017152, https://iepc2017.org/sites/default/files/speaker-papers/iepc-2017-152_final. pdf. [35] G. Williams, J.H. Gilland, H. Kamhawi, M. Choi, P.Y. Peterson, D. Herman, Wear trends of the HERMeS Thruster as a function of throttle point, 35th Int. Electric Propul. Conf. (2017). Paper No. IEPC-2017-207, https://iepc2017.org/sites/defa ult/files/speaker-papers/2017_iepc_rev6.pdf. [36] T. Meng, Z. Ning, D. Yu, Triggering of ionization oscillations in hollow cathode discharge by keeper electrode, Phys. Plasmas 26 (2019), 093510, https://doi.org/ 10.1063/1.5116830. [37] E. Chu, D.M. Goebel, R.E. Wirz, Reduction of energetic ion production in hollow cathodes by external gas injection, J. Propuls. Power 29 (2013) 1155–1163.
This work was funded by Science and Technology Commission of Shanghai Municipality under Grant. No. 17DZ2280800. References [1] A.I. Morozov, V.V. Savelyev, Fundamentals of stationary plasma thruster theory, in: B.B. Kadomtsev, V.D. Shafranov (Eds.), Rev. Plasma Phys., vol. 21, Springer Inc., Boston, 2000, p. 203. [2] D. Tilley, K. de Grys, R. Myers, Hall thruster-cathode coupling, 35th joint propulsion conf. & exhibit, Paper No. AIAA 1999-2865, https://doi.org/10.2514/ 6.1999-2865, 1999. [3] M.S. McDonald, A.D. Gallimore, Cathode position and orientation effects on cathode coupling in a 6-kw Hall thruster, 31st Int, in: Electric Propulsion Conf, 2019. Paper No. IEPC-09-113, http://erps.spacegrant.org/uploads/images/images /iepc_articledownload_1988-2007/2009index/IEPC-2009-113.pdf. [4] D.M. Goebel, K.K. Jameson, R.R. Hofer, Hall thruster cathode flow impact on coupling voltage and cathode life, J. Propuls. Power 28 (2012) 355–363, https:// doi.org/10.2514/1.B34275. [5] T. Miyasaka, et al., Influences of cathode position on operations and thrust performance of side by side Hall thruster system, Vacuum 167 (2019) 524–529, https://doi.org/10.1016/j.vacuum.2018.12.056. [6] K.K. Jameson, Investigations on Hollow Cathode Effects on Total Thruster Efficiency of a 6-kW Hall Thruster, PhD Thesis University of California, Los Angeles, 2009. [7] J.D. Sommerville, L.B. King, Hall-effect thruster–cathode coupling, part I: efficiency improvements from an extended outer pole, J. Propuls. Power 27 (2011) 744–753, https://doi.org/10.2514/1.50123. [8] J.D. Sommerville, L.B. King, Hall-effect thruster–cathode coupling, part II: ion beam and near-field plume, J. Propuls. Power 27 (2011) 754–767, https://doi.org/ 10.2514/1.50124. [9] J.A. Walker, J.D. Frieman, M.L.R. Walker, V. Khayms, D. King, P. Peterson, Electrical facility effects on Hall-effect-thruster cathode coupling: discharge oscillations and facility coupling, J. Propuls. Power 32 (2016) 844–855, https:// doi.org/10.2514/1.B35835. [10] J.A. Walker, S.J. Langendorf, M.L.R. Walker, V. Khayms, D. King, P. Peterson, Electrical facility effects on Hall current thrusters: electron termination pathway manipulation, J. Propuls. Power 32 (2016) 1365–1377, https://doi.org/10.2514/ 1.B35904. [11] T. Meng, Z. Ning, S. Eliseev, D. Yu, A. Kudryavtsev, Simulation of electron streamline distribution and coupling voltage in the coupling area of a Hall thruster, Plasma Sources Sci. Technol. 28 (2019), 035016, https://doi.org/10.1088/13616595/ab019b. [12] D. Yu, T. Meng, Z. Ning, H. Liu, Confinement effect of cylindrical-separatrix type magnetic field on the plume of magnetic focusing type Hall thruster, Plasma Sources Sci. Technol. 26 (2017), 04LT02, https://doi.org/10.1088/1361-6595/ aa6425. [13] T. Meng, Z. Ning, D. Yu, Influence of background magnetic field on hollow cathode discharge characteristics, 34th Int. Electric Propulsion Conf. (2015). Paper No. IEPC-2015-77, https://spacegrant.org/uploads/images/2015Presentations/IE PC-2015-77_ISTS-2015-b-77.pdf. [14] Y. Ding, et al., Effect of relative position between cathode and magnetic separatrix on the discharge characteristic of Hall thrusters, Vacuum 154 (2018) 167–173, https://doi.org/10.1016/j.vacuum.2018.05.005. [15] W. Li, et al., Study on the influence of change in axial position of cathode on the ignition process of a Hall thruster, Vacuum 167 (2019) 234–243, https://doi.org/ 10.1016/j.vacuum.2019.06.007. [16] X. Lu, X. Wang, H. Tang, Z. Zhang, X. Kang, Beam characteristics of HET-80 Hall thruster with 700Watts, J. Propuls. Technol. 39 (2018) 1426–1433 (In Chinese), http://mall.cnki.net/magazine/Article/TJJS201806028.htm.
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