- Email: [email protected]
Study of the catastrophic discharge phenomenon in a Hall thruster
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AID:24715 /SCO Doctopic: Plasma and fluid physics
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Study of the catastrophic discharge phenomenon in a Hall thruster
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Ding Yongjie, Su Hongbo, Li Peng, Wei Liqiu ∗ , Li Hong, Peng Wuji, Xu Yu, Sun Hezhi, Yu Daren
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Plasma Propulsion Lab, Institute of Advanced Power, Harbin Institute of Technology, Harbin, 150001, People’s Republic of China
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Article history: Received 11 January 2017 Received in revised form 24 July 2017 Accepted 2 September 2017 Available online xxxx Communicated by F. Porcelli Keywords: Hall thruster Discharge catastrophe
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In a 1350-W Hall-effect thruster, in which a technique for pushing down the magnetic field is implemented, a catastrophic discharge phenomenon is identified by varying the magnetic field strength while keeping all other operating parameters constant. According to experiments, before and after the discharge catastrophe, the plume changes from focusing state to a divergent state, and discharge parameters such as discharge current and thrust exhibit noticeable changes. The divergence half-angle of the plume increases from 22◦ to 46◦ . The oscillation amplitude and mean values of the discharge current significantly increase from 0.8 A to 4 A and from 4.6 A to 6.3 A, respectively, while the thrust increases from 89.3 mN to 91 mN. Analysis of the experimental results shows that as the maximum magnetic field of the thruster we developed is in the plume region, the acceleration occurs in the plume region and a large number of Xe2+ ions appear in the plume area, the catastrophic discharge phenomenon observed. © 2017 Elsevier B.V. All rights reserved.
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1. Introduction
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With the rapid development of various satellite and space detection technologies, electric propulsion has been widely applied to spacecraft. In particular the Hall thruster has been widely applied in station-keeping, orbit-topping, and orbit-transfer of geosynchronous communication satellites and in deep-space exploration, owing to such advantages as its high specific impulse, compact structure, small volume, and small consumption of propellant. Under specific working conditions (i.e., magnetic field, discharge voltage, cathode mass flow rate, anode mass flow rate) and when the whole vacuum system is kept unchanged, when the discharge is stable, some of the discharge parameters and performance parameters of a Hall thruster are unchanged, such as the discharge current, thrust, plume state, efficiency, etc. Under general circumstances, the values of some working parameters will only exhibit small changes, and discharge parameters and performance parameters will also exhibit small changes. However, under some special circumstances, one or a number of parameters (i.e., discharge parameters and performance parameters) will exhibit significant changes. These changes in discharge current characteristics, including the mean value and oscillation amplitude, indicate a discharge mode transition.
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Corresponding author. E-mail addresses: [email protected], [email protected] (L. Wei).
http://dx.doi.org/10.1016/j.physleta.2017.09.002 0375-9601/© 2017 Elsevier B.V. All rights reserved.
As early as the 1970s, the discharge mode transition was observed in early pioneering Russian research [1]. Tamida et al. studied mode transition in the SPT Hall thruster and the oscillation area was observed [2]. Béchu et al. varied the discharge voltage on the SPT-100ML thruster and identified four working modes, namely, the irregular mode, fluctuating mode, oscillating mode, and pulsed mode based on the shape of the discharge current signal [3]. Béchu et al. also studied the mode transition of an A53 Hall thruster—the magnetic field strength was varied by changing excitation current of the coil—and identified two different working modes [4]. Brown et al. researched the effect of changes in magnetic field strength on mode transition and found the current and plume underwent noticeable changes [5]. Recently, Hara adopted hybrid direct kinetic method to analyze the mode transition. Sekerak et al. [6,7] identified mode transition in a H6 thruster by varying the magnetic field strength. The two types of state were defined by a global oscillation mode and by a local oscillation mode. The obvious changes before and after the mode transition were in oscillation amplitude and mean values of the discharge current. The global oscillation mode and local oscillation mode were also observed by changing the discharge voltage and anode mass flow rate. The relationships between discharge current, discharge voltage, and magnetic field strength were given under different mass flow rates so as to ensure that the thruster operated stably under the optimal mode [8]. Recently, Han et al. studied the mode transition caused by a magnetic field gradient. The oscillation amplitude of the discharge current exhibited an obvious change, but the mean values remained almost unchanged [9].
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Fig. 1. Schematic diagram for the magnetic field configuration.
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Adopting prototype technology for pushing down the magnetic field [10], our team designed an experimental prototype of a 1350-W Hall thruster. Its magnetic field structure can effectively extend the ionization region and acceleration region towards the outlet of the discharge channel. Thus, the wall is only bombarded by low-energy ions and electrons and power deposition is small, which effectively reduces erosion of the walls of the channel. In such a Hall thruster, the mode changes are completely different from those observed in previous studies. That is, the change in discharge mode observed by other researchers is expressed by an approximately continuous change in the discharge current of the thruster, the discharge oscillation, or the thrust force characteristics as the discharge parameters are adjusted. The observed change in discharge mode is a sudden change in discharge current, discharge oscillation, and plume state as the excitation current increases to a specific value. Therefore, to differentiate this from previous discharge mode transitions, we term this as catastrophic discharge phenomenon. Combining the characteristics of a thruster’s magnetic field design, the reasons for the catastrophic discharge phenomenon are analyzed based on measurements of Xe2+ spectral intensity, electron temperature, and ion energy distribution before and after catastrophic discharge. The remainder of this paper is arranged as follows. Section 2 gives the experimental setup, Section 3 gives results and discussion, and the conclusion is given in Section 4.
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2. Experimental setup
Fig. 2. Experimental prototype of the 1350-W Hall thruster adopting a technology for pushing down the magnetic field.
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The wall material of the discharge channel of the thruster is boron nitride and the propellant is Xe. A heated hollow cathode with an LaB6 insert was used with a constant xenon mass flow rate of 0.3 mg/s. The cathode and the thruster were floating but unbound. The experiments were carried out in a stainless-steel vacuum chamber with an inner diameter of 1.2 m and a length of 4 m. The background chamber pressure was 4 × 10−3 Pa (for Xe) when operating the thruster at a 5 mg/s anode xenon flow rate (all these pressures are corrected for xenon). The thruster is operated under the following working conditions: discharge voltage 300 V, anode mass flow rate 5 mg/s, and cathode mass flow rate 0.3 mg/s. All experimental data were measured after the thruster was operated stably for 3 h and thermal balance was reached. A Faraday probe and arc measurement were combined to measure the ion current density. The measured bias voltage is −50 V. The probe diameter is 5 mm and the probe was installed on a rotating arm with a rotational radius of 300 mm. The probe was rotated from the center to the outer edge of thruster (90◦ scanning). The surface integral was evaluated within this 90◦ scanned area, i.e., half of the semi-spherical surface for the ion current. The position of 90% of total ion current was taken as the boundary of the divergence half-angle of the plume. The following formula describes the specific calculation process.
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A technology for pushing down the magnetic field was previously proposed by our team [10]. For traditional Hall thrusters, the ionization and acceleration zone are mainly distributed in the channel. However, our thruster is developed with a technology similar to the magnetic shielding by JPL and it has the maximum magnetic field intensity in the plume region (Brexit/Brmax ≈ 0.85). As a result, the ionization zone is near the channel exit, and the main acceleration zone is in the plume region. Fig. 1 presents a schematic diagram of the magnetic field configuration. The anode is hollow and integrated with the gas distributor. The current direction in Coil 3 is opposite to that of Coils 1 and 2, forming a zero-magnetic field configuration. Meanwhile, the excitation current of Coil 3 may be regulated to change the zero-magnetic point, and hence, the distance between the maximum to the zeromagnetic field point and the distribution of the magnetic field intensity in the channel. Therefore, the position of the ionization zone largely depends on the distance between the maximum to zero-magnetic field. Fig. 2 shows the experimental prototype of the 1350-W Hall thruster.
θdiv 0.9I b = 2π r
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j (θ) sin θ dθ,
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where I b = 2π r 02 j (θ) sin θ dθ is the ion beam current and θ div is the divergence half-angle of the plume. A retarding potential analyzer (RPA) was installed on the axial line at a position 30 cm from the thruster outlet for measuring the ion energy distribution under different working conditions. A spectral probe is located at the outlet position of the thruster, with a radial distance 20 mm to the outer edge of thruster. Measurements were made five times to find the mean value. The electron temperature and Xe2+ spectral intensity can be obtained via analysis of the measured light intensity of the spectral lines. The thrust is measured with a torsion balance [11]. The balance is calibrated with standard weights before using. The accuracy of thrust measurement is ±0.1 mN. The discharge current and oscillating peak value are monitored using a Yokogawa DL850E recorder.
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AID:24715 /SCO Doctopic: Plasma and fluid physics
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Fig. 3. Change in discharge current and thrust force before and after the discharge catastrophe.
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3. Results and discussion
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The excitation current of Coil 3 of the thruster was slowly increased from 3.5 A to 5.5 A under the following working conditions: discharge voltage 300 V, anode mass flow rate 5 mg/s, cathode mass flow rate 0.3 mg/s, and the currents of Coil 1 and Coil 2 unchanged. When the excitation current of Coil 3 is slowly changed to 5 A and an obvious catastrophic discharge phenomenon was observed; see Figs. 3 and 4. The discharge is relatively stable between 3.5 A and 5 A; however, when the current is adjusted to the working point of 5 A, the oscillation amplitude and mean values of the discharge current significantly increased: the discharge current increased from 4.6 A to 6.3 A and the oscillation amplitude increased from 0.8 A to 4 A as shown in Fig. 4 (b) and (c). The
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discharge power change designed from ∼1350 W to ∼1800 W. During the discharge catastrophe, the thrust only increased from 89.3 mN to 91 mN, but the change in current is significant, so performance parameters such as efficiency and thrust-to-power ratio display an obvious reduction. The similar discharge catastrophe phenomenon is seen either with increasing current at Coil 3 from 3.5 A to 5 A or decreasing from 5 A to 3.5 A. In addition, before and after the discharge catastrophe, the plume state underwent a noticeable change, as shown in Fig. 5. The plume state changed from the original focusing state to a divergent state, the divergence half-angle of the plume increased from 22◦ to 46◦ . Fig. 5 shows that the ionization zone (a white and bright area) is inside the channel before the mode catastrophic phenomenon; after that, the whole channel exit becomes white and bright, implying that a lot of ionization has occurred at the channel exit. In addition, the cathode plume was significantly changed as shown in Fig. 5. From a macroscopic point of view, the oscillation amplitude and mean values of the discharge current exhibit a small change before the current of the exciting Coil 3 increases close to 5 A, and plume is also in a focusing mode. The discharge mode exhibits an obvious change when the current of the exciting coil increases to 5 A. The magnetic field exhibits no significant change as the current of the exciting Coil 3 changes tiny at the value of 5 A, the overall magnetic field structure being unmodified except for in the anode region. The thruster we developed uses a hollow anode with the front end at the position corresponding to the zero-magnetic point. The subtle changes of the magnetic field in front of the zero-magnetic point do not significantly influence the discharge. However, the discharge mode exhibits a sudden change. The cathode position and its magnetic field both influence the discharge. During the test, the cathode position did not change. The current of Coil 3 mainly regulates the magnetic field in the channel while slightly influencing that of the cathode in the plume region. The magnetic field environment of the cathode also did not change.
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Fig. 4. Discharge current before and after the discharge catastrophe.
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Fig. 5. Pictures of the plumes.
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Furthermore, the cathode status almost did not change with the occurrence of the mode catastrophic phenomenon. To study the internal discharge mechanism causing the discharge catastrophe in more detail, we analyze the reasons for the change in discharge parameters and performance parameters of the thruster. We respectively used the Faraday probe, RPA, and spectrum to measure the ion current, ion energy distribution, Xe2+ spectral intensity and electron temperature distribution, making further interpretations of the catastrophic discharge phenomenon. Through the processing of spectroscopic data, the electron temperature near the channel outlet of thruster, and the Xe2+ spectral intensity before and after the discharge catastrophe can be obtained. Fig. 6 shows the changes in Xe2+ spectral intensity and ion current-to-discharge current ratio (I i / I d ) versus the excitation current of Coil 3. According to Fig. 6, after the sudden change of mode, the Xe2+ spectral intensity noticeably increases; the quantity of corresponding Xe2+ ions is large; Xe2+ could hardly be detected before catastrophe, and the ratio of Xe+ /Xe2+ spectral intensity was about 4:3 after the catastrophe. Moreover, the electron temperature of a point at the outlet of the channel was measured. The electron temperature distribution inside the channel and the plume region is continuous [12–15]. Except for the anode region, the overall magnetic field distribution is hardly affected. Although the anode characteristics show subtle changes in the magnetic field in front of the zero-magnetic point, they do not significantly influence the discharge. It can be considered that the electron temperature distribution in this region (near the ionization region) in the two modes cannot widely move. Therefore, the local electron temperature measured near the outlet of channel can quantitatively reflect the overall change of electrons in this region. The electron temperature of the measured before the sudden change of mode is approximately 21 eV; and single ionization mainly occurs. After the discharge catastrophe, the electron temperature increases to 37 eV, providing the electron energy required for double ionization of the xenon atoms, which will result in more Xe2+ ions. Though the electron temperature of the measured at only one point, it could quantitatively reflect the overall change of electrons in this region. Because the electron temperature distribution inside the channel and the plume region is continuous, the changes of magnetic field before and after the catastrophe point are tiny. The results show that the electron temperature in the channel and that in the plume region changed, causing more divalent ionization and the mode catastrophic phenomenon. According to Fig. 6, after the discharge catastrophe, I i / I d decreases significantly from 85% to 60% while the electron current noticeably increases, which further demonstrates that after the mode change, Xe2+ ions increase and a greater electron current is generated. To further analyze the change in discharge characteristics during the catastrophic discharge phenomenon. We used RPA to mea-
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Fig. 6. Change in Xe2+ spectral intensity and I i / I d before and after the discharge catastrophe.
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Fig. 7. Comparison of ion energy distributions before and after the discharge catastrophe.
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sure the ion energy distribution before and after the discharge catastrophe, as shown in Fig. 7. We can see that before the occurrence of the discharge catastrophe, the ion energy is mainly around 250 eV and it is lower than the discharge voltage (300 V). Therefore, there are only a few ions with energy over 300 eV; hence, there are few Xe2+ ions. After the discharge catastrophe, highenergy ions are clearly enhanced. In combination with the spectral processing results, significant numbers of Xe2+ ions occur at this time.
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In short, as the excitation current of Coil 3 increases, the ionization region tends to move toward the channel exit, and at 5-A current, the ionization occurs in the plume region (accompanied by the catastrophic phenomenon of the discharge mode). At this time, the channel is unable to constrain the ionization region completely if the increase of the excitation current of Coil 3 is continued. Under such circumstances, partial ionization occurs in the plume area, so that the plume changes from a focusing state to a divergent state, as shown in Fig. 5. During this process, electron temperature in the plume area increases, the electron energy required for double ionization of xenon atom is achieved, and significant numbers of Xe2+ ions occur at this time. However, there is no significant increase in thrust. This is because many faster divalent ions at the same acceleration voltage were generated upon the occurrence of the mode catastrophic phenomenon. Although some radial loss exists from the divergent plume, the thrust still increases a little after the mode catastrophic phenomenon.
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4. Conclusions
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In this paper, a 1350-W Hall-effect thruster is designed based on a technology for suppressing the magnetic field. A catastrophic discharge phenomenon was induced by varying the excitation current of Coil 3 while keeping all other operating parameters constant. Before and after the discharge catastrophe, the plume state changed from a focusing state to a divergent state. The oscillation amplitude and mean values of the discharge current significantly increased. By combining the characteristics of the characteristics of a thruster’s magnetic field design, a new catastrophic discharge phenomenon was identified. Furthermore, effective measurement and analysis tools were adopted to study in detail the internal discharge mechanism causing the discharge catastrophe. Following on from numerous studies of mode transition, this work enriches the
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literature on mode transition and provides new insights into the mode transition.
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Acknowledgements
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The authors want to gratefully acknowledge the financial support from National Natural Science Foundation of China (Grant Nos. 51477035 and 5177070578).
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References
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[1] G.N. Tilinin, Zh. Tekh. Fiz. 47 (1977) 1684–1691. [2] T. Tamida, T. Nakagawa, I. Suga, H. Osuga, T. Ozaki, K. Matsui, J. Appl. Phys. 102 (2007) 043304. [3] S. Bechu, C. Perot, N. Gascon, P. Lasgorceix, A. Hauser, M. Dudeck, in: 35th Joint Propulsion Conference, 1999, AIAA-1999-2567. [4] S. Bechu, P. Lasgorceix, N. Gascon, S. Roche, M. Prioul, L. Albarede, M. Dudeck, in: 36th Joint Propulsion Conference, 2000, AIAA-2000-3523. [5] D.L. Brown, A.D. Gallimore, in: 31st International Electric Propulsion Conference, 2009, IEPC-2009-74. [6] K. Hara, I.D. Boyd, A. Arbor, in: 31st International Electric Propulsion Conference, 2013, IEPC-2013-266. [7] K. Hara, M.J. Sekerak, I.D. Boyd, A.D. Gallimore, J. Appl. Phys. 115 (2014) 203304. [8] M.J. Sekerak, A.D. Gallimore, J. Propuls. Power 32 (2016) 1–15. [9] L. Han, L. Wei, D. Yu, J. Phys. D, Appl. Phys. 49 (2016) 375203. [10] Y.J. Ding, W.J. Peng, L.Q. Wei, G.S. Sun, H. Li, D.R. Yu, J. Phys. D, Appl. Phys. 49 (2016) 465001. [11] H. Li, Z. Ning, D. Yu, J. Appl. Phys. 113 (2013) 083303. [12] I.G. Mikellides, I. Katz, R.R. Hofer, et al., Magnetic shielding of a laboratory Hall thruster. I. Theory and validation, J. Appl. Phys. 115 (4) (2014) 043303. [13] I.G. Mikellides, I. Katz, R.R. Hofer, et al., Magnetic shielding of walls from the unmagnetized ion beam in a Hall thruster, Appl. Phys. Lett. 102 (2) (2013) 023509. [14] I.G. Mikellides, I. Katz, R.R. Hofer, et al., Magnetic shielding of the acceleration channel walls in a long-life Hall thruster, 2010. [15] I.G. Mikellides, R.R. Hofer, I. Katz, et al., The effectiveness of magnetic shielding in high-Isp Hall thrusters, in: Proc. 49th AIAA/ASME/SAE/ASEE Joint Propuls. Conf., 2013, AIAA 2013-3885.
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Physics Letters A ••• (••••) •••–•••
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Physics Letters A
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Study of the catastrophic discharge phenomenon in a Hall thruster
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Ding Yongjie, Su Hongbo, Li Peng, Wei Liqiu ∗ , Li Hong, Peng Wuji, Xu Yu, Sun Hezhi, Yu Daren
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22 23 24 25 26 27 28 29
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Plasma Propulsion Lab, Institute of Advanced Power, Harbin Institute of Technology, Harbin, 150001, People’s Republic of China
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85
a r t i c l e
i n f o
Article history: Received 11 January 2017 Received in revised form 24 July 2017 Accepted 2 September 2017 Available online xxxx Communicated by F. Porcelli Keywords: Hall thruster Discharge catastrophe
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a b s t r a c t
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In a 1350-W Hall-effect thruster, in which a technique for pushing down the magnetic field is implemented, a catastrophic discharge phenomenon is identified by varying the magnetic field strength while keeping all other operating parameters constant. According to experiments, before and after the discharge catastrophe, the plume changes from focusing state to a divergent state, and discharge parameters such as discharge current and thrust exhibit noticeable changes. The divergence half-angle of the plume increases from 22◦ to 46◦ . The oscillation amplitude and mean values of the discharge current significantly increase from 0.8 A to 4 A and from 4.6 A to 6.3 A, respectively, while the thrust increases from 89.3 mN to 91 mN. Analysis of the experimental results shows that as the maximum magnetic field of the thruster we developed is in the plume region, the acceleration occurs in the plume region and a large number of Xe2+ ions appear in the plume area, the catastrophic discharge phenomenon observed. © 2017 Elsevier B.V. All rights reserved.
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1. Introduction
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With the rapid development of various satellite and space detection technologies, electric propulsion has been widely applied to spacecraft. In particular the Hall thruster has been widely applied in station-keeping, orbit-topping, and orbit-transfer of geosynchronous communication satellites and in deep-space exploration, owing to such advantages as its high specific impulse, compact structure, small volume, and small consumption of propellant. Under specific working conditions (i.e., magnetic field, discharge voltage, cathode mass flow rate, anode mass flow rate) and when the whole vacuum system is kept unchanged, when the discharge is stable, some of the discharge parameters and performance parameters of a Hall thruster are unchanged, such as the discharge current, thrust, plume state, efficiency, etc. Under general circumstances, the values of some working parameters will only exhibit small changes, and discharge parameters and performance parameters will also exhibit small changes. However, under some special circumstances, one or a number of parameters (i.e., discharge parameters and performance parameters) will exhibit significant changes. These changes in discharge current characteristics, including the mean value and oscillation amplitude, indicate a discharge mode transition.
60 61 62 63 64 65 66
*
Corresponding author. E-mail addresses: [email protected], [email protected] (L. Wei).
http://dx.doi.org/10.1016/j.physleta.2017.09.002 0375-9601/© 2017 Elsevier B.V. All rights reserved.
As early as the 1970s, the discharge mode transition was observed in early pioneering Russian research [1]. Tamida et al. studied mode transition in the SPT Hall thruster and the oscillation area was observed [2]. Béchu et al. varied the discharge voltage on the SPT-100ML thruster and identified four working modes, namely, the irregular mode, fluctuating mode, oscillating mode, and pulsed mode based on the shape of the discharge current signal [3]. Béchu et al. also studied the mode transition of an A53 Hall thruster—the magnetic field strength was varied by changing excitation current of the coil—and identified two different working modes [4]. Brown et al. researched the effect of changes in magnetic field strength on mode transition and found the current and plume underwent noticeable changes [5]. Recently, Hara adopted hybrid direct kinetic method to analyze the mode transition. Sekerak et al. [6,7] identified mode transition in a H6 thruster by varying the magnetic field strength. The two types of state were defined by a global oscillation mode and by a local oscillation mode. The obvious changes before and after the mode transition were in oscillation amplitude and mean values of the discharge current. The global oscillation mode and local oscillation mode were also observed by changing the discharge voltage and anode mass flow rate. The relationships between discharge current, discharge voltage, and magnetic field strength were given under different mass flow rates so as to ensure that the thruster operated stably under the optimal mode [8]. Recently, Han et al. studied the mode transition caused by a magnetic field gradient. The oscillation amplitude of the discharge current exhibited an obvious change, but the mean values remained almost unchanged [9].
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Fig. 1. Schematic diagram for the magnetic field configuration.
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Adopting prototype technology for pushing down the magnetic field [10], our team designed an experimental prototype of a 1350-W Hall thruster. Its magnetic field structure can effectively extend the ionization region and acceleration region towards the outlet of the discharge channel. Thus, the wall is only bombarded by low-energy ions and electrons and power deposition is small, which effectively reduces erosion of the walls of the channel. In such a Hall thruster, the mode changes are completely different from those observed in previous studies. That is, the change in discharge mode observed by other researchers is expressed by an approximately continuous change in the discharge current of the thruster, the discharge oscillation, or the thrust force characteristics as the discharge parameters are adjusted. The observed change in discharge mode is a sudden change in discharge current, discharge oscillation, and plume state as the excitation current increases to a specific value. Therefore, to differentiate this from previous discharge mode transitions, we term this as catastrophic discharge phenomenon. Combining the characteristics of a thruster’s magnetic field design, the reasons for the catastrophic discharge phenomenon are analyzed based on measurements of Xe2+ spectral intensity, electron temperature, and ion energy distribution before and after catastrophic discharge. The remainder of this paper is arranged as follows. Section 2 gives the experimental setup, Section 3 gives results and discussion, and the conclusion is given in Section 4.
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2. Experimental setup
Fig. 2. Experimental prototype of the 1350-W Hall thruster adopting a technology for pushing down the magnetic field.
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The wall material of the discharge channel of the thruster is boron nitride and the propellant is Xe. A heated hollow cathode with an LaB6 insert was used with a constant xenon mass flow rate of 0.3 mg/s. The cathode and the thruster were floating but unbound. The experiments were carried out in a stainless-steel vacuum chamber with an inner diameter of 1.2 m and a length of 4 m. The background chamber pressure was 4 × 10−3 Pa (for Xe) when operating the thruster at a 5 mg/s anode xenon flow rate (all these pressures are corrected for xenon). The thruster is operated under the following working conditions: discharge voltage 300 V, anode mass flow rate 5 mg/s, and cathode mass flow rate 0.3 mg/s. All experimental data were measured after the thruster was operated stably for 3 h and thermal balance was reached. A Faraday probe and arc measurement were combined to measure the ion current density. The measured bias voltage is −50 V. The probe diameter is 5 mm and the probe was installed on a rotating arm with a rotational radius of 300 mm. The probe was rotated from the center to the outer edge of thruster (90◦ scanning). The surface integral was evaluated within this 90◦ scanned area, i.e., half of the semi-spherical surface for the ion current. The position of 90% of total ion current was taken as the boundary of the divergence half-angle of the plume. The following formula describes the specific calculation process.
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A technology for pushing down the magnetic field was previously proposed by our team [10]. For traditional Hall thrusters, the ionization and acceleration zone are mainly distributed in the channel. However, our thruster is developed with a technology similar to the magnetic shielding by JPL and it has the maximum magnetic field intensity in the plume region (Brexit/Brmax ≈ 0.85). As a result, the ionization zone is near the channel exit, and the main acceleration zone is in the plume region. Fig. 1 presents a schematic diagram of the magnetic field configuration. The anode is hollow and integrated with the gas distributor. The current direction in Coil 3 is opposite to that of Coils 1 and 2, forming a zero-magnetic field configuration. Meanwhile, the excitation current of Coil 3 may be regulated to change the zero-magnetic point, and hence, the distance between the maximum to the zeromagnetic field point and the distribution of the magnetic field intensity in the channel. Therefore, the position of the ionization zone largely depends on the distance between the maximum to zero-magnetic field. Fig. 2 shows the experimental prototype of the 1350-W Hall thruster.
θdiv 0.9I b = 2π r
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j (θ) sin θ dθ,
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where I b = 2π r 02 j (θ) sin θ dθ is the ion beam current and θ div is the divergence half-angle of the plume. A retarding potential analyzer (RPA) was installed on the axial line at a position 30 cm from the thruster outlet for measuring the ion energy distribution under different working conditions. A spectral probe is located at the outlet position of the thruster, with a radial distance 20 mm to the outer edge of thruster. Measurements were made five times to find the mean value. The electron temperature and Xe2+ spectral intensity can be obtained via analysis of the measured light intensity of the spectral lines. The thrust is measured with a torsion balance [11]. The balance is calibrated with standard weights before using. The accuracy of thrust measurement is ±0.1 mN. The discharge current and oscillating peak value are monitored using a Yokogawa DL850E recorder.
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AID:24715 /SCO Doctopic: Plasma and fluid physics
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Fig. 3. Change in discharge current and thrust force before and after the discharge catastrophe.
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3. Results and discussion
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The excitation current of Coil 3 of the thruster was slowly increased from 3.5 A to 5.5 A under the following working conditions: discharge voltage 300 V, anode mass flow rate 5 mg/s, cathode mass flow rate 0.3 mg/s, and the currents of Coil 1 and Coil 2 unchanged. When the excitation current of Coil 3 is slowly changed to 5 A and an obvious catastrophic discharge phenomenon was observed; see Figs. 3 and 4. The discharge is relatively stable between 3.5 A and 5 A; however, when the current is adjusted to the working point of 5 A, the oscillation amplitude and mean values of the discharge current significantly increased: the discharge current increased from 4.6 A to 6.3 A and the oscillation amplitude increased from 0.8 A to 4 A as shown in Fig. 4 (b) and (c). The
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discharge power change designed from ∼1350 W to ∼1800 W. During the discharge catastrophe, the thrust only increased from 89.3 mN to 91 mN, but the change in current is significant, so performance parameters such as efficiency and thrust-to-power ratio display an obvious reduction. The similar discharge catastrophe phenomenon is seen either with increasing current at Coil 3 from 3.5 A to 5 A or decreasing from 5 A to 3.5 A. In addition, before and after the discharge catastrophe, the plume state underwent a noticeable change, as shown in Fig. 5. The plume state changed from the original focusing state to a divergent state, the divergence half-angle of the plume increased from 22◦ to 46◦ . Fig. 5 shows that the ionization zone (a white and bright area) is inside the channel before the mode catastrophic phenomenon; after that, the whole channel exit becomes white and bright, implying that a lot of ionization has occurred at the channel exit. In addition, the cathode plume was significantly changed as shown in Fig. 5. From a macroscopic point of view, the oscillation amplitude and mean values of the discharge current exhibit a small change before the current of the exciting Coil 3 increases close to 5 A, and plume is also in a focusing mode. The discharge mode exhibits an obvious change when the current of the exciting coil increases to 5 A. The magnetic field exhibits no significant change as the current of the exciting Coil 3 changes tiny at the value of 5 A, the overall magnetic field structure being unmodified except for in the anode region. The thruster we developed uses a hollow anode with the front end at the position corresponding to the zero-magnetic point. The subtle changes of the magnetic field in front of the zero-magnetic point do not significantly influence the discharge. However, the discharge mode exhibits a sudden change. The cathode position and its magnetic field both influence the discharge. During the test, the cathode position did not change. The current of Coil 3 mainly regulates the magnetic field in the channel while slightly influencing that of the cathode in the plume region. The magnetic field environment of the cathode also did not change.
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Fig. 4. Discharge current before and after the discharge catastrophe.
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Fig. 5. Pictures of the plumes.
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Furthermore, the cathode status almost did not change with the occurrence of the mode catastrophic phenomenon. To study the internal discharge mechanism causing the discharge catastrophe in more detail, we analyze the reasons for the change in discharge parameters and performance parameters of the thruster. We respectively used the Faraday probe, RPA, and spectrum to measure the ion current, ion energy distribution, Xe2+ spectral intensity and electron temperature distribution, making further interpretations of the catastrophic discharge phenomenon. Through the processing of spectroscopic data, the electron temperature near the channel outlet of thruster, and the Xe2+ spectral intensity before and after the discharge catastrophe can be obtained. Fig. 6 shows the changes in Xe2+ spectral intensity and ion current-to-discharge current ratio (I i / I d ) versus the excitation current of Coil 3. According to Fig. 6, after the sudden change of mode, the Xe2+ spectral intensity noticeably increases; the quantity of corresponding Xe2+ ions is large; Xe2+ could hardly be detected before catastrophe, and the ratio of Xe+ /Xe2+ spectral intensity was about 4:3 after the catastrophe. Moreover, the electron temperature of a point at the outlet of the channel was measured. The electron temperature distribution inside the channel and the plume region is continuous [12–15]. Except for the anode region, the overall magnetic field distribution is hardly affected. Although the anode characteristics show subtle changes in the magnetic field in front of the zero-magnetic point, they do not significantly influence the discharge. It can be considered that the electron temperature distribution in this region (near the ionization region) in the two modes cannot widely move. Therefore, the local electron temperature measured near the outlet of channel can quantitatively reflect the overall change of electrons in this region. The electron temperature of the measured before the sudden change of mode is approximately 21 eV; and single ionization mainly occurs. After the discharge catastrophe, the electron temperature increases to 37 eV, providing the electron energy required for double ionization of the xenon atoms, which will result in more Xe2+ ions. Though the electron temperature of the measured at only one point, it could quantitatively reflect the overall change of electrons in this region. Because the electron temperature distribution inside the channel and the plume region is continuous, the changes of magnetic field before and after the catastrophe point are tiny. The results show that the electron temperature in the channel and that in the plume region changed, causing more divalent ionization and the mode catastrophic phenomenon. According to Fig. 6, after the discharge catastrophe, I i / I d decreases significantly from 85% to 60% while the electron current noticeably increases, which further demonstrates that after the mode change, Xe2+ ions increase and a greater electron current is generated. To further analyze the change in discharge characteristics during the catastrophic discharge phenomenon. We used RPA to mea-
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Fig. 6. Change in Xe2+ spectral intensity and I i / I d before and after the discharge catastrophe.
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Fig. 7. Comparison of ion energy distributions before and after the discharge catastrophe.
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sure the ion energy distribution before and after the discharge catastrophe, as shown in Fig. 7. We can see that before the occurrence of the discharge catastrophe, the ion energy is mainly around 250 eV and it is lower than the discharge voltage (300 V). Therefore, there are only a few ions with energy over 300 eV; hence, there are few Xe2+ ions. After the discharge catastrophe, highenergy ions are clearly enhanced. In combination with the spectral processing results, significant numbers of Xe2+ ions occur at this time.
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In short, as the excitation current of Coil 3 increases, the ionization region tends to move toward the channel exit, and at 5-A current, the ionization occurs in the plume region (accompanied by the catastrophic phenomenon of the discharge mode). At this time, the channel is unable to constrain the ionization region completely if the increase of the excitation current of Coil 3 is continued. Under such circumstances, partial ionization occurs in the plume area, so that the plume changes from a focusing state to a divergent state, as shown in Fig. 5. During this process, electron temperature in the plume area increases, the electron energy required for double ionization of xenon atom is achieved, and significant numbers of Xe2+ ions occur at this time. However, there is no significant increase in thrust. This is because many faster divalent ions at the same acceleration voltage were generated upon the occurrence of the mode catastrophic phenomenon. Although some radial loss exists from the divergent plume, the thrust still increases a little after the mode catastrophic phenomenon.
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4. Conclusions
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In this paper, a 1350-W Hall-effect thruster is designed based on a technology for suppressing the magnetic field. A catastrophic discharge phenomenon was induced by varying the excitation current of Coil 3 while keeping all other operating parameters constant. Before and after the discharge catastrophe, the plume state changed from a focusing state to a divergent state. The oscillation amplitude and mean values of the discharge current significantly increased. By combining the characteristics of the characteristics of a thruster’s magnetic field design, a new catastrophic discharge phenomenon was identified. Furthermore, effective measurement and analysis tools were adopted to study in detail the internal discharge mechanism causing the discharge catastrophe. Following on from numerous studies of mode transition, this work enriches the
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literature on mode transition and provides new insights into the mode transition.
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Acknowledgements
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The authors want to gratefully acknowledge the financial support from National Natural Science Foundation of China (Grant Nos. 51477035 and 5177070578).
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