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Effects of cathode operating parameters on the ignition voltage threshold of Hall thrusters
Vacuum 174 (2020) 109169
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Effects of cathode operating parameters on the ignition voltage threshold of Hall thrusters Wen-Bo Li a, Hong Li a, b, Yong-Jie Ding a, b, **, Li-Qiu Wei a, b, *, Xin-Yong Yang a, Hai-Kuo Cai a, Jian-Ning Sun a, Da-Ren Yu a, b a b
School of Energy Science and Engineering, Harbin Institute of Technology, Harbin, Heilongjiang, 150001, People’s Republic of China Key Laboratory of Aerospace Plasma Propulsion, Ministry of Industry and Information Technology, Harbin, Heilongjiang, 150001, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Keywords: Hall thruster Cathode operating parameters Ignition voltage threshold Langmuir probe
Changing the operating parameters of a cathode affects the ignition process of a Hall thruster mainly by changing the micro-plasma parameters emitted by the cathode at the moment of ignition. However, the micro-plasma parameter that dominates this effect is not clear. Therefore, the ignition voltage thresholds with different cathode operating parameters were first measured. Then, to better understand how these operating parameters affect the ignition voltage threshold of a Hall thruster, the two-dimensional distribution characteristics of the electron density, electron temperature, and plasma potential in the cathode plume region were measured using a Langmuir probe. The results showed that increasing both the keeper current and mass flow rate of the cathode causes an increase in the electron density emitted by the cathode and a decrease in the electron temperature. It was also found that increasing the cathode mass flow rate has a more pronounced effect than increasing the keeper current. Furthermore, when both the mass flow rate and keeper current of the hollow cathode are increased, the ignition voltage threshold of the Hall thruster decreases significantly. This is mainly because of a significant increase in the electron density resulting from the increases in the mass flow rate and keeper current.
1. Introduction Space exploration provides a gateway for improving our under standing of the universe [1]. In recent years, the frequency of space launches has drastically increased, especially for cube satellites with relatively low launch costs [2,3]. With the vigorous development of space industries around the world [4], human interest in space explo ration has expanded to include not only satellite constellation networks [5], but also moon exploration programs; even deep-propulsion missions and interplanetary travel, including Mars colonization, are being seri ously considered [6,7]. Because these space exploration activities usu ally require propulsion devices that work for long periods or have finely adjustable thrusts, it is difficult to use traditional chemical propulsion devices to meet these mission requirements. Therefore, more efficient thrusters that may be used for longer periods are required. Electric propulsion systems are able to achieve these qualities [7–9]. To improve these systems such that they better meet the needs of the
aforementioned space explorations, novel techniques [8], smart nano materials [9,10], and efficient information technologies [11] have been incorporated with these electric propulsion devices to take advantage of their unique features, as well as to improve their efficiencies and service lives. Owing to their moderate specific impulse value and simple structure, along with other desirable characteristics, Hall thrusters are a typical electrical propulsion device that are widely used in various space pro pulsion applications such as satellite station maintenance and orbital transfer [12–14]. As a critical component of the discharge circuit [15–17], the main purpose of the hollow cathode present in these sys tems is to provide high-energy electrons for neutralizing the ion plumes and ionizing the propellants, which have an important effect on the discharge process of Hall thrusters. Therefore, many researchers have investigated the effects of various cathode operating parameters on the steady-state discharge process and performance parameters of Hall thrusters.
* Corresponding author. Key Laboratory of Aerospace Plasma Propulsion, Ministry of Industry and Information Technology, Harbin, Heilongjiang, 150001, People’s Republic of China. ** Corresponding author. School of Energy Science and Engineering, Harbin Institute of Technology, Harbin, Heilongjiang, 150001, People’s Republic of China. E-mail addresses: [email protected] (Y.-J. Ding), [email protected] (L.-Q. Wei). https://doi.org/10.1016/j.vacuum.2020.109169 Received 1 November 2019; Received in revised form 17 December 2019; Accepted 4 January 2020 Available online 7 January 2020 0042-207X/© 2020 Elsevier Ltd. All rights reserved.
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The mitigation of the facility pressure effects by varying the cathode mass flow rate of a 9-kW magnetically shielded Hall thruster with internally and externally mounted cathodes was investigated by Cusson et al. [18]. Their results showed that when the cathode is internally mounted, the thrust was hardly affected by the pressure of the facility. The effects of the cathode placement on the performance parameters of a 10.5-kW KM-10 Hall thruster was experimentally studied by Shashkov et al. [19]. It was found that regardless of whether the cathode was internally or externally mounted, the total specific impulse of the thruster decreased as the cathode mass flow rate increased. The accel erated erosion of the keeper electrode during the coupling discharge between the Hall thruster and hollow cathode was studied by Meng et al. [20]. Their results showed that the increased mass flow rate of the cathode had little effect in the 200–300 eV range, while the total effi ciency of the thruster decreased due to the increased propellant con sumption. The operation of a hollow cathode neutralizer for sub-100-W Hall and ion thrusters was studied by Lev et al. [21]. It was found that at the same discharge current, increasing the mass flow rate of the cathode resulted in increasing the coupling power of the cathode. The effects of the cathode heat power on the beam-focusing characteristics and per formance parameters of Hall thrusters were investigated by Ning et al. [22,23]. They found that as the cathode heat power was gradually increased, the half-divergence angle of the Hall thruster initially changed over a small range and then suddenly deteriorated after reaching a critical heating power; in other words, the thrust increment first increased and then decreased. The effects of the heat power of a hot wire cathode on the performance of a cylindrical Hall thruster (CHT) were investigated by Granstedt et al. [24]. Their results showed that as the electron emission from the cathode increased due to wire heating, the divergence angle of the plume gradually decreased and the discharge current increased. Tilley et al. investigated the effects of the mass flow rate and position of the hollow cathode on the floating voltage of a laboratory BPT-4000 Hall thruster [25]. Their results showed that the floating voltage was a monotonic function of the cathode mass flow rate. The effects of the cathode mass flow rate on the cathode coupling voltage and anode efficiency of a 50-kW Hall thruster were studied by Manzella et al. [26]. They found that as the cathode mass flow rate increased, the cathode coupling voltage decreased, thereby improving the anode efficiency. Goebel et al. studied the effects of the cathode mass flow rate on the performance and cathode life of a 6-kW laboratory Hall thruster [27]. As the cathode flow fraction was gradually increased in their study, the thrust, anode efficiency, and voltage utilization effi ciency were also observed to gradually increase. Raitses et al. studied the effects of the keeper current of the cathode on the performance pa rameters of a CHT [28]. They found that as the keeper current of a CHT in the direct magnetic field configuration is gradually increased, the discharge current and propellant utilization gradually increase, while the plume divergence angle decreases dramatically. The ignition process is the first and most critical step in the safe operation of a Hall thruster [29–31]. The cathode mass flow rate and keeper current affect the densities and temperatures of electrons emitted by the cathode, which in turn significantly affect the ignition process. Because the Hall thruster ignition process typically occurs on the order of tens of microseconds, numerical simulations and high-speed time-resolved imaging are typically used to study this process. Tacco gna et al. used a two-dimensional axisymmetric model to study the plasma parameters at different times during Hall thruster ignition [32]. This model was improved by Liu et al. and successfully reproduced the ignition pulse current; the relationships between the ignition pulse current, discharge voltage, and mass flow rate were also studied [33]. Vial et al. used a high-speed charge-coupled device camera to study the dynamic behavior of the plasma plume of a SPT-100 Hall thruster. They found that the ion beam revealed oscillations in the plasma light in tensity that were associated with breathing-mode instabilities [34]. A similar method was used by Ellison et al. to capture the front image of the plume during Hall thruster ignition. The results indicated that the
hollow cathode introduced an azimuthal asymmetry, which persisted for approximately 30 μs into the ignition process [35]. To date, research on various cathode operating parameters has focused mainly on their effects on the steady-state performance pa rameters of Hall thrusters, such as the thrust, anode efficiency, and propellant utilization. Additionally, this research has largely focused on macro-level phenomena. However, the effects of changing the operating parameters of the cathode on the ignition process of these thrusters mainly results in changes in the parameters of the micro-plasma emitted by the cathode (such as the electron density and electron temperature). These changes significantly affect the ignition process of Hall thrusters, and whether the electron density or electron temperature dominates this effect is unclear. Therefore, the effects of the cathode operating pa rameters on the ignition process of Hall thrusters deserves the attention of researchers. To better understand how the cathode mass flow rate and keeper current affect Hall thruster ignition, the ignition voltage thresholds at different keeper currents and mass flow rates of the cathode were measured for a 1-kW Hall thruster. To more thoroughly analyze the effects on the plasma parameters of different mass flow rates and keeper currents, the electron density, electron temperature, and plasma po tential were measured by a Langmuir probe in the axial and radial di rections. The results showed that the cathode mass flow rate and keeper current can significantly affect the density and temperature of the electrons emitted by the cathode. This paper is organized as follows: the experimental apparatus and measurements are described in Section 2. In Section 3, the experimental results and discussion are presented. The conclusions are then presented in Section 4. 2. Experimental apparatus and measurements Our experiments were conducted in a vacuum chamber with two diffusion pumps (40,000 L/s), one rotary pump, and three mechanical booster pumps; the diameter and length of the vacuum chamber are 1.5 m and 4 m, respectively. The ultimate pressure values before and during the operation of the vacuum tank were 7 � 10 4 pa and 8.5 � 10 3 pa, respectively, for xenon. The Hall thruster used in the experiments was a 1-kW laboratory Hall thruster. The diameters of the inner and outer insulators of the experimental thruster are 70 and 100 mm, respectively. The length of the acceleration channel is 50 mm. A self-heated hollow cathode was used to provide electrons and neutralize the ejected ions. To facilitate the measurement of the plasma parameters in the radial and axial directions in the central plane of the cathode keeper exit, the cathode and discharge channel of the thruster were arranged in parallel, and the central axes of the cathode orifice and the thruster were parallel to each other. The plasma parameters at different mass flow rates and keeper cur rents of the cathode were measured by a Langmuir probe in the axial and radial directions to analyze the effects of these parameters on the plasma. The probe wires consist of a tungsten wire with a 0.3 mm diameter coated with alumina ceramic tubes with diameters of 0.4 mm, and the probe tip is flushed with the alumina ceramic tubes. To better measure the plasma parameters in the radial and axial directions in the central plane of the cathode keeper exit, a single Langmuir probe was installed on a two-dimensional stepping motor turntable. A 10 kΩ capacitor and 0.1 μF resistor were used in the measurement circuit of the single Langmuir probe. The probe voltage and scanning voltage were measured using a Yokogawa DL850 ScopeCorder. A schematic diagram of the Hall thruster and measurement circuit are shown in Fig. 1. The electrons are first energized in the electric field, and they then ionize the neutral atoms inside of the channel [8]. The generated ions are expelled by the axial electric field between the cathode and anode, thereby generating thrust. This thrust may then be applied to a wide variety in operations in space, such as maintaining the north-south position of a satellite [30,31]. 2
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Fig. 1. Schematic diagram of the Hall thruster and cathode plasma parameter measurement platform.
3. Experimental results and discussion
or 6.0 sccm. When the mass flow rate of the cathode is fixed at 3 sccm or 6 sccm, the ignition voltage threshold of the Hall thruster decreases as the cathode keeper current increases; at the higher cathode mass flow rate (6 sccm), the reduction of the ignition voltage threshold of the Hall thruster is more significant. This occurs because increasing the cathode keeper current increases the ability of the cathode to emit electrons [28], and therefore the larger the cathode keeper current, the more electrons the cathode can emit. An increased number of electrons can then pass through the magnetic field and enter the discharge channel. At the same time, the energy that approaches the ionization threshold of the xenon atoms is obtained from the axial electric field between the cathode and anode, and more high-energy electrons are generated. This makes it easier to promote the avalanche ionization effect during the ignition process of the Hall thruster. Therefore, when the cathode keeper current is relatively large, the Hall thruster can be ignited at a relatively low voltage threshold. Moreover, when the cathode mass flow is large (6 sccm), the number of electrons emitted by the cathode increases, and this effect becomes more obvious. Fig. 3 shows the ignition voltage threshold when the cathode is operated at mass flow rates of 3.0 sccm or 6.0 sccm and cathode keeper currents of 2.5 A or 3.0 A. When the keeper current of the cathode is fixed at 2.5 A or 3.0 A, the ignition voltage threshold of the Hall thruster clearly decreases as the cathode mass flow rate increases. This is mainly due to the relatively low coupling voltage between the cathode and thruster at the higher cathode mass flow rate [27]. A decrease in the coupling voltage is considered to indicate the ease with which electrons can flow from the cathode to the anode. When this coupling voltage drop is lower, the electrons can relatively easily cross the magnetic field near the exit of the thruster to reach the anode in the discharge channel. Therefore, more electrons can obtain energy in the axial electric field between the cathode and anode to reach the ionization threshold of the xenon atoms, thereby making the ignition process of the Hall thruster more likely to occur. The resulting ignition voltage threshold of the Hall thruster will then be relatively low. Furthermore, Fig. 2 also shows that by keeping the other ignition parameters constant and increasing either the mass flow rate of the cathode from 3 sccm to 6 sccm or the keeper current of the cathode from 1.5 A to 3.0 A, the ignition voltage threshold of the Hall thruster is significantly reduced. The maximum reduction of the ignition voltage threshold that was obtained is more than 72 V. This
During the experimental measurements of the ignition voltage threshold, the Hall thruster coil current is 2.0 A, the heating current of the cathode is 8.0 A, the anode mass flow rate is fixed at a certain value, and the cathode mass flow rate or keeper current acts as the control variable. The discharge voltage of the anode is continuously varied from 0 V to 600 V at a rate of 1 V/s through a controllable power supply. As the anode discharge voltage gradually increases, the Hall thruster suc cessfully ignites, and the LabVIEW control platform records the discharge voltage at this instant as the ignition voltage threshold. Fig. 2 shows the ignition voltage threshold when the cathode is operated at keeper currents of 1.5 A or 3.0 A and cathode mass flow rates of 3.0 sccm
Fig. 2. Ignition voltage threshold of the Hall thruster with different cathode keeper currents and cathode mass flow rates. The respective cathode mass flow rates and keeper currents are: 3.0 sccm and 1.5 A for the solid blue line with square marks; 3.0 sccm and 3.0 A for the dotted red line with triangular marks; 6.0 sccm and 1.5 A for the solid black line with circular marks; and 6.0 sccm and 3.0 A for the dotted green line with five-pointed star marks. (For inter pretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 3
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cathode to emit electrons, such that more electrons can pass through the magnetic field lines near the thruster exit and enter the discharge channel. This makes the thruster ignition process more likely to occur. It is therefore speculated that, compared with the reduction obtained by increasing the cathode keeper current, increasing the cathode mass flow rate can reduce the thruster ignition voltage threshold to a greater extent as more electrons will be produced by the cathode. Furthermore, changes in the cathode keeper current and mass flow rate also cause changes in the plasma parameters of the cathode plume. To better un derstand these changes in the plasma parameters, the electron density, electron temperature, and plasma potential at different keeper currents and mass flow rates were measured using a Langmuir probe in the axial (downstream of the keeper exit, over a distance ranging from 10 to 24 mm from the exit) and radial (relative to the keeper centerline from 0 to 10 mm and 0 to 10 mm) directions. The electron density contours for the different keeper currents and mass flow rates of the cathode are shown in Fig. 4. At the same cathode mass flow rate (3 sccm or 6 sccm), the electron density of the cathode plume increases significantly as the cathode keeper current increases from 1.5 A to 3.0 A, especially near the cathode keeper exit (10–12 mm from the keeper exit). In addition, the results obtained by Raitses et al. also showed that increasing the keeper current increases the electron emission capability of the cathode [28]. Fig. 4(a) and (c), as well as Fig. 4(b) and (d), show that at the same cathode keeper current (1.5 A or 3.0 A), the electron density of the cathode plume increases significantly as the cathode mass flow rate increases from 3.0 sccm to 6.0 sccm. This occurs because a significant pressure increase appears inside of the cathode when the cathode keeper current is constant and the cathode mass flow rate is increased, and the initial electrons emitted by the cathode emitter can then collide with more Xe atoms [36]. Conse quently, the ion density in the emitter barrel increases, and thus more ions bombard the emitter to produce more electrons, increasing the electron density of the cathode plume.
Fig. 3. Ignition voltage threshold of the Hall thruster with different cathode mass flow rates and cathode keeper currents. The respective keeper currents and cathode mass flow rates are: 2.5 A and 3.0 sccm for the solid blue line with square marks; 2.5 A and 6.0 sccm for the solid green line with triangular marks; 3.0 A and 3.0 sccm for the dotted red line with circular marks; and 3.0 A and 6.0 sccm for the dotted black line with five-pointed star marks. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
phenomenon may result from a significant increase in the electron density due to the increases in the mass flow rate and keeper current. According to these experimental results, as the keeper current or mass flow rate of the cathode increases, the ignition voltage threshold of the Hall thruster is significantly reduced, and the reduction in the ignition voltage threshold is more significant when the mass flow rate is increased. In previous studies [27,28], it was shown that increasing the cathode mass flow rate and keeper current enhances the ability of the
Fig. 4. Electron density (cm 3) contours for different keeper currents and mass flow rates of the cathode. The respective mass flow rates and keeper currents of the cathode are: (a) 3.0 sccm and 1.5 A; (b) 3.0 sccm and 3.0 A; (c) 6.0 sccm and 1.5 A; and (d) 6.0 sccm and 3.0 A. 4
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The plasma potential contours for different keeper currents and mass flow rates of the cathode are shown in Fig. 5. The plasma potential is lowest near the centerline of the cathode keeper exit plane, and it in creases gradually as the axial distance from the cathode keeper exit in creases. The change in the plasma potential is similar to that reported in Ref. [37]. Furthermore, at a constant cathode keeper current, the plasma potential in the cathode plume gradually decreases with increasing cathode mass flow rate. This change in the plasma potential may be related to the changes in the cathode coupling voltage observed in previous Hall thruster experiments [27], in which increasing the mass flow rate of the cathode reduced the plasma potential in the cathode plume. This resulted in a reduction of the coupling voltage drop between the thruster and cathode. These phenomena may result from an increase in the plasma density inside of the cathode and plume region due to increasing the cathode mass flow rate or keeper current. The resistivity inside of the cathode and plume region is thereby reduced, such that the plasma potential of the cathode plume region is also reduced. In particular, when the cathode mass flow rate and keeper current are increased at the same time, the electron density in the cathode plume region is significantly increased, and the plasma potential in the cathode plume region is reduced more significantly. The electron temperature contours at different keeper currents and mass flow rates of the cathode are shown in Fig. 6. The electron tem perature at different keeper currents and mass flow rates seems to be correlated with the plasma potential. As the plasma potential increases, the electrons are accelerated, causing the electron temperature to rise. Similar results have previously been reported [38]. Fig. 6 shows that at a constant cathode mass flow rate (3 sccm or 6 sccm), the electron tem perature of the cathode plume decreases as the cathode keeper current increases from 1.5 A to 3.0 A, especially near the cathode keeper exit. Moreover, Fig. 6(a) and (c), as well as Fig. 6(b) and (d) show that at a constant cathode keeper current (1.5 A or 3.0 A), the electron
temperature of the cathode plume decreases significantly as the cathode mass flow rate increases from 3.0 sccm to 6.0 sccm. As can also be seen in Fig. 4, this phenomenon occurs because as the cathode keeper current or mass flow rate is increased, the electron density in the cathode plume region significantly increases, and the electron density in the cathode emitter and orifice regions also significantly increases. When a large number of electrons flow from the orifice region to the plume region, particle collisions occur in the orifice region. When the electron density is high, the collision frequency will be significantly increased and the consumption of electron energy will also be relatively large, resulting in a lower temperature of the electrons flowing out of the cathode plume region. It can also be seen in Fig. 5 that as the cathode keeper current or mass flow rate is increased, the plasma potential in the cathode plume region is significantly reduced. The energy that can be obtained by the electrons flowing from the orifice region of the cathode to the plume region will subsequently be relatively low. The experimental results of Goebel et al. also show that increasing the mass flow rate of the cathode while holding the other parameters constant reduces the electron tem perature [39]. Furthermore, increasing the cathode mass flow rate produces a more significant reduction in the electron temperature in the cathode plume region than when the cathode keeper current is increased. The main reason for this phenomenon is that, compared with increasing the cathode keeper current, increasing the mass flow rate of the cathode increases the density of electrons emitted by the cathode. As a result, the collision frequency is higher when electrons flow through the orifice region of the cathode and the energy loss of these electrons is therefore greater, thereby lowering the electron temperature in the cathode plume. The ignition voltage thresholds measured under different cathode keeper currents and mass flow rates revealed that when the other operating parameters are constant, increasing the cathode keeper cur rent or mass flow rate results in a significant decrease in the ignition
Fig. 5. Plasma potential (V) contours for different keeper currents and mass flow rates of the cathode. The respective cathode mass flow rates and keeper currents are: (a) 3.0 sccm and 1.5 A; (b) 3.0 sccm and 3.0 A; (c) 6.0 sccm and 1.5 A; (d) 6.0 sccm and 3.0 A. 5
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Fig. 6. Electron temperature (eV) contours for different keeper currents and mass flow rates of the cathode. The respective cathode mass flow rates and keeper currents are: (a) 3.0 sccm and 1.5 A; (b) 3.0 sccm and 3.0 A; (c) 6.0 sccm and 1.5 A; and (d) 6.0 sccm and 3.0 A.
cathode to the anode. Consequently, it is relatively easy for the electrons emitted by the cathode to enter the discharge channel during the igni tion process of a Hall thruster. When the cathode keeper current and mass flow rate are increased, the Hall thruster can therefore be ignited successfully at a relatively low ignition voltage threshold. Furthermore, although increasing the cathode keeper current and mass flow rate causes the electron temperature of the cathode plume to decrease, the initial electron temperature of the cathode emission is usually low (around several electron volts [40–42]) and the electrons emitted by the cathode gain energy from the axial electric field between the cathode and anode as they enter the discharge channel. Therefore, although increasing the cathode keeper current and mass flow rate may decrease the temperature of the electrons emitted by the cathode, this decrease may be compensated for by the energy obtained from the axial electric field.
voltage threshold of the Hall thruster. In particular, the reduction in the ignition voltage threshold is larger when both parameters are increased. Increasing the cathode mass flow rate results in a larger reduction of the ignition voltage threshold than is obtained by increasing the cathode keeper current. The measured plasma parameters of the cathode plume under different cathode keeper currents and mass flow rates show that increasing the cathode keeper current and mass flow rate significantly increases the electron density near the keeper region of the cathode. This leads to a decrease in the electron temperature in the cathode plume, especially when both parameters are increased simultaneously. Increasing the cathode mass flow rate causes a larger increase in the electron density near the cathode keeper exit than is obtained by increasing the cathode keeper current. According to the ignition voltage thresholds and plasma parameters measured under different cathode keeper currents and mass flow rates, the decrease in the ignition voltage threshold of a Hall thruster resulting from increasing the cathode keeper current and mass flow rate is mainly due to a significant increase in the electron density in the cathode plume region. This increase accompanies the increases of the aforementioned parameters. The following two factors may contribute to this phenomenon. First, owing to the signifi cant increase in the electron density in the cathode plume region, more electrons near the cathode keeper exit can be bound by the nearby magnetic field lines. Therefore, energy can be obtained from the axial electric field when the electrons enter the discharge channel, and more high-energy electrons are produced. These high-energy electrons collide with neutral atoms that were previously concentrated in the thruster exit and discharge channel, enabling avalanche ionization to be established more quickly. Second, the cathode mass flow rate affects the coupling voltage drop between the cathode and thruster. When the mass flow rate of the cathode is increased, the plasma potential in the cathode plume decreases, which decreases the coupling voltage drop between the thruster and cathode [27,39]. The coupling voltage drop is considered to be an indicator of the ease with which electrons can flow from the
4. Conclusion To date, research on the operating parameters of cathodes in Hall thrusters has focused mainly on their effects towards the steady-state performance parameters of these thrusters. The effects of the cathode keeper current and mass flow rate on the ignition voltage threshold of a Hall thruster are unknown. Therefore, the ignition voltage thresholds were measured at different values of these cathode operating parame ters. To better understand how these operating parameters affect the ignition voltage threshold of Hall thrusters, the electron density, elec tron temperature, and plasma potential in the cathode plume region were measured using a Langmuir probe in the axial and radial di rections. The results showed that increasing both the keeper current and mass flow rate of the cathode increases the electron density emitted by the cathode and decreases the electron temperature. It was found that increasing the cathode mass flow rate has a more pronounced effect on these parameters than increasing the keeper current. In addition, when 6
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the mass flow rate and keeper current of the hollow cathode increase simultaneously, the electron density increases and the ignition voltage threshold of the Hall thruster decreases significantly.
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Declaration of competing interest The author(s) declare no competing interests. Acknowledgments The authors acknowledge the support of the National Natural Sci ence Foundation of China (Grant Nos. 51736003 and 51777045) and the Science and Technology Innovation Project of Hunan Province (Project No. 2018RS3146). References [1] B. Zypries, Space, the public, and politics, Space Policy 41 (2017) 73–74. [2] C. Nieto-Peroy, M.R. Emami, Cubesat mission: from design to operation, Appl. Sci. 9 (2019) 3110. [3] I. Levchenko, M. Keidar, J. Cantrell, Y.L. Wu, H. Kuninaka, K. Bazaka, S.Y. Xu, Explore space using swarms of tiny satellites, Nat 562 (2018) 185–187. [4] N. Kishi, Management analysis for the space industry, Space Policy 39–40 (2017) 1–6. [5] S.Y. Ye, Y. Zhang, W. Yao, Q. Chen, X.Q. Chen, An effective surrogate ensemble modeling method for satellite coverage traffic volume prediction, Appl. Sci. 9 (2019) 3689. [6] I. Levchenko, S.Y. Xu, S. Mazouffre, M. Keidar, K. Bazaka, Mars colonization: beyond getting there, Glob. Chall. 3 (2019) 1800062. [7] E.Y. Choueiri, A critical history of electric propulsion: the first 50 years (19061956), J. Propuls. Power 20 (2) (2004) 193–203. [8] S. Mazouffre, Electric propulsion for satellites and spacecraft: established technologies and novel approaches, Plasma Sources Sci. Technol. 25 (2016), 033002. [9] I. Levchenko, S.Y. Xu, G. Teel, D. Mariotti, M.L.R. Walker, M. Keidar, Recent progress and perspectives of space electric propulsion systems based on smart nanomaterials, Nat. Commun. 9 (1) (2018) 879. [10] I. Levchenko, K. Bazaka, T. Belmonte, M. Keidar, S.Y. Xu, Advanced materials for next-generation spacecraft, Adv. Mater. 30 (2018) 1802201. [11] C. Wang, Y.S. Cao, W.D. Wang, Data-aided frequency offset estimation for CEOFDM broadband satellite systems, Appl. Sci. 9 (2019) 2310. [12] I. Levchenko, K. Bazaka, Y.J. Ding, Y. Raitses, S. Mazouffre, T. Henning, P.J. Klar, S. Shinohara, J. Schein, L. Garrigues, M. Kim, D. Lev, F. Taccogna, R.W. Boswell, C. Charlesal, H. Koizumi, Y. Shen, C. Scharlemann, M. Keidar, S.Y. Xu, Space micropropulsion systems for Cubesats and small satellites: from proximate targets to furthermost frontiers, Appl. Phys. Rev. 5 (2018), 011104. [13] L.Q. Wei, J. Li, L. Han, Z.Y. Yang, D.R. Yu, C.H. Zhang, X.B. He, Study on the peak current of power supply during a Hall thruster start-up, Vacuum 123 (2016) 126–130. [14] P. Duan, X. Li, H.J. Shen, L. Chen, E. Peng, Characteristics of a sheath with secondary electron emission in the double walls of a Hall thruster, Plasma Sci. Technol. 14 (9) (2012) 837–841. [15] D.M. Goebel, R.M. Watkins, K.K. Jameson, LaB6 hollow cathodes for ion and Hall thrusters, J. Propuls. Power 23 (3) (2007) 552–558. [16] I. Levchenko, K. Bazaka, S. Mazouffre, S.Y. Xu, Prospects and physical mechanisms for photonic space propulsion, Nat. Photonics 12 (11) (2018) 649–657. [17] A.L. Ortega, B.A. Jorns, I.G. Mikellides, Hollow cathode simulations with a firstprinciples model of ion-acoustic anomalous resistivity, J. Propuls. Power 34 (4) (2018) 1–13. [18] S.E. Cusson, M.P. Byrne, B.A. Jorns, A.D. Gallimore, Investigation into the use of cathode flow fraction to mitigate pressure-related facility effects on a magnetically shielded Hall thruster, in: 50th AIAA Propulsion and Energy Forum, Indianapolis, 2019. AIAA PEF-4077.
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Vacuum journal homepage: http://www.elsevier.com/locate/vacuum
Effects of cathode operating parameters on the ignition voltage threshold of Hall thrusters Wen-Bo Li a, Hong Li a, b, Yong-Jie Ding a, b, **, Li-Qiu Wei a, b, *, Xin-Yong Yang a, Hai-Kuo Cai a, Jian-Ning Sun a, Da-Ren Yu a, b a b
School of Energy Science and Engineering, Harbin Institute of Technology, Harbin, Heilongjiang, 150001, People’s Republic of China Key Laboratory of Aerospace Plasma Propulsion, Ministry of Industry and Information Technology, Harbin, Heilongjiang, 150001, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Keywords: Hall thruster Cathode operating parameters Ignition voltage threshold Langmuir probe
Changing the operating parameters of a cathode affects the ignition process of a Hall thruster mainly by changing the micro-plasma parameters emitted by the cathode at the moment of ignition. However, the micro-plasma parameter that dominates this effect is not clear. Therefore, the ignition voltage thresholds with different cathode operating parameters were first measured. Then, to better understand how these operating parameters affect the ignition voltage threshold of a Hall thruster, the two-dimensional distribution characteristics of the electron density, electron temperature, and plasma potential in the cathode plume region were measured using a Langmuir probe. The results showed that increasing both the keeper current and mass flow rate of the cathode causes an increase in the electron density emitted by the cathode and a decrease in the electron temperature. It was also found that increasing the cathode mass flow rate has a more pronounced effect than increasing the keeper current. Furthermore, when both the mass flow rate and keeper current of the hollow cathode are increased, the ignition voltage threshold of the Hall thruster decreases significantly. This is mainly because of a significant increase in the electron density resulting from the increases in the mass flow rate and keeper current.
1. Introduction Space exploration provides a gateway for improving our under standing of the universe [1]. In recent years, the frequency of space launches has drastically increased, especially for cube satellites with relatively low launch costs [2,3]. With the vigorous development of space industries around the world [4], human interest in space explo ration has expanded to include not only satellite constellation networks [5], but also moon exploration programs; even deep-propulsion missions and interplanetary travel, including Mars colonization, are being seri ously considered [6,7]. Because these space exploration activities usu ally require propulsion devices that work for long periods or have finely adjustable thrusts, it is difficult to use traditional chemical propulsion devices to meet these mission requirements. Therefore, more efficient thrusters that may be used for longer periods are required. Electric propulsion systems are able to achieve these qualities [7–9]. To improve these systems such that they better meet the needs of the
aforementioned space explorations, novel techniques [8], smart nano materials [9,10], and efficient information technologies [11] have been incorporated with these electric propulsion devices to take advantage of their unique features, as well as to improve their efficiencies and service lives. Owing to their moderate specific impulse value and simple structure, along with other desirable characteristics, Hall thrusters are a typical electrical propulsion device that are widely used in various space pro pulsion applications such as satellite station maintenance and orbital transfer [12–14]. As a critical component of the discharge circuit [15–17], the main purpose of the hollow cathode present in these sys tems is to provide high-energy electrons for neutralizing the ion plumes and ionizing the propellants, which have an important effect on the discharge process of Hall thrusters. Therefore, many researchers have investigated the effects of various cathode operating parameters on the steady-state discharge process and performance parameters of Hall thrusters.
* Corresponding author. Key Laboratory of Aerospace Plasma Propulsion, Ministry of Industry and Information Technology, Harbin, Heilongjiang, 150001, People’s Republic of China. ** Corresponding author. School of Energy Science and Engineering, Harbin Institute of Technology, Harbin, Heilongjiang, 150001, People’s Republic of China. E-mail addresses: [email protected] (Y.-J. Ding), [email protected] (L.-Q. Wei). https://doi.org/10.1016/j.vacuum.2020.109169 Received 1 November 2019; Received in revised form 17 December 2019; Accepted 4 January 2020 Available online 7 January 2020 0042-207X/© 2020 Elsevier Ltd. All rights reserved.
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The mitigation of the facility pressure effects by varying the cathode mass flow rate of a 9-kW magnetically shielded Hall thruster with internally and externally mounted cathodes was investigated by Cusson et al. [18]. Their results showed that when the cathode is internally mounted, the thrust was hardly affected by the pressure of the facility. The effects of the cathode placement on the performance parameters of a 10.5-kW KM-10 Hall thruster was experimentally studied by Shashkov et al. [19]. It was found that regardless of whether the cathode was internally or externally mounted, the total specific impulse of the thruster decreased as the cathode mass flow rate increased. The accel erated erosion of the keeper electrode during the coupling discharge between the Hall thruster and hollow cathode was studied by Meng et al. [20]. Their results showed that the increased mass flow rate of the cathode had little effect in the 200–300 eV range, while the total effi ciency of the thruster decreased due to the increased propellant con sumption. The operation of a hollow cathode neutralizer for sub-100-W Hall and ion thrusters was studied by Lev et al. [21]. It was found that at the same discharge current, increasing the mass flow rate of the cathode resulted in increasing the coupling power of the cathode. The effects of the cathode heat power on the beam-focusing characteristics and per formance parameters of Hall thrusters were investigated by Ning et al. [22,23]. They found that as the cathode heat power was gradually increased, the half-divergence angle of the Hall thruster initially changed over a small range and then suddenly deteriorated after reaching a critical heating power; in other words, the thrust increment first increased and then decreased. The effects of the heat power of a hot wire cathode on the performance of a cylindrical Hall thruster (CHT) were investigated by Granstedt et al. [24]. Their results showed that as the electron emission from the cathode increased due to wire heating, the divergence angle of the plume gradually decreased and the discharge current increased. Tilley et al. investigated the effects of the mass flow rate and position of the hollow cathode on the floating voltage of a laboratory BPT-4000 Hall thruster [25]. Their results showed that the floating voltage was a monotonic function of the cathode mass flow rate. The effects of the cathode mass flow rate on the cathode coupling voltage and anode efficiency of a 50-kW Hall thruster were studied by Manzella et al. [26]. They found that as the cathode mass flow rate increased, the cathode coupling voltage decreased, thereby improving the anode efficiency. Goebel et al. studied the effects of the cathode mass flow rate on the performance and cathode life of a 6-kW laboratory Hall thruster [27]. As the cathode flow fraction was gradually increased in their study, the thrust, anode efficiency, and voltage utilization effi ciency were also observed to gradually increase. Raitses et al. studied the effects of the keeper current of the cathode on the performance pa rameters of a CHT [28]. They found that as the keeper current of a CHT in the direct magnetic field configuration is gradually increased, the discharge current and propellant utilization gradually increase, while the plume divergence angle decreases dramatically. The ignition process is the first and most critical step in the safe operation of a Hall thruster [29–31]. The cathode mass flow rate and keeper current affect the densities and temperatures of electrons emitted by the cathode, which in turn significantly affect the ignition process. Because the Hall thruster ignition process typically occurs on the order of tens of microseconds, numerical simulations and high-speed time-resolved imaging are typically used to study this process. Tacco gna et al. used a two-dimensional axisymmetric model to study the plasma parameters at different times during Hall thruster ignition [32]. This model was improved by Liu et al. and successfully reproduced the ignition pulse current; the relationships between the ignition pulse current, discharge voltage, and mass flow rate were also studied [33]. Vial et al. used a high-speed charge-coupled device camera to study the dynamic behavior of the plasma plume of a SPT-100 Hall thruster. They found that the ion beam revealed oscillations in the plasma light in tensity that were associated with breathing-mode instabilities [34]. A similar method was used by Ellison et al. to capture the front image of the plume during Hall thruster ignition. The results indicated that the
hollow cathode introduced an azimuthal asymmetry, which persisted for approximately 30 μs into the ignition process [35]. To date, research on various cathode operating parameters has focused mainly on their effects on the steady-state performance pa rameters of Hall thrusters, such as the thrust, anode efficiency, and propellant utilization. Additionally, this research has largely focused on macro-level phenomena. However, the effects of changing the operating parameters of the cathode on the ignition process of these thrusters mainly results in changes in the parameters of the micro-plasma emitted by the cathode (such as the electron density and electron temperature). These changes significantly affect the ignition process of Hall thrusters, and whether the electron density or electron temperature dominates this effect is unclear. Therefore, the effects of the cathode operating pa rameters on the ignition process of Hall thrusters deserves the attention of researchers. To better understand how the cathode mass flow rate and keeper current affect Hall thruster ignition, the ignition voltage thresholds at different keeper currents and mass flow rates of the cathode were measured for a 1-kW Hall thruster. To more thoroughly analyze the effects on the plasma parameters of different mass flow rates and keeper currents, the electron density, electron temperature, and plasma po tential were measured by a Langmuir probe in the axial and radial di rections. The results showed that the cathode mass flow rate and keeper current can significantly affect the density and temperature of the electrons emitted by the cathode. This paper is organized as follows: the experimental apparatus and measurements are described in Section 2. In Section 3, the experimental results and discussion are presented. The conclusions are then presented in Section 4. 2. Experimental apparatus and measurements Our experiments were conducted in a vacuum chamber with two diffusion pumps (40,000 L/s), one rotary pump, and three mechanical booster pumps; the diameter and length of the vacuum chamber are 1.5 m and 4 m, respectively. The ultimate pressure values before and during the operation of the vacuum tank were 7 � 10 4 pa and 8.5 � 10 3 pa, respectively, for xenon. The Hall thruster used in the experiments was a 1-kW laboratory Hall thruster. The diameters of the inner and outer insulators of the experimental thruster are 70 and 100 mm, respectively. The length of the acceleration channel is 50 mm. A self-heated hollow cathode was used to provide electrons and neutralize the ejected ions. To facilitate the measurement of the plasma parameters in the radial and axial directions in the central plane of the cathode keeper exit, the cathode and discharge channel of the thruster were arranged in parallel, and the central axes of the cathode orifice and the thruster were parallel to each other. The plasma parameters at different mass flow rates and keeper cur rents of the cathode were measured by a Langmuir probe in the axial and radial directions to analyze the effects of these parameters on the plasma. The probe wires consist of a tungsten wire with a 0.3 mm diameter coated with alumina ceramic tubes with diameters of 0.4 mm, and the probe tip is flushed with the alumina ceramic tubes. To better measure the plasma parameters in the radial and axial directions in the central plane of the cathode keeper exit, a single Langmuir probe was installed on a two-dimensional stepping motor turntable. A 10 kΩ capacitor and 0.1 μF resistor were used in the measurement circuit of the single Langmuir probe. The probe voltage and scanning voltage were measured using a Yokogawa DL850 ScopeCorder. A schematic diagram of the Hall thruster and measurement circuit are shown in Fig. 1. The electrons are first energized in the electric field, and they then ionize the neutral atoms inside of the channel [8]. The generated ions are expelled by the axial electric field between the cathode and anode, thereby generating thrust. This thrust may then be applied to a wide variety in operations in space, such as maintaining the north-south position of a satellite [30,31]. 2
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Fig. 1. Schematic diagram of the Hall thruster and cathode plasma parameter measurement platform.
3. Experimental results and discussion
or 6.0 sccm. When the mass flow rate of the cathode is fixed at 3 sccm or 6 sccm, the ignition voltage threshold of the Hall thruster decreases as the cathode keeper current increases; at the higher cathode mass flow rate (6 sccm), the reduction of the ignition voltage threshold of the Hall thruster is more significant. This occurs because increasing the cathode keeper current increases the ability of the cathode to emit electrons [28], and therefore the larger the cathode keeper current, the more electrons the cathode can emit. An increased number of electrons can then pass through the magnetic field and enter the discharge channel. At the same time, the energy that approaches the ionization threshold of the xenon atoms is obtained from the axial electric field between the cathode and anode, and more high-energy electrons are generated. This makes it easier to promote the avalanche ionization effect during the ignition process of the Hall thruster. Therefore, when the cathode keeper current is relatively large, the Hall thruster can be ignited at a relatively low voltage threshold. Moreover, when the cathode mass flow is large (6 sccm), the number of electrons emitted by the cathode increases, and this effect becomes more obvious. Fig. 3 shows the ignition voltage threshold when the cathode is operated at mass flow rates of 3.0 sccm or 6.0 sccm and cathode keeper currents of 2.5 A or 3.0 A. When the keeper current of the cathode is fixed at 2.5 A or 3.0 A, the ignition voltage threshold of the Hall thruster clearly decreases as the cathode mass flow rate increases. This is mainly due to the relatively low coupling voltage between the cathode and thruster at the higher cathode mass flow rate [27]. A decrease in the coupling voltage is considered to indicate the ease with which electrons can flow from the cathode to the anode. When this coupling voltage drop is lower, the electrons can relatively easily cross the magnetic field near the exit of the thruster to reach the anode in the discharge channel. Therefore, more electrons can obtain energy in the axial electric field between the cathode and anode to reach the ionization threshold of the xenon atoms, thereby making the ignition process of the Hall thruster more likely to occur. The resulting ignition voltage threshold of the Hall thruster will then be relatively low. Furthermore, Fig. 2 also shows that by keeping the other ignition parameters constant and increasing either the mass flow rate of the cathode from 3 sccm to 6 sccm or the keeper current of the cathode from 1.5 A to 3.0 A, the ignition voltage threshold of the Hall thruster is significantly reduced. The maximum reduction of the ignition voltage threshold that was obtained is more than 72 V. This
During the experimental measurements of the ignition voltage threshold, the Hall thruster coil current is 2.0 A, the heating current of the cathode is 8.0 A, the anode mass flow rate is fixed at a certain value, and the cathode mass flow rate or keeper current acts as the control variable. The discharge voltage of the anode is continuously varied from 0 V to 600 V at a rate of 1 V/s through a controllable power supply. As the anode discharge voltage gradually increases, the Hall thruster suc cessfully ignites, and the LabVIEW control platform records the discharge voltage at this instant as the ignition voltage threshold. Fig. 2 shows the ignition voltage threshold when the cathode is operated at keeper currents of 1.5 A or 3.0 A and cathode mass flow rates of 3.0 sccm
Fig. 2. Ignition voltage threshold of the Hall thruster with different cathode keeper currents and cathode mass flow rates. The respective cathode mass flow rates and keeper currents are: 3.0 sccm and 1.5 A for the solid blue line with square marks; 3.0 sccm and 3.0 A for the dotted red line with triangular marks; 6.0 sccm and 1.5 A for the solid black line with circular marks; and 6.0 sccm and 3.0 A for the dotted green line with five-pointed star marks. (For inter pretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 3
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cathode to emit electrons, such that more electrons can pass through the magnetic field lines near the thruster exit and enter the discharge channel. This makes the thruster ignition process more likely to occur. It is therefore speculated that, compared with the reduction obtained by increasing the cathode keeper current, increasing the cathode mass flow rate can reduce the thruster ignition voltage threshold to a greater extent as more electrons will be produced by the cathode. Furthermore, changes in the cathode keeper current and mass flow rate also cause changes in the plasma parameters of the cathode plume. To better un derstand these changes in the plasma parameters, the electron density, electron temperature, and plasma potential at different keeper currents and mass flow rates were measured using a Langmuir probe in the axial (downstream of the keeper exit, over a distance ranging from 10 to 24 mm from the exit) and radial (relative to the keeper centerline from 0 to 10 mm and 0 to 10 mm) directions. The electron density contours for the different keeper currents and mass flow rates of the cathode are shown in Fig. 4. At the same cathode mass flow rate (3 sccm or 6 sccm), the electron density of the cathode plume increases significantly as the cathode keeper current increases from 1.5 A to 3.0 A, especially near the cathode keeper exit (10–12 mm from the keeper exit). In addition, the results obtained by Raitses et al. also showed that increasing the keeper current increases the electron emission capability of the cathode [28]. Fig. 4(a) and (c), as well as Fig. 4(b) and (d), show that at the same cathode keeper current (1.5 A or 3.0 A), the electron density of the cathode plume increases significantly as the cathode mass flow rate increases from 3.0 sccm to 6.0 sccm. This occurs because a significant pressure increase appears inside of the cathode when the cathode keeper current is constant and the cathode mass flow rate is increased, and the initial electrons emitted by the cathode emitter can then collide with more Xe atoms [36]. Conse quently, the ion density in the emitter barrel increases, and thus more ions bombard the emitter to produce more electrons, increasing the electron density of the cathode plume.
Fig. 3. Ignition voltage threshold of the Hall thruster with different cathode mass flow rates and cathode keeper currents. The respective keeper currents and cathode mass flow rates are: 2.5 A and 3.0 sccm for the solid blue line with square marks; 2.5 A and 6.0 sccm for the solid green line with triangular marks; 3.0 A and 3.0 sccm for the dotted red line with circular marks; and 3.0 A and 6.0 sccm for the dotted black line with five-pointed star marks. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
phenomenon may result from a significant increase in the electron density due to the increases in the mass flow rate and keeper current. According to these experimental results, as the keeper current or mass flow rate of the cathode increases, the ignition voltage threshold of the Hall thruster is significantly reduced, and the reduction in the ignition voltage threshold is more significant when the mass flow rate is increased. In previous studies [27,28], it was shown that increasing the cathode mass flow rate and keeper current enhances the ability of the
Fig. 4. Electron density (cm 3) contours for different keeper currents and mass flow rates of the cathode. The respective mass flow rates and keeper currents of the cathode are: (a) 3.0 sccm and 1.5 A; (b) 3.0 sccm and 3.0 A; (c) 6.0 sccm and 1.5 A; and (d) 6.0 sccm and 3.0 A. 4
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The plasma potential contours for different keeper currents and mass flow rates of the cathode are shown in Fig. 5. The plasma potential is lowest near the centerline of the cathode keeper exit plane, and it in creases gradually as the axial distance from the cathode keeper exit in creases. The change in the plasma potential is similar to that reported in Ref. [37]. Furthermore, at a constant cathode keeper current, the plasma potential in the cathode plume gradually decreases with increasing cathode mass flow rate. This change in the plasma potential may be related to the changes in the cathode coupling voltage observed in previous Hall thruster experiments [27], in which increasing the mass flow rate of the cathode reduced the plasma potential in the cathode plume. This resulted in a reduction of the coupling voltage drop between the thruster and cathode. These phenomena may result from an increase in the plasma density inside of the cathode and plume region due to increasing the cathode mass flow rate or keeper current. The resistivity inside of the cathode and plume region is thereby reduced, such that the plasma potential of the cathode plume region is also reduced. In particular, when the cathode mass flow rate and keeper current are increased at the same time, the electron density in the cathode plume region is significantly increased, and the plasma potential in the cathode plume region is reduced more significantly. The electron temperature contours at different keeper currents and mass flow rates of the cathode are shown in Fig. 6. The electron tem perature at different keeper currents and mass flow rates seems to be correlated with the plasma potential. As the plasma potential increases, the electrons are accelerated, causing the electron temperature to rise. Similar results have previously been reported [38]. Fig. 6 shows that at a constant cathode mass flow rate (3 sccm or 6 sccm), the electron tem perature of the cathode plume decreases as the cathode keeper current increases from 1.5 A to 3.0 A, especially near the cathode keeper exit. Moreover, Fig. 6(a) and (c), as well as Fig. 6(b) and (d) show that at a constant cathode keeper current (1.5 A or 3.0 A), the electron
temperature of the cathode plume decreases significantly as the cathode mass flow rate increases from 3.0 sccm to 6.0 sccm. As can also be seen in Fig. 4, this phenomenon occurs because as the cathode keeper current or mass flow rate is increased, the electron density in the cathode plume region significantly increases, and the electron density in the cathode emitter and orifice regions also significantly increases. When a large number of electrons flow from the orifice region to the plume region, particle collisions occur in the orifice region. When the electron density is high, the collision frequency will be significantly increased and the consumption of electron energy will also be relatively large, resulting in a lower temperature of the electrons flowing out of the cathode plume region. It can also be seen in Fig. 5 that as the cathode keeper current or mass flow rate is increased, the plasma potential in the cathode plume region is significantly reduced. The energy that can be obtained by the electrons flowing from the orifice region of the cathode to the plume region will subsequently be relatively low. The experimental results of Goebel et al. also show that increasing the mass flow rate of the cathode while holding the other parameters constant reduces the electron tem perature [39]. Furthermore, increasing the cathode mass flow rate produces a more significant reduction in the electron temperature in the cathode plume region than when the cathode keeper current is increased. The main reason for this phenomenon is that, compared with increasing the cathode keeper current, increasing the mass flow rate of the cathode increases the density of electrons emitted by the cathode. As a result, the collision frequency is higher when electrons flow through the orifice region of the cathode and the energy loss of these electrons is therefore greater, thereby lowering the electron temperature in the cathode plume. The ignition voltage thresholds measured under different cathode keeper currents and mass flow rates revealed that when the other operating parameters are constant, increasing the cathode keeper cur rent or mass flow rate results in a significant decrease in the ignition
Fig. 5. Plasma potential (V) contours for different keeper currents and mass flow rates of the cathode. The respective cathode mass flow rates and keeper currents are: (a) 3.0 sccm and 1.5 A; (b) 3.0 sccm and 3.0 A; (c) 6.0 sccm and 1.5 A; (d) 6.0 sccm and 3.0 A. 5
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Fig. 6. Electron temperature (eV) contours for different keeper currents and mass flow rates of the cathode. The respective cathode mass flow rates and keeper currents are: (a) 3.0 sccm and 1.5 A; (b) 3.0 sccm and 3.0 A; (c) 6.0 sccm and 1.5 A; and (d) 6.0 sccm and 3.0 A.
cathode to the anode. Consequently, it is relatively easy for the electrons emitted by the cathode to enter the discharge channel during the igni tion process of a Hall thruster. When the cathode keeper current and mass flow rate are increased, the Hall thruster can therefore be ignited successfully at a relatively low ignition voltage threshold. Furthermore, although increasing the cathode keeper current and mass flow rate causes the electron temperature of the cathode plume to decrease, the initial electron temperature of the cathode emission is usually low (around several electron volts [40–42]) and the electrons emitted by the cathode gain energy from the axial electric field between the cathode and anode as they enter the discharge channel. Therefore, although increasing the cathode keeper current and mass flow rate may decrease the temperature of the electrons emitted by the cathode, this decrease may be compensated for by the energy obtained from the axial electric field.
voltage threshold of the Hall thruster. In particular, the reduction in the ignition voltage threshold is larger when both parameters are increased. Increasing the cathode mass flow rate results in a larger reduction of the ignition voltage threshold than is obtained by increasing the cathode keeper current. The measured plasma parameters of the cathode plume under different cathode keeper currents and mass flow rates show that increasing the cathode keeper current and mass flow rate significantly increases the electron density near the keeper region of the cathode. This leads to a decrease in the electron temperature in the cathode plume, especially when both parameters are increased simultaneously. Increasing the cathode mass flow rate causes a larger increase in the electron density near the cathode keeper exit than is obtained by increasing the cathode keeper current. According to the ignition voltage thresholds and plasma parameters measured under different cathode keeper currents and mass flow rates, the decrease in the ignition voltage threshold of a Hall thruster resulting from increasing the cathode keeper current and mass flow rate is mainly due to a significant increase in the electron density in the cathode plume region. This increase accompanies the increases of the aforementioned parameters. The following two factors may contribute to this phenomenon. First, owing to the signifi cant increase in the electron density in the cathode plume region, more electrons near the cathode keeper exit can be bound by the nearby magnetic field lines. Therefore, energy can be obtained from the axial electric field when the electrons enter the discharge channel, and more high-energy electrons are produced. These high-energy electrons collide with neutral atoms that were previously concentrated in the thruster exit and discharge channel, enabling avalanche ionization to be established more quickly. Second, the cathode mass flow rate affects the coupling voltage drop between the cathode and thruster. When the mass flow rate of the cathode is increased, the plasma potential in the cathode plume decreases, which decreases the coupling voltage drop between the thruster and cathode [27,39]. The coupling voltage drop is considered to be an indicator of the ease with which electrons can flow from the
4. Conclusion To date, research on the operating parameters of cathodes in Hall thrusters has focused mainly on their effects towards the steady-state performance parameters of these thrusters. The effects of the cathode keeper current and mass flow rate on the ignition voltage threshold of a Hall thruster are unknown. Therefore, the ignition voltage thresholds were measured at different values of these cathode operating parame ters. To better understand how these operating parameters affect the ignition voltage threshold of Hall thrusters, the electron density, elec tron temperature, and plasma potential in the cathode plume region were measured using a Langmuir probe in the axial and radial di rections. The results showed that increasing both the keeper current and mass flow rate of the cathode increases the electron density emitted by the cathode and decreases the electron temperature. It was found that increasing the cathode mass flow rate has a more pronounced effect on these parameters than increasing the keeper current. In addition, when 6
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the mass flow rate and keeper current of the hollow cathode increase simultaneously, the electron density increases and the ignition voltage threshold of the Hall thruster decreases significantly.
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