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Abstract It is commonly accepted that high neutral density or low bulk axial velocity will efficiently enhance ionization. At present, all the operation of neutral gas flow inside the Hall thruster is an attempt to reduce axial velocity of neutral gas and extend residence time. Based on the idea, we proposed the azimuthal diversion rail (ADR) Hall thruster for the first time in preliminary work, and provided a kind of brand new method to reduce axial velocity of neutral gas. Compared to other modes, such kind of mode features simple structure, light weight and significant effect, in which axial movement is transformed to circumferential movement through elastic collision, thus the residence time and neutral density in the channel can be increased, ionization degree and discharge performance are improved accordingly. However, we found in the ADR research process made at later stage, the location of ADR in the discharge channel exerts significant impact on discharge. Therefore, this paper studies the effect of the ADR location on the discharge performance of Hall thrusters. The location of the ADR can affect the distribution of the propellant density in the channel of the Hall thruster, thereby affecting the propellant utilization and performance. The thrust, specific impulse, efficiency and propellant utilization of the ADR are measured in three different locations in the channel, respectively. The propellant density in the ionization zone is higher when the ADR is flush with the anode top surface. Furthermore, the propellant utilization can be increased by 12% and 9% compared with that at the bottom and the middle of the anode, respectively, resulting in the thrust being increased by up to 3.2 mN (10%) and 2 mN (8.5%) and the anode efficiency being increased by up to 9% and 6%. The location of the ADR has a significant effect on the performance of the thruster. The sensible placement of the rail can effectively increase the propellant density in the ionization zone to improve the performance. The results of the study can be used to guide the design of Hall thrusters with ADR. Highlights • Effects of the azimuthal diversion rail location on performance of Hall thrusters is studied. • It is proved the rail placement can increase the propellant density in the ionization zone. • Azimuthal diversion rail location can be used to improve the hall thruster performance. [ABSTRACT FROM AUTHOR]

To improve the performance of thrusters by using two permanent ring-shaped magnets to push a magnetic field downward, we propose an annular–cylindrical combined-channel Hall thruster and numerically study its performance. The results indicate that, compared with simple annular channels, the annular–cylindrical combined channel can effectively increase the neutral gas density contained within it and increase the degree of ionization, efficiency, thrust, and specific impulse. Moreover, the sputtering zone of discharge channel is in front of the permanent magnets, i.e., wall erosion occurs upon ion bombardment during discharge. However, ions cannot bombard the permanent magnetic rings, thus guaranteeing a constant magnetic field and long discharge lifetime. [ABSTRACT FROM AUTHOR]

Abstract A 200-W Hall thruster was designed using permanent magnets placing the axial maximum magnetic field outside of the channel, and the anode/gas distributor as an integrated U-shaped structure. A split structure is adopted for the discharge channel to conveniently change the wall material. With the initial conditions that the anode mass flow is maintained at 0.8 mg/s, 1.0 mg/s and 1.2 mg/s over a voltage range of 150–400 V (at 50 V intervals), the discharge properties were determined for the two thruster models with the channel walls composed of graphite and boron nitride (BN). The results demonstrate that under identical operating parameters, the properties of the thruster with the graphite channel walls are similar to the properties of the thruster with the BN channel walls. The maximum difference of the discharge current between the two wall materials is 6.2%; the maximum difference of the thrust and the specific impulse is 3.3%, and that of the anode efficiency is 1.7% (absolute value). These differences are smaller than the corresponding parameter differences observed from changing wall materials in other common Hall thrusters. Highlights • A permanent magnet Hall thruster with graphite channel wall is studied. • Results show differences are smaller with graphite and BN wall. • This study given a new choice of Hall thruster discharge wall to prolong life. [ABSTRACT FROM AUTHOR]

In this paper, a new “convex” external magnetic pole structure is proposed. With this structure, two different configurations of the cathode inside and outside the magnetic separatrix can be realized by changing the position of the magnetic separatrix instead of changing the position of the cathode in the radial direction. Furthermore, the discharge plume, coupling voltage, and performance of the thruster under different configuration conditions are studied experimentally. The experimental results indicate that, when the cathode is located inside the magnetic separatrix, the effective acceleration voltage is higher, the coupling voltage between the cathode and the thruster is lower, and the thruster performance (including thrust, specific impulse, and efficiency) is higher. When the cathode is located outside the magnetic separatrix, the effective acceleration voltage is lower, the coupling voltage between the cathode and the thruster is higher, and the thruster performance is degraded. When the cathode is located inside the magnetic separatrix, the thrust and specific impulse can be increased up to 2.3%, and the efficiency—up to 4.6%. This study provides a new method for the design of the magnetic separatrix and is of great importance for the coupling-matching design of the cathode and thruster. [ABSTRACT FROM AUTHOR]

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 Xe 2+ ions appear in the plume area, the catastrophic discharge phenomenon observed. [ABSTRACT FROM AUTHOR]

A hollow indented anode is proposed for increasing the neutral gas density in a discharge channel, in order to improve the performance of the thruster. The experimental results show that a hollow indented anode structure can effectively improve the performance, compared to a hollow straight anode under similar operating conditions, in terms of thrust, propellant utilization, ionization rate, and anode efficiency. Furthermore, simulations show that the indented anode can effectively increase the neutral gas density in a discharge channel and on the centerline of the channel, compared to a hollow straight anode. In addition, it can increase the ionization rate in the channel and the pre-ionization in the anode. Therefore, the hollow indented anode could be considered as an important design idea for improving thruster performance. [ABSTRACT FROM AUTHOR]

The high amplitude pulse current during Hall thruster start-up is an important source of electromagnetic interference, which impacts the safety and reliability of the propulsion system. The characteristics of the pulse current, including the pulse current's integrated area, peak value, and transient time, are studied experimentally and theoretically under different Hall thruster discharge parameters. Results show that the pulse current's integrated area is determined by the atom density in the channel before Hall thruster start-up; the effects of coil current on the peak value and transient time of the pulse current are negligible. As the discharge voltage and mass flow rate increase, the pulse current peak value increases, the pulse current duration is inversely proportional to the discharge voltage and directly proportional to the mass flow rate. [ABSTRACT FROM AUTHOR]

This paper proposes increasing the width of the discharge channel to improve the specific impulse of Hall thrusters. In our study, we significantly increased the achievable channel width by optimizing the magnetic circuit topology, and the design freedom of the Hall thruster was improved. A prototype of a wide-channel high-specific-impulse Hall thruster was designed and experimentally verified. The effects of channel width on the discharge process and the characteristics of an aft-loaded magnetic field Hall thruster were investigated. The high-intensity luminous region did not diffuse radially with an increase in the channel width, and the plasma was always concentrated near the channel centerline. A wider channel is more conducive for improving the propellant utilization efficiency, thereby effectively improving the specific impulse of the Hall thruster. By optimizing the discharge channel width, the upper limit of the discharge voltage was increased, and the maximum specific impulse was increased from 3170 to 3794 s, a relative improvement of 20%. The effect of the specific impulse improvement at the same discharge voltage was analyzed and discussed. The results of this study are significant for the design of high-specific-impulse Hall thrusters. [ABSTRACT FROM AUTHOR]

The theoretical specific impulse of Hall thrusters using a krypton propellant is much higher than that of those using xenon. However, the actual benefits of this improvement are limited by the difficulty in achieving krypton atom ionization, and its efficiency is much lower than that of xenon. In this study, the anode specific impulse and anode efficiency of Hall thrusters using krypton propellant were improved by adding a small quantity of xenon. A dual-channel supply of xenon and krypton was achieved by optimizing the discharge channel structure based on the design of the aft-loaded magnetic field. The effects of the xenon and krypton supply method and xenon fraction on the discharge characteristics of the Hall thruster were investigated by conducting discharge experiments using the mixed propellant. The results indicated that injecting a small quantity of xenon significantly improved the ionization of krypton, improved the ionization and acceleration efficiency of the thruster, and fully utilized the high specific impulse characteristics of krypton. Furthermore, the addition of a small quantity of xenon also significantly increased the upper limit of the discharge voltage for stable operation using krypton from 500 V. At the maximum tested xenon fraction, the maximum discharge voltage was loaded to 900 V. At a discharge voltage of 800 V, the optimal anode efficiency and anode specific impulse increased by 4.5 % and 534 s, respectively, compared with those for pure xenon propellant (with an anode efficiency of 54 % and anode specific impulse of 3034 s). The results of this study are of great significance for the design of high-efficiency, high-specific-impulse, and low-cost Hall thrusters. • A dual-channel gas supply structure is proposed, which can supply two kinds of gases into the discharge channel individually. • Thruster performance is optimal when krypton is supplied downstream and auxiliary xenon is supplied upstream. • High-voltage operating ability under krypton is greatly improved with the assistance of a small quantity of xenon. [ABSTRACT FROM AUTHOR]

Using a permanent magnet to realize a design with a large height-radius ratio can effectively reduce the radial size of the thruster. The experimental results show that a Hall thruster with a large height-radius ratio also has significant advantages in its discharge performance. Taking the 1.35 kW Hall thruster HEP-1350PM, which has a large height-radius ratio, as an example, the thrust, specific impulse, and efficiency of HEP-1350PM are increased by up to 4.3%, 10.4%, and 7%, respectively, as compared to SPT-100, while its weight is only 56% of that of SPT-100. Numerical simulations were performed to understand the discharge characteristics of a Hall thruster with a large height-radius ratio and the internal mechanism of its performance improvement. The results show that the small middle diameter and wide channel structure concentrate the distribution of the neutral gas in the channel, accelerate the decrease of the propellant density along the axial direction, and move the main ionization zone closer to the channel upstream. These factors increase the ionization concentration, ionization rate peak, working medium utilization rate, and voltage utilization rate. They also reduce the ion loss on the wall surface and plume divergence, thereby improving the comprehensive discharge performance. This study verifies the feasibility of a large height-radius ratio in the design of a Hall thruster, and demonstrates that the design would not only be valuable for reducing the volume and weight of the thruster, but also has the advantages of enhancing its discharge performance and prolonging its working life. • Large height-radius ratio design can reduce the volume and weight of the thruster. • A large height-radius ratio can improve the discharge performance of the thruster. • Hall thruster with a large height-radius ratio has a higher acceleration efficiency. [ABSTRACT FROM AUTHOR]

The use of krypton in the field of electric propulsion has become a key research topic owing to its high specific impulse and high reserves. However, because krypton has features such as a small ionization cross section and high ionization energy, its ionization performance is relatively low, thereby increasing the actual specific impulse loss of the krypton Hall thruster in comparison with its theoretical value. Therefore, to use of krypton as the propellant of the Hall thruster instead of xenon, it is necessary to study the optimization of its ionization process. In this study, a gas distributor with a rotating propellant supply mode is designed, considering factors such as the velocity and density of the velocity of neutral atoms, which affects the ionization process. The distribution of neutral atom parameters in the discharge channel is changed by the rotation; and its influences on the propellant ionization and other performance parameters of the Hall thruster are experimentally analyzed with respect to the magneto-ampere, volt-ampere, and flow rate-ampere characteristics. The results demonstrate that the rotating propellant supply reduces the axial velocity of neutral atoms, increases the density and circumferential motion distance, and thus improves the ionization efficiency of the krypton Hall thruster. Furthermore, the performance parameters, such as thrust and efficiency, are also improved, and the plume divergence angle is reduced. When the discharge voltage is 450 V and the anode flow rate is 60 sccm, the rotating propellant supply can increase the anode efficiency of the axial propellant supply mode from 46.2% to 53.2%. However, the plume divergence half-angle reduces from 39.2° to 35.5°. In addition, an increase in the anode flow rate and discharge voltage continuously increases the performance promotion in the rotating propellant supply mode. • Rotating propellant supply is achieved by a particular gas distributor design. • Rotating propellant supply slows down krypton neutrals inside the channel and improves ionization efficiency. • Performance increment under rotating propellant supply increases with both discharge voltage and krypton flow rate. [ABSTRACT FROM AUTHOR]

Abstract The reliability of cathode is critical to electric propulsion systems. Although researchers have carried out many life tests on hollow cathode, the discharge current is almost steady when the cathode is independently tested for life. Actually, when it is coupled with the thruster, the discharge current is with large amplitude low-frequency oscillation. Therefore, we suggest a new external circuit for a cathode-independent life test, which could simulate the characteristics of discharge current oscillation when the cathode is coupled with Hall thruster. We carried out 160 h of life test using the normal external circuit and the new external circuit, and the variation characteristics of the cathode orifice area are also studied during the life test. The results show that under the normal external circuit, the growth rate of the cathode orifice area initially increases and then decreases gradually with the increase of the time of life test, whilst the cathode orifice area in the new external circuit linearly increases. The new external circuit can provide a new method to simulate the discharge current oscillation when the cathode is coupled with Hall thruster during the independent life test of the cathode. [ABSTRACT FROM AUTHOR]

Hall thrusters generally use a U-shaped insulated wall to form an annular discharge channel. As high-power and high-specific-impulse Hall thrusters develop, the disadvantages of this conventional topology—in terms of material and mechanical strength, processing, and manufacturing—are gradually being exposed. Therefore, in this study, a new wall material was employed, and its effects were investigated numerically and experimentally. A boron nitride ceramic wall was retained in the strong discharge area owing to its excellent secondary electron emission characteristics; while a metallic wall was applied upstream of the channel, forming a segmented hybrid wall. A Hall thruster with an aft-loaded magnetic field, which offers a long lifetime, was adopted for this study. The particle-in-cell simulation shows that the length of the metallic wall has a significant impact on thruster performance. The same trend was observed in the experiments. Under the optimal metallic wall length, the anode efficiency is improved by approximately 4% compared to that of the ceramic-wall-only case. The physics involved is analyzed and qualified by comparing the simulated and measured results. The findings of this study provide valuable guidance for the optimal design of high-power, high-specific-impulse Hall thrusters. • Metal-ceramic hybrid channel wall improves high-voltage performance of Hall thrusters. • Optimal length of metallic segment exists and is strongly related to magnetic field. • Too long metallic segment augments electron wall loss and then ionization loss. [ABSTRACT FROM AUTHOR]

In this study, the effect of pulse frequency on the discharge and plume characteristics of Hall thrusters under pulsating operation is investigated. The experimental results show that the discharge current and thrust decrease as the frequency increases to the range of 30–90 kHz. The efficiency of the thruster is negatively correlated with the pulse frequency, and the pulse frequency minimally affects the plume divergence angle and ion current. It is found that an increase in the pulse frequency decreases the secondary electron emission flux on the wall and the near-wall conduction rate of electrons in the channel, thus consequently decreasing the electron current. An increase in the electron current share improves the current-utilization efficiency. In addition, as the frequency increases, the ion power loss on wall increases and the ion acceleration becomes less effective, thus resulting in a decrease in thrust. The combined effect of these two factors results in an increase in the anode efficiency as the frequency increases. • The effect of pulse frequency on the discharge and plume characteristics of Hall thrusters under pulsating operation were investigated. • Increasing the pulse frequency decreases the secondary electron emission flux, which reduces the rate of electron-in-wall conduction in the channel, thus resulting in a lower electron current. • As the pulse frequency increases, the ions power loss on wall increases leading to poorer ion acceleration and a reduction in thrust. [ABSTRACT FROM AUTHOR]

Electric propulsion systems with high-specific impulses have replaced chemical propulsion systems in many space missions. The aft-loaded magnetic field technology of Hall thrusters moves the ionization and acceleration processes downstream of the channel by varying the magnetic field distribution. Consequently, ion bombardment of the channel walls is inhibited, and the service life of the Hall thrusters is substantially increased. However, neutral gas leaks because of the rapid diffusion near the channel outlet, where strong ionization occurs. Thus, the propellant atoms are ionized insufficiently, resulting in a decline in the performance. Therefore, an aft-loading design for the neutral gas flow field was developed to optimization distribution of the neutral gas in this study. A 1.5 kW Hall thruster was verified experimentally. The experimental results show that the aft-loaded neutral gas flow field significantly improved the ionization and acceleration processes. The propellant utilization efficiency increased by 4–8% within the discharge voltage range of 300–900 V. The ion energy distribution moved toward the high-energy direction significantly, promoting an overall improvement in the performance, with a thrust increase of 7.3–11.5 %. The anode efficiency increased from 49–57 % to 58–61 %. The results of this study are significant for the design of high-performance, long-life Hall thrusters. • The aft-loading design of gas flow is realized using the special topology of the hybrid-wall Hall thruster. • The ionization distribution of aft-loaded neutral gas flow field is concentrated in higher potential region. • The aft-loaded neutral gas flow moves the ionization region downstream of the channel. [ABSTRACT FROM AUTHOR]

Prior experiments have demonstrated that the pole in a magnetically shielded Hall thruster sputters and erodes visibly, in contrast with that in an unshielded thruster. To determine the reason for this, a particle-in-cell simulation was conducted, and the difference between shielded and unshielded thrusters in terms of plasma behavior near the exit plane was studied. The results show that in the case of the magnetically shielded thruster, the ion flux peak is on the magnetic pole face with higher ion energy, indicating severer erosion at the pole. By contrast, the flux peak of the unshielded thruster is on the head face of the channel wall, indicating severer bombardment at the wall and milder erosion at the pole. Therefore, pole erosion occurs in the magnetically shielded thruster, and channel head face erosion can be observed in the unshielded thruster. In addition, the simulation results indicate that the greater the outward shift of the magnetic field, the severer the pole erosion. [ABSTRACT FROM AUTHOR]

The inward deflection of the excitation magnetic field was found in a Hall thruster voltage-varying experiment. The influence of Hall current magnitude and radial distribution on the total magnetic field was analyzed through magnetic field simulations. The probable reason why the excitation magnetic field deflected inward evidently at high discharge voltages (＞600 V) was obtained by combining the simulation results and theoretical analysis. This study provides qualitative guidance on the design of the excitation magnetic field in high-specific-impulse Hall thrusters. • Hall current induces an outward deflected magnetic field. • Magnitude and asymmetry of Hall current have an equivalent effect on magnetic field deflection. • An inward deflected excitation magnetic field is necessary to obtain a symmetrical total field. [ABSTRACT FROM AUTHOR]

The large height-radius ratio design of the Hall thruster increases the difference of radial distributions of magnetic field and plasma parameters in the channel, so that the optimal discharge effect of the thruster cannot be guaranteed under the symmetrical magnetic field configuration relative to the channel centerline. In this work, the effects of the unsymmetrical magnetic field on discharge characteristics of Hall thruster with large height-radius ratio were studied through experiments and PIC simulations. The results show that the thruster can realize stable discharge in a wide power range under unsymmetrical magnetic fields. Also, the unsymmetrical magnetic field plays a positive role in improving the discharge performance and prolonging the service life of the thruster. The inward-inclined magnetic field can effectively improve the discharge performance of the thruster, and the discharge efficiency can be increased by 1.5% under the rated working condition compared with the symmetrical magnetic field. The advantage in performance is mainly attributed to the improvement of ionization efficiency of propellant. The outward-inclined magnetic field is more conducive to prolong the service life of the thruster, and the peak value of ion power deposition on the inner wall can be reduced by 40% under the rated working condition due to the outward inclination of the ionization zone and the reduction of the radial electric field. However, under unsymmetrical magnetic field, the ionization zone deviates from the channel center, and the high ionization rate zone is more dispersed in distribution, thus leading to the increase in plume divergence angle. This study verifies that the unsymmetrical magnetic field has certain advantages under the structure of large height-radius ratio, which could be used as a guidance for the optimal design of Hall thruster with large height-radius ratio. • Hall thruster can operate in a wide power range under unsymmetrical magnetic fields. • Inward-inclined magnetic field can improve thruster discharge performance. • Outward-inclined magnetic field is conducive to prolong thruster service life. [ABSTRACT FROM AUTHOR]

In this study, a high-speed camera was used to capture the ionization process of a Hall thruster with slits on the side of its discharge channel, and the change in light intensity was used to study ionization variations in the thruster channel. The light intensity was distributed along the axis, following a normal distribution. The peak position of the ionization intensity, ionization width fluctuation, and low-frequency oscillation of the Hall thruster was investigated. Experiments and simulations show that the oscillation frequency of the peak position of ionization intensity and the width of the main ionization zone are the same as the frequency of low-frequency oscillations. When discharge current increases, the peak position of ionization intensity moves towards the channel exit and the ionization width narrows during the decay of discharge current oscillations. Moreover, theoretical analyses show that the characteristics of the ionization distribution in the channel under a low-frequency-oscillation time scale are primarily governed by the ionization and supplementation of the mass flow rate. • The plasma distribution image in the inner channel of the Hall thruster is studied. • The discharge situation of the Hall thruster under low frequency oscillation is given. • The channel ionization parameters in the Hall thruster change periodically. • There is a coupling relationship between channel ionization in Hall thruster and low frequency oscillation. [ABSTRACT FROM AUTHOR]

In recent years, high-voltage and high-specific-impulse Hall thrusters are being developed. However, the efficiency of the Hall thruster does not increase monotonically with the discharge voltage, but reaches peak at an intermediate voltage, which limits the ability of high-voltage Hall thruster. Previous studies indicate that the sharp increase of the wall energy loss at high voltages dominates the occurrence of the efficiency peak. Therefore, a peak-pushed-outside (PPO) magnetic field is designed in this study, which is expected to diminish the wall energy loss. Experimental results show that the PPO magnetic field can push the propellant ionization and acceleration process downstream, reduce the wall energy deposition at high voltages effectively, and therefore improve the efficiency peak value and increase the voltage corresponding to the efficiency peak. This is helpful to enlarge the high-voltage operation range of Hall thrusters. However, the discharge oscillation rises up significantly at high voltages, and the thruster performance degrades at low voltages, which requires further investigation. • Peak-pushed-outside magnetic field reduces wall energy deposition in Hall thrusters greatly. • Peak-pushed-outside magnetic field promotes the efficiency peak and the corresponding discharge voltage in Hall thrusters. • Peak-pushed-outside magnetic field is suitable for high-voltage Hall thrusters. [ABSTRACT FROM AUTHOR]

A curved, convex towards the channel bottom magnetic field is an important feature of an advanced Hall thruster that allows confining the plasma flow in the channel center, reducing the divergence angle of the ejected ion beam, and improving the discharge performance. In this article, the discharge behaviour of a Hall thruster in magnetic fields with different degrees of curvature is simulated with a particle‐in‐cell numerical method, and the effect of curved magnetic field on the ion bombardment and wall erosion and the associated mechanisms are studied and analysed. The results show that, as the curvature of the magnetic field increases, the propellant ionization becomes more confined at the channel center, the potential drop inside the channel decreases, and the acceleration region shifts outside the channel, which lead to the attenuation of the ion energy bombarding the wall and the deviation of the bombardment angle from the optimal sputtering angle. Conversely, the ion flux bombarding the wall near the channel exit increases. Nevertheless, the bombardment energy and angle are the dominant factors for the wall erosion, and the wall erosion rate clearly decreases with the increasing curvature of the magnetic field. These findings are closely related to the behaviour of electron conduction under a curved magnetic field; the relevant mechanisms are clarified in this article. [ABSTRACT FROM AUTHOR]