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Increasing the specific impulse has become one of the main trends in the development of Hall thrusters and may be achieved by increasing the proportion of multiply charged ions in the plume. In this study, the multiple ionization characteristics of Hall thrusters with large height–radius ratios and their effects on the specific impulse and other performance parameters were examined via numerical simulations. The simulation results show that the proportion of multiply charged ions in a Hall thruster with a large height–radius ratio increases by 7.8 percentage points, compared with that of the traditional Hall thruster, and the multiply charged ions can obtain a higher average jet velocity under an accelerating voltage, thereby improving the specific impulse and thrust of the thruster. Further analyses show that a structure with a large height–radius ratio reduces the cooling effect of the channel wall on the electrons, and the increase in electron temperature effectively promotes the occurrence of the multiple ionization process. In addition, the smaller wall area and the larger magnetic field intensity in the near-wall region inhibit the electron near-wall conduction and prevent the potential negative effect of the increase in electron current caused by multiply charged ionization, which ensures a high level of efficiency. It was proved that multiple ionizations contribute positively to the specific impulse and efficiency of Hall thrusters with a large height–radius ratio. This serves as a reference for the high specific impulse design of Hall thrusters with large height–radius ratios and provides a new optimization perspective for improving the specific impulse of Hall thrusters. [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]

A low-power Hall-effect thruster was designed with two permanent magnet rings. This Hall-effect thruster contains a symmetrical double-peak magnetic field with a gradient larger than that of conventional Hall-effect thrusters. The matching of the anode position with the magnetic field determines the performance of the thruster to a very large extent. This study investigates the laws and mechanisms that govern the front end of a U-shaped hollow anode at the inner magnetic separatrix, the outer magnetic separatrix, and the area between them and influence the discharge characteristics of the thruster. The study shows that with the increase of anode length, the ionization and acceleration zones are pushed down toward the channel outlet and even into the plume region. At the same operating point, both the thrust and the efficiency are the largest when the anode is placed between the inner and outer magnetic separatrices. When the anode is placed at the inner magnetic separatrix, both the thrust and the efficiency are small because of the heavy energy loss on the walls, despite the high degree of ionization. Finally, when the anode is at the outer magnetic separatrix, the performance of the Hall-effect thruster is the poorest because of the lower degree of ionization and larger divergence angle of the plume upon the shift of the ionization zone toward the plume region. [ABSTRACT FROM AUTHOR]

The characteristics of low-frequency oscillations of double-stage Hall thrusters are quite different from those of conventional single-stage Hall thrusters. In this paper, the effects of double-stage discharge on the low-frequency oscillations are experimentally investigated. The results indicate that the amplitude significantly decreases with increasing magnetic field strength and voltage during the ionization stage. Meanwhile, data analysis reveals that ionization occurs in both stages and that the transport ion current between the two stages is the key factor that affects the amplitude and main frequency of the oscillations. Two new processes, namely the ion transport and ion recombination caused by double-stage discharge, are found to be relevant to the change in the current oscillations. To summarize, the ionization stage relieves the oscillations in the acceleration stage, leading to a reduction in the amplitude of the discharge current. [ABSTRACT FROM AUTHOR]

The strength and shape of the magnetic field are the core factors in the design of the Hall thruster. However, Hall current can affect the distribution of static magnetic field. In this paper, the Particle-In-Cell (PIC) method is used to obtain the distribution of Hall current in the discharge channel. The Hall current is separated into a direct and an alternating part to calculate the induced magnetic field using Finite Element Method Magnetics (FEMM). The results show that the direct Hall current decreases the magnetic field strength in the acceleration region and also changes the shape of the magnetic field. The maximum reduction in radial magnetic field strength in the exit plane is 10.8 G for an anode flow rate of 15 mg/s and the maximum angle change of the magnetic field line is close to 3° in the acceleration region. The alternating Hall current induces an oscillating magnetic field in the whole discharge channel. The actual magnetic deformation is shown to contain these two parts. [ABSTRACT FROM AUTHOR]

The magnetic field distribution of Hall thrusters has a crucial impact on its performance. Studies have mostly focused on the magnetic field distribution within the channel and near the channel exit, but less attention has been paid to the magnetic field distribution in the near-field plume region. Two different magnetic field distributions were designed. The influence of the magnetic inclination in the plume region on the performance of a low-power Hall thruster was investigated using a particle-in-cell (PIC) numerical simulation and experimental study. The results indicate that the magnetic field with inclined inward can effectively improve the performance of Hall thrusters, relatively increasing the thrust by 3.6%–9.6%, anode specific impulse by 3.8%–9.8%, and anode efficiency by 2.5%–5.5% at a fixed flow rate. It can also reduce the plume divergence angle, and the maximum decrease in the plume divergence half-angle is approximately 7°. The main reason is that the radial electric field in the plume region under an inward-inclined magnetic field is lower than that with an outward-inclined magnetic field, thus making the radial velocity of ions in the former magnetic field lower than that in the latter magnetic field, ultimately driving more ions to the thruster center. • The inclination of magnetic field lines in the plume region can affect performance of Hall thrusters. • The magnetic field with inclined inward in the plume region can enhance performance and reduce plume divergence angle of Hall thrusters. • When designing Hall thrusters, the magnetic field lines in the plume region should be kept inclined towards the center of thruster. [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]

During a long-term discharge of a Hall thruster, local overheating on the anode surface occurs and its location shifts randomly. The asymmetrical supply of anode potential is thought to be the cause. To investigate their relationship, a bias voltage is applied on one piece of a quartered anode to realize the asymmetrical supply of anode potential. Variations of both the current and temperature of each anode piece are measured and analyzed. The results indicate that the disruption of the anode potential symmetrical supply does not affect the total anode current; however, it enhances the current oscillation. As the bias voltage increases, the current concentrates gradually on the high-potential part of the anode, accompanied by an increase in the local temperature. Changes in the Ez × Br drift direction, magnetic field strength, discharge voltage, and anode flow rate do not affect the above-mentioned laws. Preliminary analysis shows that the formation of a local electric field near the anode and the generation of an extra Eθ × Br drift are the main causes, which can affect the movement of electrons in the near-anode region greatly. [ABSTRACT FROM AUTHOR]

• On-orbit non-intrusive perturbation measurement of Hall effect thrusters. • 10 kHz to 100 kHz analysis better than full frequency domain. • Perturbation signal amplified by scatter numbers in the far-zero region. • The physical core represented by the scatter numbers is the dwell time. • Dwell time is mainly related to breathing oscillations. The failure of Hall effect thruster (HET) is usually the result of the accumulation of several tiny abnormal disturbances during operation. Timely and accurate early warning of perturbations is an important guarantee for the normal operation of thrusters. In this study, when the perturbation rate of HET signal (anode voltage or anode flow rate) is −4% to 4%, the time scale signal of breathing oscillation (BO) in the frequency range of 10 kHz to 100 kHz is statistically analyzed. Experiments show that when the signal is perturbed, the balance between ionization and acceleration of plasma in the discharge channel is destroyed, resulting in the dwell time of the wave peaks and valleys of BO in the far-zero region with the perturbation law, which visually shows that the number of scattered points at the corresponding current value changes regularly. According to the above statistical characteristics, this paper proposes a new condition monitoring index, namely the number of scattered points in the wave peaks side of the far-zero point area, and designs a perturbation indicator for the operating condition of the HET. Compared with direct monitoring of discharge current in engineering, this indicator can amplify the influence of the perturbation signal on BO, and has better identification and accuracy. Meanwhile, the indicator does not use invasive measurement and complex equipment, and is promising to be applied to the monitoring and diagnosis of the perturbation operation state of the on-orbit thruster. [ABSTRACT FROM AUTHOR]

With the development of electric space propulsion and all-electric propulsion technologies, the application range of the Hall effect thruster (HET) has gradually expanded. As an important part of the HET, the magnetic field plays a decisive role in its performance. In the development of versatile HETs, it is necessary to comprehensively and thoroughly understand the effects of the magnetic field on the HET discharge characteristics. In this paper, a HEP-100X thruster with multiple degrees of freedom in terms of magnetic field control is used to study the effects of the peak magnetic field position on the thruster discharge characteristics. The results indicate that the peak magnetic field position can effectively control the main ionization zone position and affect the ionization rate. The thruster has the best comprehensive discharge performance when the peak magnetic field position is located near the channel exit. Further analysis reveals that the performance improvement is mainly due to the reduced ion loss on the wall surface and plume divergence half-angle, and the increased current utilization efficiency and voltage utilization efficiency. The HEP-100X shows the above trends under various flow conditions, and the anode efficiency is the highest when the anode flow is 50 sccm. In this condition, the propellant is sufficiently ionized and the ion loss on the wall surface is small. This research provides the necessary support for a comprehensive understanding of the role and significance of the magnetic field in the thruster discharge process, and has important significance for the design optimization of multifunctional Hall thrusters and subsequent research. [ABSTRACT FROM AUTHOR]

This paper experimentally evaluated the effect of the disruption of the symmetrical distribution of the propellant on the characteristics of the anode current. The change in the asymmetry degree of the propellant distribution is achieved by supplying gas with a dual-cavity gas distributor. The results show that as the asymmetry degree increases, the magnitude of the anode current changes monotonically from a slow growth to a rapid growth, while the peak-to-peak value of the anode current exhibits a non-monotonic behavior. A preliminary analysis shows that the asymmetrical distribution of the propellant causes a nonuniform plasma generation along the azimuthal direction and consequently the appearance of an azimuthal electric field. The effect of the azimuthal electric field on the electron azimuthal drift as well as the induction of the electron axial drift are key factors that account for the change of the anode current and its oscillation. • Transition in discharge current occurs as propellant asymmetry degree increases. • Transition point drops down as propellant flow rate or discharge voltage increases. • Azimuthal electric field induced by asymmetrical discharge accounts for transition. [ABSTRACT FROM AUTHOR]

A type of Hall thruster without wall losses is designed by adding two permanent magnet rings in the magnetic circuit. The maximum strength of the magnetic field is set outside the channel. Discharge without wall losses is achieved by pushing down the magnetic field and adjusting the channel accordingly. The feasibility of the Hall thrusters without wall losses is verified via a numerical simulation. The simulation results show that the ionization region is located in the discharge channel and the acceleration region is outside the channel, which decreases the energy and flux of ions and electrons spattering on the wall. The power deposition on the channel walls can be reduced by approximately 30 times. [ABSTRACT FROM AUTHOR]

A typical Hall thruster is powered by a constant-voltage power supply. In this study, to ensure the normal operation of Hall thrusters at lower power conditions, we applied pulse modulation technology to supply power to a 600-W Hall thruster. The results show that, when the input power of the thruster was within the rated power range, the thrust and efficiency of the thruster were slightly reduced under pulsating mode compared with those of direct current (DC) operation. However, the thrust-to-power ratio of the thruster under pulsating mode was superior to that under DC operation at low power (<350 W). The thrust-to-power ratio under pulsating mode was improved to 77.5 mN/kW at 210 W compared with that under DC operation (70 mN/kW). Meanwhile, we found that the stable operating boundary of the thruster under pulsating mode significantly widened at high frequencies. A 600-W Hall thruster under pulsating mode can operate at a lowest power of 66.8 W, which is an order of magnitude lower than the rated power. The thrust and anode efficiency under pulsating mode at 68.8 W were 4.3 mN and 7.8%, respectively. [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]

A magnetically shielded Hall thruster (MSHT) has a significantly longer service life than a conventional unshielded Hall thruster (USHT) as it shifts the magnetic field of maximum strength from the channel interior to the exterior. In this study, it was experimentally determined that the influence of the radial position of an externally mounted hollow cathode on the performance of an MSHT is quite different from that of a USHT. The measured plasma parameters demonstrate that in addition to the coupling voltage loss and plume divergence loss, the electron cross-field transport loss (not considered previously and introduced by the outward shifting of the magnetic field) is also important to explaining the performance variation in the MSHT. This study refines the coupling theory between the cathode and Hall thruster and provides a valuable reference for determining the radial position of an externally mounted hollow cathode in an MSHT. [ABSTRACT FROM AUTHOR]

In steady-state operation of Hall thrusters excited by permanent magnets, the existence of temperature difference between the inner and outer permanent magnets will cause the deviation of the magnetic field configuration from the design value, thus affecting the discharge process. The simulation results show that the ion power loss on the inner wall increases by over 7.5% when the temperature difference between the inner and outer permanent magnets reaches to 140 ℃. The main reason is that the coercivity of the inner permanent magnet decreases more at higher temperature, and the magnetic field lines will incline toward the inner wall, resulting in the increment of the radial component of the electric field pointing to the inner wall. An optimized magnetic field, obtained by moving the inner permanent magnet outward in an appropriate manner, is proposed to alleviate the inner wall erosion. The optimized magnetic field has a near-symmetrical configuration, especially the nearly equal length of intersection lines of the magnetic field lines with the inner and outer walls, which is contributed to reduce the radial component of the electric field and preventing the inclination of the main ionization zone toward the inner wall, so that the peak value of ion power deposition on inner wall can be reduced by more than 50%. Based on the illustrated features and mechanism of wall erosion, an effective and easy-to-implement optimization scheme was proposed in this work to provide a useful reference for the design of long-life Hall thrusters with large height-radius ratio. [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]

Traditional Hall thrusters can ionize gas inside the channel and accelerate ions outside it by moving the peak magnetic field out of the channel, which improves the thruster performance. In this study, the effects of the peak magnetic field location on the discharge characteristics of a 100-W Hall thruster with large-gradient magnetic field were investigated by changing the ratio of the magnetic field intensity at the channel exit to the peak magnetic field intensity (Br exit /Br max). The experimental results show that the discharge performance decreases significantly when Br exit /Br max < 1. Moreover, the PIC numerical simulation and neutral gas simulation results demonstrate that the ionization rate inside the channel reduces and the ionization proportion outside the channel increases when Br exit /Br max decreases, thereby reducing the performance. When Br exit /Br max = 1, under the anode flow of 5.5 sccm and discharge voltage of 250 V, the thrust, anode specific impulse, propellant utilization efficiency, and anode efficiency of the 100-W thruster were 6.4 mN, 1210 s, 68.9%, and 38.5%, respectively, which represent respective improvements of 16.2%, 16.4%, 13.0%, and 30.0% (relative value) compared with the condition when Br exit /Br max = 0.9. These results provide a reference for the performance optimization of 100-W Hall thrusters with a large-gradient magnetic field. • The Effects of peak magnetic field location on performance of a 100W large gradient magnetic field Hall thruster is studied. • The ionization and discharge performance parameters of thruster under different Br exit /Br max conditions are given. • When the value of Br exit /Br max decreases from 1, the overall performance of the thruster decreases significantly. • The decrease of Br exit /Br max worsens the ionization process and reduces the overall performance of the thruster. [ABSTRACT FROM AUTHOR]

In an ATON-type Hall effect thruster, a ring-shaped anode and a cusped magnetic field intersect the match between the anode shape and the field topology thus must be clarified to optimize the electron transport to the anode and consequently the design of a high-efficiency thruster. By changing the match pattern with both the change in the length of the anode ring and the axial displacement of the cusp magnetic field, this study experimentally investigated the influence of the match pattern on the discharge characteristics of an ATON-type thruster—P100—under the condition of a moderate discharge voltage. The experimental results show that there is a match pattern that always optimizes the performance of the P100 thruster. At the rated operation parameters (300 V of discharge voltage and 5 mg/s of propellant mass flow rate) and the rated magnetic field strength, the observed improvements on thrust (∼79 mN to ∼85 mN) and anode efficiency (∼46% to ∼55%) are significant. Through further theoretical analysis, this study revealed that the change in the characteristics of electron momentum and energy transfer in the near-anode region, induced by the change of the match pattern, is the basic reason. The findings of this work are instructive for both understanding the electron motion in a cusp magnetic field and guiding the design of the anode ring intersecting with a cusp magnetic field in an ATON-type Hall effect thruster. [ABSTRACT FROM AUTHOR]

This study proposes a newly designed ignition circuit that does not change the ignition starting parameters of a Hall thruster, reduces the critical ignition voltage, and improves ignition reliability. The complexity and weight of the Hall thruster power supply system are reduced by multiplexing the ignition and discharge power supplies. Under the condition that the design parameters are satisfied, that is, R 3 + R E C 2 is much larger than RCC1, the residual energy in the circuit after the cathode breakdown can be fed into the thruster discharge channel to improve the ignition energy supply. The experimental results show that when the anode voltage is 98 V, only nine of the 100 ignition tests are successful using the traditional ignition circuit, whereas the number of successes using the newly designed ignition circuit is as high as 99, which provides a simple solution method for improving the ignition reliability. [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]

Poor performance limits the wider application of 100-W-power class Hall thrusters. The effects of position matching between the anode and the zero-magnetic-field region (ZMFR) on the discharge process and performance improvement of 100-W Hall thrusters were investigated. The experimental results demonstrate that appropriately widening the ZMFR downstream of the anode improves the ion current and thrust, promotes plume focusing, and optimizes thruster performance. The thrust and anode efficiency is enlarged by 7.6 % and 20 % (relative value), respectively, when the ZMFR downstream of the anode is expanded by 0.2 L (L is the channel length). The simulation results indicated that widening the ZMFR promoted the expansion of the high-ionization zone to the bottom of the channel, and increased the overall ionization rate. Moreover, as the ZMFR increases, the effective ion acceleration voltage increases. However, widening the ZMFR also aggravated the ion-wall interaction and reduced the propellant utilization rate. Therefore, the performance of the thrusters exhibited an optimal range as the length of the ZMFR increased. In addition, increasing the ZMFR did not affect the life of the thrusters. • The effects of position matching between the anode and zero magnetic field region on discharge characteristics are studied. • The discharge characteristic parameters of thruster with different lengths of ZMFR downstream of the anode are given. • Appropriately increasing the ZMFR can improve the ion current and thrust, thereby improving the anode efficiency. • The position matching between the anode and ZMFR affects the ionization process and propellant utilization ratio. [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]

In this study, the evolution of the plasma parameters of a Hall thruster under the pulsating operation was investigated using the particle-in-cell (PIC) model. The results show that the current waveform of the simulation results was almost the same as that of the experimental results under the same pulse parameters and working conditions. The average thrust of the simulation results was close to the experimentally measured thrust, so the accuracy of the simulation model was confirmed. The peak thrust of the Hall thruster under pulsating operation was approximately three times that under direct current (DC) operation. The simulation results show that the ionization rate increases rapidly under pulsating operation and is several times higher than that under DC operation. An increase in the number of ions led to an increase in the discharge current. Meanwhile, owing to the increase in the potential drop in the acceleration zone of the Hall thruster under pulsating operation, the ion acceleration effect was improved. These are the main reasons for the rapid increase in the instantaneous thrust during the pulsating operation. • A PIC model for Hall thruster under pulsating operation was proposed. • Comparison of simulation and experimental results of discharge characteristics and thrust verified the accuracy of the model. • The evolution of the plasma parameters of a Hall thruster under the pulsating operation was investigated. • The increase in the potential drop and ionization rate are the main reasons for the rapid increase in the instantaneous thrust. [ABSTRACT FROM AUTHOR]

Abstract HET72-VC is a kW-class Hall effect thruster with a variable channel characterized by high thrust and power densities. In this work, an experimental study of the effect of magnetic field configuration on the discharge characteristics of HET72-VC is carried out. By separately adjusting the current through the front and rear coils to change the magnetic field configuration inside the channel, a change in the plume is observed and the discharge performance parameters are measured. Experimental results show that the effects of reducing the front coil current and increasing the rear coil current on the thruster discharge are consistent. The plume gradually changes from a hollow cone with a clear boundary into a solid cone, the thrust increases by about 3%, and the anode efficiency increases by about 2.5%. Theoretical analysis shows that the degree of inclination of the magnetic lines of force plays an important role on the electron conduction and ion acceleration. When the angle between the magnetic lines of force and the channel centerline becomes larger, the discharge performance improves. This study helps in providing a deeper understanding of the working characteristics of this type of thruster, and can provide important guidelines for its structure design and optimization. Highlights • Plume shape is strongly modulated by magnetic field configuration. • Plume shape of a solid cone rather than a hollow cone represents good performance. • Magnetic field configuration impeding electrons strongly yields good performance. [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]

A kilowatt-order Hall effect thruster with a variable channel, the HET72-VC, is proposed. The discharge channel consists of an inner divergent annular segment and a cylindrical segment near the exit. A discharge comparison experiment was conducted on this thruster, in which the propellant gas is supplied along the axial direction and along the channel centerline. The results show that under the condition of gas supply along the channel centerline, both the current utilization and propellant utilization are higher, the plume divergence angle is smaller, the ion energy distribution curve moves to the high-energy portion, and the overall discharge performance, including the thrust and anode efficiency, is improved. A theoretical analysis shows that these results are attributed to the change in the ionization and acceleration characteristics caused by the different gas supply directions. The HET72-VC is demonstrated to be able to operate stably at an electric power of 2.5 kW; compared with those Hall effect thrusters with the same outer diameter or flow area of the channel, its thrust density and power density are both greatly improved. This provides a new perspective for the development of a high-power Hall effect thruster with a compact structure. [ABSTRACT FROM AUTHOR]

The channel wall shape of a magnetically shielded Hall thruster is typically designed to be tangential to the near-wall magnetic field line, which significantly constrains wall design freedom. In this study, the wall chamfer was expanded based upon tangent matching, and the effectiveness of magnetic shielding was determined based on the wall color after the experiments. The experiments indicated that the thruster would maintain the same high performance and no visible erosion at constant power levels and constant anode mass flow rate levels in the expanded chamfer wall, even if the wall was not perfectly tangent to the grazing line. The results provide new insights into the design of the topology of magnetically shielded Hall thrusters. • Good magnetically shielded effect remains when both endpoints of chamfered wall is over-tangent to magnetic field line. • Over tangent of wall downstream endpoint affects the discharge little and so a high level of thruster performance remains. • Over-tangent wall shape is much easier to achieve than the perfect tangent one, thereby expanding wall design freedom. [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]