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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.

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.

Ionic wind has been widely used in the fields of flow control, heat cooling, thruster, and so on. In this work, an investigation of ionic wind under different magnetic field configurations is processed. By changing the position relationship between the electrode and the magnetic field, the magnetic field configuration in the discharge region is modified. With the injection of different magnetic field configurations to the discharge region, the amplitude of Trichel pulse is decreased, and the pulse frequency is increased. Besides, the ionic wind velocities’ distribution is deflected and the electrical-to-kinetic energy conversion efficiency is enhanced. The mechanism is analyzed by considering the movement path of particles in the ionization region and drift region under different magnetic field configurations. These results illustrate that the magnetic field configuration would be a method to adjust the direction and improve the electrical-to-kinetic energy conversion efficiency of ionic wind. This method provides a feasible way to regulate the performance of ionic wind without additional energy consumption.

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.

Hall thrusters must be frequently ignited and extinguished during operation in orbit. The thruster is typically extinguished by terminating the anode voltage. However, the equilibrium state of the plasma in the discharge channel is lost when the external electric field disappears. This consequently changes the state of the plasma, causing rotating spokes to appear. However, spoke rotation ceases after nine rotations. Therefore, in this study, a high-speed camera was used to study the instantaneous spoke rotations of a Hall thruster during the extinguishing process. Additionally, the characteristics of the azimuthal rotating spoke during this process were studied. It was found that, because the anode voltage does not instantaneously decrease to 0 V during the extinguishing process, the spoke mode changes from multi-mode to single-mode. Furthermore, as the voltage dropped from 250 to 0 V, a single-mode spoke appeared, and the spoke speed linearly decreased.

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.

A high speed charged coupled device (CCD) camera was used to examine the plume features over time when the cathode was located inside and outside the magnetic separatrix. Due to the different positions of the cathode, there are obvious differences in the electron impact excitation process, the characteristics of the plasma bridge, and the transition process from the end of ignition to the steady-state discharge process. The main reason for these differences may be due to the different conduction paths followed by emitted electrons into the acceleration channel. The CCD images method can be used as a technique to characterize the ignition process of Hall thruster.

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.

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.

A time-resolved optical measurement method is applied to examine the gas discharge process inside the discharge channel during a Hall thruster ignition. The results show that the excitation and ionization processes in the working gas begin near the exit of the channel and then propagate upstream along the channel. The change in energy of electrons moving toward the anode leads to the alternating dominance of ionization and excitation processes, which is the main reason for the changes in light intensity in different regions of the channel during the thruster startup.

Discharge characteristics of a non-wall-loss Hall thruster were studied under different channel lengths using a design based on pushing a magnetic field through a double permanent magnet ring. The effect of different magnetic field intensities and channel lengths on ionization, efficiency, and plume divergence angle were studied. The experimental results show that propellant utilization is improved for optimal matching between the magnetic field and channel length. While matching the magnetic field and channel length, the ionization position of the neutral gas changes. The ion flow is effectively controlled, allowing the thrust force, specific impulse, and efficiency to be improved. Our study shows that the channel length is an important design parameter to consider for improving the performance of non-wall-loss Hall thrusters.

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.

In this paper, we investigated how magnetic field influenced Trichel discharges by performing a statistical analysis of the current amplitude and pulse interval of Trichel discharges at various magnetic field intensities. Experimental results indicate that the amplitude and pulse intervals always obey lognormal functions. As the magnetic field intensity increased, the current amplitudes almost decreased and the pulse intervals increased significantly. The paths of negative ions produced in the Trichel discharge were curved by the magnetic field, weakening their shielding effect. A magnetic field can enhance the possibility of Trichel discharge with small energy release. Moreover, the characteristics of the Trichel pulse could be controlled with only magnetic field and without any energy addition.

The effect of the tube materials on the characteristics of an argon atmospheric pressure cold plasma jet is investigated through a single-electrode configuration driven by ac power supply. Two kinds of tube materials, Pyrex glass and 95% alumina ceramic, are applied in the experiments. Results show that, with the change of tube material, both the discharge current and the plume length are barely affected, whereas the plume volume exhibits an enormous distinction. In comparison with the results obtained in the ceramic tube, the peak volume can increase by 65% in the glass tube, while the increment of the peak length is only 2%. Through the analysis, the difference in the thermal conductivity between the glass and the ceramic is regarded as the main reason.

High-energy ions in the plume of Hall thrusters can affect a spacecraft by sputtering, deposition contamination, or deflection torque. Thus, the plume divergence angle is an important aspect to evaluate for the application of Hall thrusters. Because of the variation in the plume divergence angle measurement with different measuring radii when the arc measurement method is adopted, a torus-source model was established in this study for error analysis. This variation, because of the different gyration radii, was analyzed, and a corresponding correction method was proposed. In addition, the accuracy and scope of the arc measurement method were improved.

The core–nanoshell composite materials with magnetic fly-ash hollow cenosphere as core and nano SmFeO3 as shell were synthesized by high-energy ball milling method. The magnetic fly-ash hollow cenosphere, samarium nitrate, and iron nitrate were used as raw materials. The synthesis and growth kinetics of the composite materials were investigated using the thermogravimetry and differential thermal analysis (TG–DTA) at different heating rates. The results show that the precursor of the composite materials decomposes in three steps. The apparent activation energy of each stage was calculated using the Doyle–Ozawa and Kissinger methods. The reaction order, frequency factor, and rate equations were also determined. The activation energy of the nano crystallite growth is calculated to be 16.12 kJ mol−1 according to kinetics theory of nano crystallite growth. It can be inferred that the crystallite grows primarily by means of an interfacial reaction during the thermal treatment. The magnetic properties and microwave absorbing properties of samples were analyzed by the vibrating sample magnetometer analysis and vector network analyzer. The results indicated that the exchange coupling interaction happens between ferrite of magnetic fly-ash hollow cenosphere and nanosized ferrite coating, which cause outstanding magnetic properties. In the frequency between 1 MHz and 1 GHz, the absorbing effectiveness of the composite absorbers can achieve −32 dB. The magnetic properties of the composite material are better than those of single phase. So it is consistent with requirements of the microwave absorbing material at the low-frequency absorption., National Natural Science Foundation (China) (No. 20976018), Liaoning Sheng (China) (Science Research Plan Project, Higher Education Department (No. L2013176)), China. Ministry of Education (Key Laboratory of Industrial Ecology and Environmental Engineering, KLIEEE-12-03), China. Ministry of Education (National Project of China ‘‘Innovation and Entrepreneurship Training Program of Undergraduate students’’ (201310150002))

Surrounded with distilled water, a classic argon atmospheric pressure discharge device is employed to study the propagation of the plasma. By examining the high-speed photographs, a phenomenon of interruption is found at different flow rates. At 3 L/min, the interruption appears only at the downward stage of the applied voltage after the plasma jet has run into the ambient air. As the flow rate reaches up to 15 L/min, however, the interruption can be obtained at a rather low applied voltage and exists in both the upward and downward stages. It is believed that the active species flowing from the upstream to the downstream region may be the main cause.

This paper illustrates a repetitive nanosecond volume discharge in the airflows at different velocities. The plasma images presented here show that the volume discharge modes vary with the changes of airflow speeds, and a diffuse and homogeneous volume discharge occurs at the airflow speed of 30 m/s. The discharge is promoted in the airflows at a lower velocity, and the discharge tends to be inhibited at a speed to 40 m/s. However, the inhibited mode disappears at higher repetitive frequency of the pulse.

The structure of an azimuthal diversion rail causes the neutral gas in the Hall effect thruster to possess a certain circumferential velocity. Two different directions of the magnetic field can be formed in the discharge channel, where the direction of the magnetic field depends on the direction of the excitation current. The aim of this work is to study the effect of magnetic field directionality on the discharging characteristics of a Hall effect thruster with an azimuthal diversion rail. The experimental results demonstrate that a change in the magnetic field direction corresponds to a change in the electronic Hall drift direction. When the electronic Hall drift direction is opposite to the circumferential velocity of the neutral gas, its thrust, specific impulse, and efficiency are higher than the case in which the electronic Hall drift direction is in the same direction as that of the circumferential velocity of neutral gas. The increase in performances is due to the increase in the propellant utilization. The reasons for the increase in the propellant utilization are explained. The experimental result provides a guide for the selection of the direction of excitation current of the Hall effect thruster with an azimuthal diversion rail.The structure of an azimuthal diversion rail causes the neutral gas in the Hall effect thruster to possess a certain circumferential velocity. Two different directions of the magnetic field can be formed in the discharge channel, where the direction of the magnetic field depends on the direction of the excitation current. The aim of this work is to study the effect of magnetic field directionality on the discharging characteristics of a Hall effect thruster with an azimuthal diversion rail. The experimental results demonstrate that a change in the magnetic field direction corresponds to a change in the electronic Hall drift direction. When the electronic Hall drift direction is opposite to the circumferential velocity of the neutral gas, its thrust, specific impulse, and efficiency are higher than the case in which the electronic Hall drift direction is in the same direction as that of the circumferential velocity of neutral gas. The increase in performances is due to the increase in the propellant utiliz...

In order to increase the degree of ionization and improve the efficiency, the effects of different cylindrical outer wall lengths on the discharge characteristics of the annular-cylindrical combined channel Hall thruster are studied. The cylindrical outer wall is set to the following positions: 95% (with positive magnetic field gradient), and 100%, 95%, 90%, 75%, and 50% (with negative magnetic field gradient) of the maximum magnetic field. The ionization gain and wall energy loss under different positions are analyzed through numerical simulations. The simulation result shows that when the cylindrical outer wall is set to the position where it is 100%–90% of the maximum magnetic field (negative magnetic field gradient zone), the power deposition on the channel wall is small, the propellant utilization can be largely improved, and the comprehensive performance is optimal for the annular-cylindrical combined channel Hall thruster.

By examining the affect, behavior and cognition (the ABCs) involved in the writing process of a Chinese high school student composing application essays in English (a foreign language) as a case study by means of ethnographic approach and under the notion of small culture, this study aims to illustrate how different cultural forces interact with one another and how they come to play in the shaping of rhetorical differences between learner's native language (L1/NL )and second or foreign language (L2/FL ) writing. Data were collected and analyzed from various sources --essay drafts, interviews, personal reflections, and email exchanges. The findings show that multiple cultural forces ranging from national culture (Chinese culture, for example) to small cultures such as L1/NL and L2 /EFL writing instruction and family education and the like, interact with one another and co-influence L2/EFL learner's writing practice. And the intensity of struggle in L2/EFL writing somewhat relates to the interaction between the mentioned cultures.

Seen as “assessment for learning”, formative assessment has attracted the attention of more and more educators and practitioners around the globe. Based on the writer’s experience with the implementation of formative assessment in her classrooms in China, this article describes the operational procedures of formative assessment use in classrooms. It first presents guidelines for the proportion of summative assessment and formative assessment in the course’s final scores, for formative assessment plan formulating and for feedback and grading, then describes the procedures, from need analysis and goal setting to formative assessment plan drafting, communicating, implementing and evaluating and revising.

As a learning tool, formative assessment has attracted the attention of more and more researchers. Based on the writer’s own innovation with the tool and relevant research, this paper first describes the use of formative assessment in college English classrooms in China, then provides recommendations and discusses future directions.