Numerical study of viscosity and heat flux role in heavy species dynamics in Hall thruster discharge

A two-and three-dimensional velocity space axisymmetric hybrid-PIC model of Hall thruster discharge called Hybrid2D has been developed.The particle-in-cell(PIC) method was used for neutrals and ions(heavy species),and fluid dynamics on a magnetic field-aligned(MFA) mesh was used for electrons.A time-saving method for heavy species moment interpolation on a MFA mesh was developed.The method comprises using regular rectangle and irregular triangle meshes,connected to each other on a pre-processing...     

The far-field plasma characterization in a 600W Hall thruster plume by laser-induced fluorescence

Non-intrusive characterization of the singly ionized xenon velocity in Hall thruster plume using laser induced fluorescence(LIF) is critical for constructing a complete picture of plume plasma,deeply understanding the ion dynamics in the plume, and providing validation data for numerical simulation. This work presents LIF measurements of singly ionized xenon axial velocity on a grid ranging from 100 to 300 mm in axial direction and from 0 to 50 mm in radial direction for a600 W Hall thruster operating at the nominal condition of discharge voltage 300 V and discharge current 2 A, the influence of discharge voltage is investigated as well. The ion velocity distribution function(IVDF) results in the far-field plume demonstrate a profile of bimodal IVDFs, especially prominent at radial distances greater than channel inner radius of 22 mm at axial position of 100 mm, which is quite different from that of the near-field plume where bimodal IVDFs occur in the central core region for the same power Hall thruster when compared to previous LIF measurements of BHT-600 by Hargus(2010 J. Propulsion Power 26 135).Beyond 100 mm, only single-peak IVDFs are measured. The two-dimensional ion velocity vector field indicates the bimodal axial IVDF is merely a geometry effect for the annular discharge channel in the far-field plume. Results about the IVDF, the most probable velocity and the accelerating potential profile along the centerline all indicate that ions are still accelerating at axial distances greater than 100 mm, and the maximum most probable velocity measured at300 mm downstream of the exit plane is about 19 km s-1. In addition, the most probable velocity of ions along radial direction changes a little except the lower velocity ion populations in the bimodal IVDF cases. The ion temperature at axial distances of 10 and 300 mm oscillates along the radial direction, while the ion temperature first increases, and then decreases for the 200 mm case. Finally, the axial position for the ion peak axial velocity on the thruster centerline is shifted upstream for higher discharge voltages, and the velocity curve is becoming steeper with the discharge voltage before reaching the maximum. This observation can be used as a criterion to optimize the thruster performance.     

Life test research of a high specific impulse Hall thruster HEP-140MF

In this study, a high specific impulse Hall thruster, HEP-140MF, having a high discharge voltage, was used to accelerate ions. We aimed to obtain a high specific impulse and an acceleration zone moving downstream toward the channel exit to reduce wall sputtering erosion of the walls of the discharge channel, hence ensuring an enhanced lifetime. To study the lifetime characteristics of the high specific impulse Hall thruster, a life test was performed on the HEP- 140MF thruster for the first time, and performance parameters, such as thrust, specific impulse, and efficiency, were measured. Changes in the performance parameters and evolutions in the surface profiles of the discharge channel wall were summarized. The reasons contributing to these changes during the life test were analyzed. Moreover, the accelerated life test method was validated on the HEP-140MF.     

Effect of Segmented Electrode Length on the Performances of an Aton-Type Hall Thruster

The influences of the low-emissive graphite segmented electrode placed near the channel exit on the discharge characteristics of a Hall thruster are studied using the particlein-cell method.A two-dimensional physical model is established according to the Hall thruster discharge channel configuration.The effects of electrode length on the potential,ion density,electron temperature,ionization rate and discharge current are investigated.It is found that,with the increasing of the segmented electrode length,the equipotential lines bend towards the channel exit,and approximately parallel to the wall at the channel surface,the radial velocity and radial flow of ions are increased,and the electron temperature is also enhanced.Due to the conductive characteristic of electrodes,the radial electric field and the axial electron conductivity near the wall are enhanced,and the probability of the electron-atom ionization is reduced,which leads to the degradation of the ionization rate in the discharge channel.However,the interaction between electrons and the wall enhances the near wall conductivity,therefore the discharge current grows along with the segmented electrode length,and the performance of the thruster is also affected.     

Particle-in-cell simulation for effect of anode temperature on discharge characteristics of a Hall effect thruster

Propellant gas flow has an important impact on the ionization and acceleration process of Hall effect thrusters (HETs). In this paper, a particle-in-cell numerical method is used to study the effect of the anode temperature, i.e., the flow speed of the propellant gas, on the discharge characteristics of a HET. The simulation results show that, no matter the magnitude of the discharge voltage, the calculated variation trends of performance parameters with the anode temperature are in good agreement with the experimental ones presented in the literature. Further mechanism analysis indicates that the magnitude of the electron temperature is responsible for the two opposing variation laws found under different discharge voltages. When the discharge voltage is low, the electron temperature is low, and so is the intensity of the propellant ionization; the variation of the thruster performance with the anode temperature is thereby determined by the variation of the neutral density that affects the propellant utilization efficiency. When the discharge voltage is high, the electron temperature is large enough to guarantee a high degree of the propellant utilization no matter the magnitude of the anode temperature. The change of the thruster performance with the anode temperature is thus dominated by the change of the electron temperature and consequently the electron-neutral collisions as well as the electron cross-field mobility that affect the current utilization efficiency.     

Numerical simulation of hydrogen arcjet thruster with coupled sheath model

In the present work, a complete 2D chemical and thermal non-equilibrium numerical model coupled with a relatively simple sheath model is developed for hydrogen arcjet thruster. Conduction heat transfer in the anode wall is also included in the model. The operating voltages predicted by the model are compared with those in the literature and are found to be in close agreement. Power distributions for the various operating conditions are obtained, anode radiation loss primarily determines the thruster efficiency. Higher thruster efficiency was found to be associated with longer arc length. At cathode ion diffusion contribution dominates except at low input current where thermo-field electron current is dominant.     

Porosity dependence of thermal conductivity of uranium mononitride

Thermal diffusivity of sintered uranium mononitrides of various densities 70–95% TD were measured over the temperature range from 300 to 1400 °C. The curve of thermal diffusivity versus porosity at each temperature exhibited a break at porosity near 0.1. Thermal conductivity obtained from thermal diffusivity was found to be well fitted to a set of the simplified Maxwell-Eucken equations. The coefficient β in the equation was obtained as 1.68–1.86 for the porosity range above 0.10 and that for the smaller porosity range varied from 1.38 at 300 °C to ?0.09 at 1300 °C. The porosity dependence of thermal conductivity of the porous material is discussed, taking account of the effect of pore conduction in a similar way as conducted by Marino. The calculated result for the larger porosity is in agreement with the experimental ones, whereas that for the smaller porosity is not wholly consistent with the experiments.     

Study on the influence of the discharge voltage on the ignition process of Hall thrusters

A high-speed charge-coupled device camera was used to capture images of the plume and acceleration channel of a Hall effect thruster during ignition at different discharge voltages. To better understand the influence of changes in the discharge voltage on the plasma parameters during thruster ignition, a particle-in-cell numerical model was used to calculate the distribution characteristics of the ion density and electric potential at different ignition moments under different discharge voltages. The results show that when the discharge voltage is high, the ion densities in the plume and acceleration channel are significantly higher at the initial phase of thruster ignition; with the gradual strengthening of the ignition process, the propellant avalanche ionization during thruster ignition occurs earlier and the pulse current peak increases. The main reason for these phenomena is that the change in the discharge voltage results in different energy acquisitions of the emitted electrons entering the thruster channel.     

Upward vertical two-phase flow local flow regime identification using neural network techniques

Traditionally, the flow regimes in two-phase flow are considered in a global sense. However, a local flow regime is required to understand and model the interfacial structures present in the flow. In this work, a new approach has been used to identify both global and local flow regimes in a two-phase upward flow in a 50.8 mm internal diameter pipe under adiabatic conditions. In the present method, the bubble chord length distributions, which are measured simultaneously with three double-sensor conductivity probes, have been used to feed a self-organized neural network. The global flow regime identification results show a reasonable agreement with the visual observation for all the flow conditions. Nonetheless, only the local flow regimes measured at the center of the pipe agree with the global ones. The local flow regime combinations found are analyzed using the flow map information, cross-correlations between the probe signals, and previous correlations. In this way, it is possible to identify eight different global flow regime configurations.     

Study on the influences of ionization region material arrangement on Hall thruster channel discharge characteristics

There exists strong interaction between the plasma and channel wall in the Hall thruster,which greatly affects the discharge performance of the thruster.In this paper,a two-dimensional physical model is established based on the actual size of an Aton P70 Hall thruster discharge channel.The particle-in-cell simulation method is applied to study the influences of segmented low emissive graphite electrode biased with anode voltage on the discharge characteristics of the Hall thruster channel.The influences of segmented electrode placed at the ionization region on electric potential,ion number density,electron temperature,ionization rate,discharge current and specific impulse are discussed.The results show that,when segmented electrode is placed at the ionization region,the axial length of the acceleration region is shortened,the equipotential lines tend to be vertical with wall at the acceleration region,thus radial velocity of ions is reduced along with the wall corrosion.The axial position of the maximal electron temperature moves towards the exit with the expansion of ionization region.Furthermore,the electron-wall collision frequency and ionization rate also increase,the discharge current decreases and the specific impulse of the Hall thruster is slightly enhanced.     

The far-field plasma characteristics of LHT40 low-power Hall thruster for commercial aerospace applications

To achieve a better insight into the far-field plasma spatial distribution and evolution characteristics of the 300 W class low-power Hall thruster (LHT) for commercial aerospace applications,a dedicated and integrated plasma diagnostic system composed of seventeen Faraday cups (FC) and two triple Langmuir probes (TLP) is established to investigate the time-averaged in situ spatial distribution characteristics of far-field ions and electrons.The ion current density (ICD),plasma potential,plasma density,and electron temperature at 1000 mm downstream of 300 W class LHT for commercial aerospace applications in the azimuthal angle range of-90° to 90° were investigated under the conditions of different anode mass flow rates and discharge voltages.The results demonstrated that ICD,beam divergence angle,and mass utilization efficiency increased with increasing anode mass rate.The double-wings phenomenon was observed in the spatial distribution of ICD at large angles from the thruster axis,which is attributed to charge exchange collisions at increasing vacuum backpressure.The plasma electron temperature,electron density,and plasma potential parameters derived from the TLP decreased rapidly in the angle range from 0° to 30° and did not exhibit significant variations above 30°,which was also in good agreement with the results of the measured divergence angle of the FC.The discrepancy of average ion speed was calculated.The maximum error is better than 31.5％which checks the consistency between the TLP's results and that of FC to some extent.     

Study on the characteristics of non-Maxwellian magnetized sheath in Hall thruster acceleration region

The secondary electron emission (SEE) and inclined magnetic field are typical features at the channel wall of the Hall thruster acceleration region (AR), and the characteristics of the magnetized sheath have a significant effect on the radial potential distribution, ion radial acceleration and wall erosion. In this work, the magnetohydrodynamics model is used to study the characteristics of the magnetized sheath with SEE in the AR of Hall thruster. The electrons are assumed to obey non-extensive distribution, the ions and secondary electrons are magnetized. Based on the Sagdeev potential, the modified Bohm criterion is derived, and the influences of the non-extensive parameter and magnetic field on the AR sheath structure and parameters are discussed. Results show that, with the decrease of the parameter q, the high-energy electron leads to an increase of the potential drop in the sheath, and the sheath thickness expands accordingly, the kinetic energy rises when ions reach the wall, which can aggravate the wall erosion. Increasing the magnetic field inclination angle in the AR of the Hall thruster, the Lorenz force along the $x$ direction acting as a resistance decelerating ions becomes larger which can reduce the wall erosion, while the strength of magnetic field in the AR has little effect on Bohm criterion and wall potential. The propellant type also has a certain effect on the values of wall potential, secondary electron number density and sheath thickness.     

Hybrid–PIC simulation of sputtering product distribution in a Hall thruster

Hall thrusters have been widely used in orbit correction and the station-keeping of geostationary satellites due to their high specific impulse,long life,and high reliability.During the operating life of a Hall thruster,high-energy ions will bombard the discharge channel and cause serious erosion.As time passes,this sputtering process will change the macroscopic surface morphology of the discharge channel,especially near the exit,thus affecting the performance of the thruster.Therefore,it is necessary to carry out research on the motion of the sputtering products and erosion process of the discharge wall.To better understand the moving characteristics of sputtering products,based on the hybrid particle-in-cell (PIC) numerical method,this paper simulates the different erosion states of the thruster discharge channel in different moments and analyzes the moving process of different particles,such as B atoms and B+ ions.In this paper,the main conclusion is that B atoms are mainly produced on both sides of the channel exit,and B+ ions are mainly produced in the middle of the channel exit.The ionization rate of B atoms is approximately 1％.     

A method to measure the in situ magnetic field in a Hall thruster based on the Faraday rotation effect

This paper presents a method to measure the in situ magnetic field in a Hall thruster by optical non-invasive means, based on the optical Faraday rotation effect. This method does not affect the discharge of the thruster. Furthermore, its time resolution depends on the speed of the photodetector, and measurement at a MHz scale can be achieved.     

Numerical study of low-frequency discharge oscillations in a 5kW Hall thruster

A two-dimensional particle-in-cell plasma model is built in the R–Z plane to investigate the lowfrequency plasma oscillations in the discharge channel of a 5 kW LHT-140 Hall thruster. In addition to the elastic, excitation, and ionization collisions between neutral atoms and electrons,the Coulomb collisions between electrons and electrons and between electrons and ions are analyzed. The sheath characteristic distortion is also corrected. Simulation results indicate the capability of the built model to reproduce the low-frequency oscillation with high accuracy. The oscillations of the discharge current and ion density produced by the model are consistent with the existing conclusions. The model predicts a frequency that is consistent with that calculated by the zero-dimensional theoretical model.     

Modelling of the HFR-EU1BIS experiment and thermomechanical evaluation

Numerical modelling has been successfully applied during the design and evaluation of the HFREU1bis HTR fuel pebble irradiation experiment conducted by the Joint Research Centre Institute for Energy (JRC) in the HFR Petten, The Netherlands. HFR-EU1bis contains 5 HTR fuel pebbles that are placed in a graphite shroud. This assembly is placed in a containment tube, which is again placed in another containment, which is in contact with the HFR cooling water.The experimental requirement is to maintain a central temperature of 1250 °C for all pebbles throughout the irradiation time of 250 effective full power days (efpd). This was achieved by tailoring the gas gaps in the sample holder such that the axial heat generation profile is compensated. Determining the dimensions of the components of the experiment has been done using a thermomechanical finite element model. A combination of numerical and analytical techniques has been applied to minimise calculation times. The thermomechanical finite element model includes the thermal influence of changing gas gap dimensions due to expansion. Heat transfer by radiation through the gas gap has been modelled as well.Results of the model have been compared with measured temperatures. After some adjustments of the model parameters, good agreement has been found between calculated and measured temperatures. Additionally, the influence of irradiation on graphite thermal conductivity and thermal expansion has been included in the model. Due to irradiation damage the thermal conductivity of graphite decreases and thermal expansion generally increases, leading to increasing thermal stresses. Due to burn-up however, the heat flux decreases during irradiation, which leads to a decrease in thermal stresses. To evaluate the influence of these competing mechanisms, the thermal stress evolution during irradiation has been calculated and will be discussed.     

Effect of low-frequency oscillation on performance of Hall thrusters

In this paper,a direct connection between the discharge current amplitude and the thruster performance is established by varying solely the capacitance of the filter unit of the Hall thrusters.To be precise,the variation characteristics of ion current,propellant utilization efficiency,and divergence angle of plume at different low-frequency oscillation amplitudes are measured.The findings demonstrate that in the case of the propellant in the discharge channel just meets or falls below the full ionization condition,the increase of low-frequency oscillation amplitude can significantly enhance the ionization degree of the neutral gas in the channel and increase the thrust and anode efficiency of thruster.On the contrary,the increase in the amplitude of low-frequency oscillation will lead to increase the loss of plume divergence,therefore the thrust and anode efficiency of thruster decrease.     

Matching characteristics of magnetic field configuration and chamfered channel wall in a magnetically shielded Hall thruster

To date,the selection of the magnetic field line used to match the chamfered inner and outer channel walls in a magnetically shielded Hall thruster has not been quantitatively studied.Hence,an experimental study was conducted on a 1.35 kW magnetically shielded Hall thruster with a xenon propellant.Different magnetic field lines were chosen,and corresponding tangentially matched channel walls were manufactured and utilized.The results demonstrate that high performance and a qualified anti-sputtering effect cannot be achieved simultaneously.When the magnetic field lines that match the chamfered wall have a strength at the channel centerline of less than 12％ of the maximum field strength,the channel wall can be adequately protected from ion sputtering.When the magnetic field lines have a strength ratio of 12％-20％,the thruster performance is high.These findings provide the first significant quantitative design reference for the match between the magnetic field line and chamfered channel wall in magnetically shielded Hall thrusters.     

An efficient static correction algorithm with application to the hydrodynamic loading of thin shell structures

This paper treats the problem of correctly accounting for the quasi-static component of the response of a thin-shell structure to a dynamic loading which originates from a pressure disturbance within the body of a contained fluid. The importance of quasi-static structure response has been observed in BWR safety/relief valve (SRV) actuation tests which produce a pressure oscillation in the suppression pool as a consequence of the expansion and contraction of bubbles formed from air expelled from the SRV pipeline. The problem is treated within the context of a finite-element methodology which has been previously developed for the coupled analysis of a linear elastic structure in contact with a linear acoustic fluid. For problems involving a large number of degrees of freedom, the efficient application of this method requires reduction to a set of normal modes approximating the response of the coupled fluid-structure system. The coupled modes are synthesized from the fluid properties and a set of normal modes describing the behavior of the structure in the absence of the fluid. The proposed solution to the problem is to append a shape function, representing the quasi-static component of the structural response, to the set of modes computed by standard eigenvalue extraction. The success of the method is demonstrated by comparison of calculated results with measured structural response from a typical SRV test.     

Experimental and numerical simulation study of the effect for the anode positions on the discharge characteristics of 300 W class low power Hall thrusters

Low-power Hall thruster(LHT) generally has poor discharge efficiency characteristics due to the large surface-to-volume ratio.Aiming to further refine and improve the performance of 300 W class LHT in terms of thrust and efficiency,and to obtain the most optimal operating point,the experimental study of the discharge characteristics for three different anode positions was conducted under the operation of various discharge voltages(100-400 V) and anode mass flow rates(0.65 mg·s^-1 and 0...