A study of the influence of different grid structures on plasma characteristics in the discharge chamber of an ion thruster

The grid structure has significant effects on the discharge characteristics of an ion thruster.The discharge performances of a 30 cm diameter ion thruster with flat,convex and concave grids are studied.The analysis results show that the discharge chamber with a convex grid has a larger’magnetic-field free area’than the others,and the parallelism of the magnetic-field isopotential lines and anode is generally the same in the three models.Plasma densities of the three structures at the grid outlet are in the range of 3.1×1016-6.9×1017m-3.Along the thruster axis direction,the electron temperature in the chamber with the convex and concave grids is in the range of 3.3-3.5 eV,while that with a flat grid is lower,in the range of 3.1-3.5 eV.In addition,the convex and the concave grids have better uniform distribution of electron temperature.Moreover,the collision frequency ratios show that the axial degree of ionization of the three models is the highest,and the flat grid has the highest discharge efficiency,followed by the convex grid and the concave grid is the least efficient.The test and simulation results of the 30 cm diameter ion thruster with the convex grid show that the measurement and calculation results are 3.67 A and 3.44 A,respectively,and the error above mainly comes from the ignorance of the doubly charged ions and parameter settings in the model.The comparison error between the simulation and measurement of beam current density is mainly caused by the actual thermal deformation of the grids during the discharge process,which leads to the change in electric potential distribution and variation of the focusing characteristics of the grids.Upon consideration of discharge performance and the thermal grid gap variation,it can be concluded that the flat and concave grids are more suitable for small-diameter ion thrusters,while the convex grid is a more reasonable choice for the higher-power and larger-diameter thrusters.

Experimental and numerical simulation study of the effect for the anode positions on the discharge characteristics of 300W 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-1and 0.95 mg·s-1).The experimental results indicated that the thruster has the most excellent performance in terms of thrust and efficiency etc at a channel length of 27 mm for identical operating conditions.In addition,particle in cell simulations,employed to reveal the underlying physical mechanisms,show that the ionization and acceleration zone is pushed downwards towards the channel exit as the anode moves towards the exit.At the identical operating point,when the channel length is reduced from 32 to 27 mm,the ionization and acceleration zone moves towards the exit,and the parameters such as thrust and efficiency increase due to the high ionization rate,ion number density,and axial electric field.When the channel length is further moved to 24 mm,the parameters in terms of thrust(F) and efficiency(ηa) are reduced as a result of the decreasing ionization efficiency(ηm) and the larger plume divergence angle(α).In this paper,the results indicated that an optimum anode position(ΔL=27 mm) exists for the optimum performance.

Study of beam divergence and thrust vector eccentricity characteristics of the Hall thruster based on dual Faraday probe array planes and its applications

The accurate knowledge of the thrust vector eccentricity and beam divergence characteristics of Hall thrusters are of significant engineering value for the beneficial integration and successful application of Hall thrusters on spacecraft.For the characteristics of the plume bipolar diffusion due to the annular discharge channel of the Hall thruster,a Gaussian-fitted method for thrust vector deviation angle and beam divergence of Hall thrusters based on dual Faraday probe array planes was proposed in respect of the Hall thruster beam characteristics.The results show that the ratios of the deviation between the maximum and minimum values of the beam divergence angle and the thrust vector eccentricity angle using a Gaussian fit to the optimized Faraday probe dual plane to the mean value are 1.4%and 11.5%,respectively.The optimized thrust vector eccentricity angle obtained has been substantially improved,by approximately 20%.The beam divergence angle calculated using a Gaussian fitting to the optimized Faraday probe dual plane is approximately identical to the non-optimized one.The beam divergence and thrust vector eccentricity angles for different anode mass flow rates were obtained by averaging the beam divergence and thrust vector eccentricity angles calculated by the dual-plane,Gaussian-fitted ion current density method for different cross-sections.The study not only allows for an immediate and effective tool for determining the design of thrust vector adjustment mechanisms of spacecraft with different power Hall thrusters but also for characterizing the 3D spatial distribution of the Hall thruster plume.

Molecular Dynamics Simulation of Interface Properties between Water-Based Inorganic Zinc Silicate Coating Modified by Organosilicone and Iron Substrate

The interface properties of Fe(101)/zinc silicate modified by organo-siloxane(KH-570)was studied by using the method of molecular dynamics simulation.By calculating the temperature and energy fluctuation of equilibrium state,equilibrium concentration distribution,MSD of layer and different groups,and interaction energy of two interface models,the influencing mechanism on the interface properties of adding organosiloxane into coating system was studied at the atomic scale.It shows that the temperature and energy of interface oscillated in a small range and it was exited in a state of dynamic equilibrium within the initial simulation stage(t<20 ps).It can be seen from the multiple peak states of concentration distribution that the iron substrate,organo-siloxane and zinc silicate are distributed in the form of a concentration gradient in the real environment.The rapid diffusion of free zinc powder in zinc silicate coating was the essential reason that affected the comprehensive properties of coating.The interface thickness decreased from 7.45 to 6.82Å,the MSD of free zinc powder was effectively reduced,and the interfacial energy was increased from 104.667 to 347.158 kcal/mol after being modified by organo-siloxane.

A study of grid failure mode drivers and methods for accelerated life testing of a 30cm diameter ion thruster

In view of the high cost caused by the 1:1 lifetime verification test of ion thrusters,the lifetime acceleration test should be considered.This work uses the PIC-MCC(Particle-in-Cell MonteCarlo Collision)method to analyze the five failure factors that lead to the failure of the accelerator grid of a 30 cm diameter ion thruster under the working mode of 5 k W.Meanwhile,the acceleration stress levels corresponding to different failure factors are obtained.The results show that background pressure has the highest stress level on the grid's erosion.The accelerator grid aperture's mass sputtering rate under the rated vacuum degree(1×10^(-4)Pa)of 5 k W work mode is 8.78 times that of the baseline vacuum degree(1×10^(-6)Pa),and the mass sputtering rate under worse vacuum degree(5×10^(-3)Pa)is 5.08 times that of 1×10^(-4)Pa.Under the influence of the other four failure factors,namely,the voltage of the accelerator grid,upstream plasma density,the screen grid voltage and mass utilization efficiency,the mass sputtering rates of the accelerator grid hole are 2.32,2.67,1.98 and 2.51 times those of the accelerator grid hole under baseline condition,respectively.The ion sputtering results of two 30 cm diameter ion thrusters(both installed with new grids assembly)after working for 1000 h show that the mass sputtering rate of the accelerator grid hole under vacuum conditions of 5×10^(-3)Pa is 4.54 times that under the condition of 1×10^(-4)Pa,and the comparison error between simulation results and test results of acceleration stress is about 10%.In the subsequent ion thruster lifetime verification,the working vacuum degree can be adjusted according to the acceleration stress level of background pressure,so as to shorten the test time and reduce the test cost.

Influence of temperature on the microstructure of V_2O_5 film prepared by DC magnetron sputtering

V2O5 films were prepared on silicon wafers by DC magnetron sputtering and post-annealing under various conditions. The influence of depositing and post-annealing temperatures on microstructure of V2O5 films was studied by XRD and Raman scattering measurements. The results reveal that sputtered V2O5 films show preferred growth orientation along (001) planes and the c-axis is perpendicular to the silicon substrate surface. It is interesting to find that both the V2O5 film deposited at temperature of 511 ℃ and the one annealed at 500 ℃ exhibit desirable layer-type structure of orthorhombic symmetry. Such layer-typed V2O5 films are promising candidates for cathodes of rechargeable lithium or magnesium thin-film batteries.

Characteristics and properties of metal aluminum thin films prepared by electron cyclotron resonance plasma-assisted atomic layer deposition technology

Metal aluminum (Al) thin films are prepared by 2450 MHz electron cyclotron resonance plasma-assisted atomic layer deposition on glass and p-Si substrates using trimethylaluminum as the precursor and hydrogen as the reductive gas. We focus our attention on the plasma source for the thin-film preparation and annealing of the as-deposited films relative to the surface square resistivity. The square resistivity of as-deposited Al films is greatly reduced after annealing and almost reaches the value of bulk metal. Through chemical and structural analysis, we conclude that the square resistivity is determined by neither the contaminant concentration nor the surface morphology, but by both the crystallinity and crystal size in this process.

Analysis of Numerical Simulation Results of LIPS-200 Lifetime Experiments

Accelerator grid structural and electron backstreaming failures are the most important factors affecting the ion thruster's lifetime.During the thruster's operation,Charge Exchange Xenon(CEX) ions are generated from collisions between plasma and neutral atoms.Those CEX ions grid's barrel and wall frequently,which cause the failures of the grid system.In order to validate whether the 20 cm Lanzhou Ion Propulsion System(LIPS-200) satisfies China's communication satellite platform's application requirement for North-South Station Keeping(NSSK),this study analyzed the measured depth of the pit/groove on the accelerator grid's wall and aperture diameter's variation and estimated the operating lifetime of the ion thruster.Different from the previous method,in this paper,the experimental results after the 5500 h of accumulated operation of the LIPS-200 ion thruster are presented firstly.Then,based on these results,theoretical analysis and numerical calculations were firstly performed to predict the on-orbit lifetime of LIPS-200.The results obtained were more accurate to calculate the reliability and analyze the failure modes of the ion thruster.The results indicated that the predicted lifetime of LIPS-200's was about 13218.1 h which could satisfy the required lifetime requirement of 11000 h very well.

Study on the influence of three-grid assembly thermal deformation on breakdown times and an ion extraction process

In order to study the influence of three-grid assembly thermal deformation caused by heat accumulation on breakdown times and an ion extraction process,a hot gap test and a breakdown time test are carried out to obtain thermal deformation of the grids when the thruster is in 5 k W operation mode.Meanwhile,the fluid simulation method and particle-in-cell-Monte Carlo collision(PICMCC) method are adopted to simulate the ion extraction process according to the previous test results.The numerical calculation results are verified by the ion thruster performance test.The results show that after about 1.2 h operation,the hot gap between the screen grid and the accelerator grid reduce to 0.25–0.3 mm,while the hot gap between the accelerator grid and the decelerator grid increase from 1 mm to about 1.4 mm when the grids reach thermal equilibrium,and the hot gap is almost unchanged.In addition,the breakdown times experiment shows that 0.26 mm is the minimal safe hot gap for the grid assembly as the breakdown times improves significantly when the gap is smaller than this value.Fluid simulation results show that the plasma density of the screen grid is in the range 6?×10^(17)–6?×?10^(18) m^(13) and displays a parabolic characteristic,while the electron temperature gradually increases along the axial direction.The PIC-MCC results show that the current falling of an ion beam through a single aperture is significant.Meanwhile,the intercepted current of the accelerator grid and the decelerator grid both increase with the change in the hot gap.The ion beam current has optimal perveance status without thermal deformation,and the intercepted current of the accelerator grid and the decelerator grid are 3.65 m A and 6.26 m A,respectively.Furthermore,under the effect of thermal deformation,the ion beam current has over-perveance status,and the intercepted current of the accelerator grid and the decelerator grid are 10.46 m A and 18.24 m A,respectively.Performance test results indicate that the breakdown times increase obviously.The intercepted current of the accelerator grid and the decelerator grid increases to 13 m A and 16.5 m A,respectively,due to the change in the hot gap after 1.5 h operation.The numerical calculation results are well consistent with performance test results,and the error comes mainly from the test uncertainty of the hot gap.

Numerical research of a 2D axial symmetry hybrid model for the radio-frequency ion thruster

Since the high efficiency discharge is critical to the radio-frequency ion thruster(RIT), a 2D axial symmetry hybrid model has been developed to study the plasma evolution of RIT. The fluid method and the drift energy correction of the electron energy distribution function(EEDF) are applied to the analysis of the RIT discharge. In the meantime, the PIC-MCC method is used to investigate the ion beam current extraction character for the plasma plume region. The beam current simulation results, with the hybrid model, agree well with the experimental results, and the error is lower than 11%, which shows the validity of the model. The further study shows there is an optimal ratio for the radio-frequency(RF) power and the beam current extraction power under the fixed RIT configuration. And the beam extraction efficiency will decrease when the discharge efficiency beyond a certain threshold(about 87 W). As the input parameters of the hybrid model are all the design values, it can be directly used to the optimum design for other kinds of RITs and radio-frequency ion sources.

Particle-in-Cell Simulation Study on the Floating Potential of Spacecraft in the Low Earth Orbit

In order to further understand the characteristics of the floating potential of low earth orbit spacecraft,the effects of the electron current collection area,background electron temperature,photocurrent emission,spacecraft wake,and the shape of spacecraft on spacecraft floating potential were studied here by particle-in-cell simulation in the low earth orbit.The simulation results show that the electron current collection area and background electron temperature impact on the floating potential by changing the electron current collection of spacecraft.By increasing the electron current collection area or background electron temperature,the spacecraft will float at a lower electric potential with respect to the surrounding plasma.However,the spacecraft wake affects the floating potential by increasing the ion current collected by spacecraft.The emission of the photocurrent from the spacecraft surface,which compensates for the electrons collected from background plasma,causes the floating potential to increase.The shape of the spacecraft is also an important factor influencing the floating potential.

2D hybrid-PIC simulation of the two and three-grid system of ion thruster

A 2D hybrid-PIC simulation model is proposed to investigate the beam extraction phenomena of the ion thruster. In which the electrons of the plasma sheath upstream the accelerator grid are assumed as particles while the downstream are fluid for improving the calculation efficiency. The ion transparency, plasma sheath formation, ion beam extraction characteristic of a two-and three-grid system have been compared in detail in this paper. From the comparison of the appearing time of the under-perveance phenomena in the two-and three-grid system, it illustrated that the two grid system has the wider operation range of the plasma densities than the three-grid one.

Lattice structures and electronic properties of CIGS/CdS interface:First-principles calculations

Using first-principles calculations within density functional theory, we study the atomic structures and electronic properties of the perfect and defective(2VCu+ InCu) CuInGaSe2/CdS interfaces theoretically, especially the interface states.We find that the local lattice structure of(2VCu+ InCu) interface is somewhat disorganized. By analyzing the local density of states projected on several atomic layers of the two interfaces models, we find that for the(2VCu+ InCu) interface the interface states near the Fermi level in CuInGaSe2and CdS band gap regions are mainly composed of interfacial Se-4p,Cu-3d and S-3p orbitals, while for the perfect interface there are no clear interface states in the CuInGaSe2region but only some interface states which are mainly composed of S-3p orbitals in the valance band of CdS region.

Investigation of variable aperture on the performance and lifetime of ion thruster

Beam flatness is an important parameter that determines the performance and the lifetime of a gridded ion thruster.To improve the beam flatness of the 30 cm(LIPS-300)ion thruster,variable aperture ion optics that adapts to the decreasing ion density as the radius increases is proposed.It is the ion optics that the screen grid surface is divided into several zones,where the aperture diameter in each zone is determined by the ion density and the electron temperature upstream of the screen grid.The beam current density in the central area is artificially reduced.A particle in cell-Monte Carlo collision model is applied in this work to investigating the effect of variable aperture on the perveance and the maximum beam current per aperture by simulating the extraction,focusing and acceleration processes of ions.Taking into account the engineering implementability,the screen grid surface is divided into four zones.The hole diameter in each zone is decreased from 1.95 mm to 1.8 mm,1.9 mm,1.8 mm and 1.7 mm,respectively.The simulation results show that the maximum ion density in the center area of grid is decreased by10.6%and 6.99%,while it is increased by 6.49%and 22.3%in the edge region,respectively.The beam flatness of the variable aperture ion optics is improved from 0.69 to 0.88.The erosion rate is decreased by 31.9%,but the total beam current is also decreased by 7.15%.The simulation results can provide a valuable reference of the development of the ion thruster.

Three Dimensional Simulation of Ion Thruster Plume-Spacecraft Interaction Based on a Graphic Processor Unit

Based on the three-dimensional particle-in-cell (PIC) method and Compute Unified Device Architecture (CUDA), a parallel particle simulation code combined with a graphic processor unit (GPU) has been developed for the simulation of charge-exchange (CEX) xenon ions in the plume of an ion thruster. Using the proposed technique, the potential and CEX plasma distribution are calculated for the ion thruster plume surrounding the DS1 spacecraft at different thrust levels. The simulation results are in good agreement with measured CEX ion parameters reported in literature, and the GPU's results are equal to a CPU's. Compared with a single CPU Intel Core 2 E6300, 16-processor GPU NVIDIA GeForce 9400 GT indicates a speedup factor of 3.6 when the total macro particle number is 1.1×10 6 . The simulation results also reveal how the back flow CEX plasma affects the spacecraft floating potential, which indicates that the plume of the ion thruster is indeed able to alleviate the extreme negative floating potentials of spacecraft in geosynchronous orbit.

Study of the key factors affecting the triple grid lifetime of the LIPS-300 ion thruster

In order to ascertain the key factors affecting the lifetime of the triple grids in the LIPS-300 ion thruster,the thermal deformation,upstream ion density and component lifetime of the grids are simulated with finite element analysis,fluid simulation and charged-particle tracing simulation methods on the basis of a 1500 h short lifetime test.The key factor affecting the lifetime of the triple grids in the LIPS-300 ion thruster is obtained and analyzed through the test results.The results show that ion sputtering erosion of the grids in 5 kW operation mode is greater than in the case of 3 kW.In 5 kW mode,the decelerator grid shows the most serious corrosion,the accelerator grid shows moderate corrosion,and the screen grid shows the least amount of corrosion.With the serious corrosion of the grids in 5 kW operation mode,the intercept current of the acceleration and deceleration grids increases substantially.Meanwhile,the cold gap between the accelerator grid and the screen grid decreases from 1 mm to 0.7 mm,while the cold gap between the accelerator grid and the decelerator grid increases from 1 mm to 1.25 mm after 1500 h of thruster operation.At equilibrium temperature with 5 k W power,the finite element method(FEM)simulation results show that the hot gap between the screen grid and the accelerator grid reduces to 0.2 mm.Accordingly,the hot gap between the accelerator grid and the decelerator grid increases to 1.5 mm.According to the fluid method,the plasma density simulated in most regions of the discharge chamber is 1?×?10^(18)-8?×?10^(18)m^(-3).The upstream plasma density of the screen grid is in the range 6?×?10^(17)-6?×?10^(18)m^(-3)and displays a parabolic characteristic.The charged particle tracing simulation method results show that the ion beam current without the thermal deformation of triple grids has optimal perveance status.The ion sputtering rates of the accelerator grid hole and the decelerator hole are 5.5?×?10^(-14)kg s^(-1)and 4.28?×?10^(-14)kg s^(-1),respectively,while after the thermal deformation of the triple grids,the ion beam current has over-perveance status.The ion sputtering rates of the accelerator grid hole and the decelerator hole are 1.41?×?10^(-13)kg s^(-1)and 4.1?×?10^(-13)kg s^(-1),respectively.The anode current is a key factor for the triple grid lifetime in situations where the structural strength of the grids does not change with temperature variation.The average sputtering rates of the accelerator grid and the decelerator grid,which were measured during the 1500 h lifetime test in5 k W operating conditions,are 2.2?×?10^(-13)kg s^(-1)and 7.3?×?10^(-13)kg s^(-1),respectively.These results are in accordance with the simulation,and the error comes mainly from the calculation distribution of the upstream plasma density of the grids.

Analysis of the primary experimental results on a 5 cm diameter ECR ion thruster

An ECR ion thruster with a diameter of 5 cm has been developed and tested. Four different antenna positions were experimentally and numerically investigated, and the results suggest that the optimal location for the antenna is where it is perfectly surrounded by the electron cyclotron resonance layer. We also evaluated two different antenna configurations, and found that the star configuration is preferable to the circular configuration, and also that the circular antenna is only 40% as efficient as the star antenna. The experimental curve of the ion beam current and voltage agrees with the fitting results from the analytic solution. The simulation of the magnetic topology in the discharging chamber with different back yoke heights indicates that it needs to be further verified.

Surface Charging Controlling of the Chinese Space Station with Hollow Cathode Plasma Contactor

A highly charged manned spacecraft threatens the life of an astronaut and extravehicular activity, which can be effectively reduced by controlling the spacecraft surface charging.In this article, the controlling of surface charging on Chinese Space Station(CSS) is investigated,and a method to reduce the negative potential to the CSS is the emission electron with a hollow cathode plasma contactor. The analysis is obtained that the high voltage(HV) solar array of the CSS collecting electron current can reach 4.5 A, which can be eliminated by emitting an adequate electron current on the CSS. The theoretical analysis and experimental results are addressed,when the minimum xenon flow rate of the hollow cathode is 4.0 sccm, the emission electron current can neutralize the collected electron current, which ensures that the potential of the CSS can be controlled in a range of less than 21 V, satisfied with safety voltage. The results can provide a significant reference value to define a flow rate to the potential controlling programme for CSS.