Study On The 3D Plume Flowfield And Its Contamination Effect Of Stationary Plasam Thruster

Stationary Plasma Thruster(SPT)is a kind of electric propulsion system with quite mature development and most in-orbit applications at present.Depending on the bound of electron by magnetic field,a virtual cathode is formed to generate a static orthogonal electric and magnetic field.The aforementioned electromagnetic field can accelerate the speed of ionized propellant,which is engendered for the ionization caused by collisions between neutral propellant atoms and electrons from external cathodes,to 5-10 times of the outlet speed of traditional chemical thrusters.Meanwhile,its specific impulse is absolutely higher than that of the chemical thruster.Up to now,SPT has been applied to more than 100 satellites successfully,and was most used by Russia,secondly used by America and EU.The application of SPT has been covering many kinds of satellites including comsat.Although China has finished the test of in-orbit operation of SPT,there is no practical application with the SPT.The advantage of SPT 's high specific impulse can bring a great profit to commercial operation,while its some features such as the micro-thrust and to be easy to control can realize the precise control to spacecraft movements,which is of great strategic importance.In other countries,SPT has been studied for more than forty years while its in-orbit operating has also lasted for two decades.However,because of the complexity of its operation principles,increasingly demands on its performance and application and some new problems found in its operation,some investigations on SPT have still been an important aspect in electric propulsion field.Recently,the interaction between SPT plasma plume and spacecraft body has become one very issue including additional heat,force,moment,sputtering and deposition pollution as well as the more serious suspension potential and parasitic current on the insulation surface(such as solar panels and so on).These problems has been there before SPT was applied,and some researchers had tried to solve them at that time.However,the poor condition of computing and testing at that time and some increasing demand on SPT application made these problems very tougher and sharper.For example,all the SPTs on service now are equipped with a power supply system.In order to reduce the weight of SPT system,NASA tended to drive the SPT with solar panel directly,which means the panel should provide a high voltage,even up to 500 V,though it can now only provide a 30 V voltage.As the result,some problems became more serious including sputtering and deposition pollution,surface floating potential,parasitic current and even overheat problem on solar panels.This dissertation has carried out an experimental measurement and numerical simulation in order to investigate the compatibility between electric propulsion system and whole spacecraft system.A 3-D model based on PIC-DIMC hybrid algorithm for SPT plume flowfield has been built up and the computational code is written in FORTRAN language.The CUDA parallel processing technology based on GPU has been employed to improve the computational efficiency.The comparisons between the SPT plume experimental data and 2-D simulating results show that relative error of electron number density is not more than 14% and the maximum relative error of ion current density is not more than 15%,which indicate that the plume model and code are reliable and effective.To completely investigate the acting relationship between plasma plume and spacecraft surface wall,some parameters on the ion,propellant atomic and electron are needed to know as well as that of sputtered insulating material atomic,which is one of input conditions to compute plume field and sputtering effect caused by interaction between ion and surface wall.As a result,a vacuum chamber experiment system has been developed,which is to measure the average number of sputtered Ceramic atoms after a short time operating of SPT.Meanwhile,a semi-rotational formula from the cited papers is already modified.The inside and outside diameters of the discharge chamber insulation layer section along the axis with the CMM has been measured under the condition that the SPT has worked for a short time in the vacuum chamber,by which the average number of sputtered Ceramic atoms during the test time can be gotten.The measured results show that there is about 3×1013 atoms getting into the field per mm*s in the experiment.As sputtered atom number varies with SPT work time,in order to acquire parameter of any time bucket,a semi-rational formula has been modified with probability density function.And a comparison with the data from other's papers verifies that the shape of discharge chamber section after the SPT works for a long time can be predicted accurately.The result shows that the absolute value of the internal and external face error is less than 0.51 mm.Short duration lifetime test can provide the input parameters which can estimate the section shape of the plasma channel after the SPT worked for 4000 hours,and the mean relative error is 1.93%.On these basis,the number of Ceramic atoms which went into fluid field from the thruster during any time frame can be gotten.The sputtering yield is one fundamental parameter to evaluate thruster's lifetime and to analyze the sputtering effect on spacecraft body caused by plasma plume.This dissertation took SPT as an ion source,and acquired the plasma current density and energy at the position where the test specimen is installed with a Faraday cup and a RPA probe.A test specimen with a support system and a graphite beam plate has been designed.The specimen are basically similar to the SPT discharge chamber insulation layer system.The data of specimen sputtering yield with weight method is acquired.The advantage of the experimental scheme is that the sputtering yield can be measured on different specimen or at different angles on the same specimen through only once measure.In order to overcome the shortcomings of high consumption of time and finance caused by relying solely on experimental measurements and of large error caused by the measurement because of low-energy ion sputtering,the effect incident angle and incoming energy on the sputtering yield of Hall thruster discharge chamber wall have been investigated deeply at the condition that the incoming energy is lower than 300 e V by combining the previous measurements and the software SRIM based on Monte Carlo method.The results show that the tendency showed by the measurements data is consistent with related results in other references.However,according to the difference between the ceramic from China and one from foreign countries,there is a little difference in some detail datum.After that,in order to provide foundation for the following plume pollution calculation,simulation is conducted about the sputtering characteristics of Fe and Si,which are the main materials of spacecraft surface wall.The result shows that,with the incident angle increasing,the sputtering yield first grows,and then reduces.And with the incoming energy increasing,the sputtering yield grows,but when the energy is less than 100 e V,the sputtering yield tends to be a very small value.On these basis,the main pollution condition has been investigated including heat,force,moment,sputtering and other pollutions.Some conclusions can be obtained including the top heat flux(329W/m2)occurred on the SPT pedestal,most additional force is in the numerical order of 10-7N,and the max additional torque is about 10-6Nm.After uninterrupted operation lasts for 2000 hours,SPT can obtain a deposition amount of 10-5g/cm2 on its solar panel,and the sputtering amount is 1.2×10-5g/cm2.The damage to the surface finish of solar panels will influence the angle between sunlight and surface,and can result in its performance to degrade.The SPT plasma plume may make some solid wall to generate floating potential and parasitic current.Based on the simulation of 3-D plume flowfield,a model to calculate the floating potential and the parasitic current on solar panel is developed,and the influence of solar panels voltage is also inspected.The results show that,under the given calculation conditions,the floating potential is about-30V~-40 V,by which the surface current caused is less than 10 m A/m2,meanwhile the relative position between the panels and the plume has an effect on the floating potential and the surface current.Therefore,when considering the operating policy of SPT,the solar panels should be assigned in the proper position and orientation.