Research On Spectrum Characteristics And Spectroscopic Diagnostic Methods Of Hall Effect Thruster Discharge Process

Hall effect thrusters?HETs?are widely used in the aerospace field,as advanced electric propulsion devices.Nowadays,new types of HETs are emerging and accurate measurements of the plasma parameters in the discharge process are necessary to study their working mechanisms.The traditional probe measurement methods interfere with the plasma,difficult to be used in the internal discharge channel,and can not diagnose neutral components,which can no longer meet the research needs.Spectral diagnosis methods can avoid disadvantages of probes in principle,but the applications in HET research are limited,due to the incomplete understanding of optical radiation mechanisms and the imperfect physical model of spectral radiation.As a result,the spectral diagnosis area is confined to regions within low density plasma,with few measurable particle types and low accuracy.In view of the shortcomings of HET spectral diagnosis,this paper will summarize and analyze the changing laws of HET spectral characteristics in discharging process,starting from the study of the dynamic behaviors of key particles,then builting spectral diagnosis models for all discharge regions and establishing corresponding diagnosis methods as well as error evaluations.Finally,the measurements of key plasma parameters in all discharging regions are realized,within HET in steady working state.The discharging mechanism of HET is obviously different from that of ordinary DC plasma,so it is necessary to systematically summarize and analyze the spectral characteristics and clarify the particularities of emission dynamics process.The distributions of spectral lines intensities in the discharge channel and plume region were measured experimentally,and the variation law in frequency domain was also analyzed.The relationship between the angular momentum quantum numbers of upper levels in transitions and the proportion of intensity counts was obtained in different discharge areas.The difference of the electron energy distribution function inside and outside the channel was confirmed.The distribution of spectral lines strengths along the axis of the discharge channel was studied and its spatial variation law was analyzed.Based on the emission characteristics in frequency and spatial domains,a spectral method for dividing the discharge areas of HET-P70 was established by using 823.16 and 828.01 nm atomic lines and 484.43 nm ion lines.The transmission characteristics of photons inside and outside the channel was studied experimentally.The variation of the ratio of the branching coefficients of lines transition from 2p₆ level under different anode flux conditions was obtained,and the existence of metastable atomic self-absorption effect was confirmed.The optical thicknesses of plasma were calculated by the self-absorption theory.The fluctuation phenomenon of emission spectrum signal was studied experimentally.It is found that the fluctuation in HET steady state operation is caused by the quantum property of luminescence mechanism,the characteristics of plasma oscillation and the noise of photoelectric converter.By using the semiconductor refrigerated CCD detector kuro-sCMOS,processing the data by interpolation and integration near the central wavelength of spectral line and the square of the mean value,the fluctuation of signal intensity can be suppressed.The corona model and double spectrum lines ratio diagnostic method commonly used in HET plume region were tested,using the latest Xenon atomic interaction cross-section data obtained by the relativistic B-spline R matrix algorithm.The analysis of spectral residual errors and the comparison with the results of the probe confirmed that the coronal model could not accurately describe the radiation process in the plume.By comparing the resluts of theoretical calculation and experimental measurements,it is found that the excitation of ions can not be neglected in the plume region and acceleration region.Combining with the conclusion of the second chapter,the generation and consumption frequencies of metastable atoms in the plume region and acceleration region were calculated,and it is found that this type of neutral particles also participate in the emission process.On the basis of the improved coronal model,new rate equilibrium equations were constructed and a"multi-spectral line ratio"spectral diagnostic method for the plume and acceleration regions was developed.The sensitivity and reliability of multi-group spectral pairs were analyzed.Finally,it was confirmed that three groups of atomic spectral pairs can be used to calculate plasma parameters.The spectral measurement results were verified with other experimental methods and numerical simulation results.In order to solve the problem of spectrally quantitative measurements in near-anode and ionizing regions,a complete collision-radiation model was established.The collision-radiation model consists of 73 xenon atomic levels,the main dynamic processes and the corresponding rate equilibrium equations.The kinetic behaviors of metastable atoms and Paschen 2p level atoms,which play an important role in the luminescence process,were calculated and analyzed by using this model.The generation and consumption pathways and frequency information were obtained.By studying the dynamic characteristics of the main excitation levels in the collision-radiation model,a diagnostic method of"multi-spectral line ratio sum"was developed.By analyzing the sensitivity and reliability of the spectrum lines,10 atomic lines were selected to calculate the plasma parameters.By comparing the spectral diagnosis results with the numerical simulation results under typical working conditions of HET-P70,it is found that the spectral diagnosis results are more according with the actual situation,basing on the measured spectral information.On the basis of above-mentioned spectral diagnosis methods,the variation laws of plasma parameters such as electron temperature and density with various working conditions were measured,especially in the discharge channel.The influences of macroscopic discharge parameters on the ionization process of thruster were analyzed.Using hydrogen as tracer particle,the influence of background gas reflux process on the working characteristics of HET-P70 in ground vacuum facility was analyzed,by spectral measurements combined with probe and simulation methods.It is found that background gas would enter the discharge channel and participate in the ionization process.Basing on the high time resolution of spectral diagnosis,the transient ionization characteristics of the thruster start-up was studied by using a high-speed camera.It is found that the energy and density of electrons during the first pulse period of ignition from cathode to anode are similar to those of glow discharge.Aiming at the difficulties of plasma parameters measurement of HET,complete spectral diagnostic models and emission spectrum diagnostic methods for different discharge areas were constructed,of which the accuracy,sensitivity,stability and application range were studied.The spectral diagnostic methods and related spectral physical models are also useful for other kinds of electric propulsion devices and the study of xenon plasma discharge process.