| In recent years,with the increasing demand for vacuum arc plasma applications,the ablation problem caused by cathode temperature rise has become the bottleneck of cathode vacuum arc in many application fields.For example,cathode ablation affects the electrode life of ion thruster discharge chamber,the insulation performance of vacuum switch decreases due to ablation particles,and the electrode life and coating quality of MEVVA source.The vacuum arc plasma is generated from the moving cathode spot on the electrode surface.The high-energy ion bombardment under the sheath pressure drop and the explosive emission process of the electrode material form the temperature rise change of the cathode spot,and the periodic appearance and random position jump of the cathode spot become the temperature rise and ablation phenomenon of the cathode surface.Due to the factors such as fast cathode temperature rise process(Hundred nanoseconds level),small spatial scale(micron level)and the interference of plasma,the research on cathode temperature rise at home and abroad is still mainly based on theoretical analysis and thermodynamic simulation,and there is no corresponding online and real-time measurement and analysis method,especially the lack of experimental data on the temperature rise characteristics of vacuum arc cathode region.Because vacuum arc discharge changes with the discharge environment,electrode structure and current parameters,it must be measured online.The accurate acquisition of very high temperature,especially the measurement of transient target temperature field,leads to the slow progress in the research on the ablation life of cathode materials and related physical problems of ablation products,which cannot provide effective guidance for the design improvement of vacuum switch,plasma rocket propulsion device and MEVVA source,and has become a key basic scientific problem in the application field of cathode vacuum arc at present.Therefore,the research on the real temperature measurement technology in the cathode region of pulsed vacuum arc has great scientific research value and practical significance for promoting the in-depth development of temperature rise and ablation research of vacuum arc electrode.It is a common non-contact temperature measurement method,which uses bright temperatures at different wavelengths to calculate and finally get the real temperature of the measured target.This method is especially suitable for the real temperature measurement of this kind of pulsed vacuum arc discharge and this kind of high temperature transient target.In this paper,the true surface temperature of pulsed vacuum arc discharge cathode is measured by multi-spectral thermometry,aiming at:exploring the theoretical basis of multi-spectral thermal imager detection wavelength selection method under pulse interference,and solving the problem that pulsed vacuum arc discharge plasma spectral line interferes with multi-spectral thermal imager detection wavelength;A multi-spectral thermal imager suitable for measuring the true temperature on the cathode surface of pulsed vacuum arc discharge is developed,and a light splitting structure is designed to solve the problem that monochromatic high-speed camera cannot realize two-dimensional distribution of temperature and simultaneous measurement of true temperature.The temperature rise model of cathode surface under different discharge conditions was studied,and the discharge experimental platform was built.The effectiveness of the established discharge model was verified by experiments.The main research contents of this paper are as follows:(1)Aiming at the practical problem that plasma spectral lines interfere with the effective selection of detection wavelength of multi-spectral thermal imager during pulsed vacuum arc discharge,an effective selection method of detection wavelength of multi-spectral thermal imager is determined by the effective wavelength calibration method based on temperature data.Combined with the temperature calibration data of multi-spectral thermal imager,based on Planck’s law,the intrinsic relationship between the temperature calibration data at different temperature points and the detection wavelength of multi-spectral thermal imager is established.Based on the effective wavelength calibration method based on temperature data,the effective selection model of optical filter under the interference of pulsed vacuum arc discharge arc is established,and the detection wavelength selection method of multispectral thermal imager is determined by the temperature calibration data of optical filter.Using this method,the detection wavelength of multi-spectral thermal imager can be effectively selected,and the time-consuming selection process of traditional pyrometer detection wavelength can be avoided.(2)To solve the problem that pulsed vacuum arc plasma radiation affects the two-dimensional temperature distribution and simultaneous measurement of true temperature of high-speed camera,a multi-spectral thermal imager based on highspeed camera(referred to as multispectral thermal imager)is developed.The radiation characteristics of plasma in the process of discharge and the actual requirements for measuring the true temperature of cathode surface in pulsed vacuum arc discharge are analyzed,and the optical system and application program of multispectral thermal imager are designed.In order to realize the true temperature measurement of cathode surface temperature under plasma radiation,a new fourchannel spectral resolution imaging beam splitting structure is designed,which realizes the two-dimensional distribution of cathode surface temperature and the simultaneous measurement of true temperature in pulsed vacuum arc discharge.(3)Aiming at the lack of research on the surface ablation characteristics of pulsed vacuum arc discharge cathode,the research work is carried out based on COMSOL Multiphysics numerical simulation software.The formation of weld pits on the cathode surface is a complicated process coupled with multiple physical fields.A heat transfer model on the cathode surface of pulsed vacuum arc discharge is established,and the phase transformation,deformation and surface temperature distribution of pulsed vacuum arc discharge on the cathode surface are studied.The model takes into account the bombardment heating of plasma cloud on the cathode surface and the influence of phase change on the cathode surface during heating,and simulates the temperature change of the cathode surface.The simulation results show the distribution of cathode surface temperature under different conditions.Under different discharge time conditions,the cathode surface temperature first increases and then decreases with discharge time,and under different discharge current conditions,the cathode surface temperature increases with the increase of discharge current.Finally,the experimental platform of pulsed vacuum arc discharge is built to carry out pulsed vacuum arc discharge experiments,and the effectiveness of the model is verified by experiments.(4)The experimental verification and uncertainty analysis of the multi-spectral thermal imager based on high-speed camera are carried out.Laboratory and field experiments were carried out with the developed multi-spectral thermal imager.Experiments of tungsten halogen lamp temperature measurement at different temperatures were carried out in the laboratory,and the laboratory results verified the accuracy of the design of multi-spectral thermal imager based on high-speed camera.In the field experiment,a test platform for cathode surface temperature of pulsed vacuum arc discharge was built,and pulsed vacuum arc discharge experiments under different conditions were carried out.In the experiment,a multi-spectral thermal imager was developed to measure the whole process of cathode surface temperature change during pulsed vacuum arc discharge.The measurement uncertainty of the multi-spectral thermal imager is analyzed and its extended uncertainty is 2.92%. |
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