A system and method for in-situ real-time measurement of charge distribution at gas-solid interface of solid insulation materials

Abstract

The application discloses a kind of solid insulation material gas-solid interface charge distribution in-situ real-time measurement system and method.The system uses nanosecond pulse laser;With convex lens, high lens, high reflection mirror, filter, dispersion prism composition optical path unit;With needle-plate electrode and solid insulation material are placed in airtight cavity as discharge unit;With infrared photodiode, photomultiplier as laser receiving device;With oscilloscope and computer as data processing end.The system is based on electric field induced second harmonic generation effect, the second harmonic signal intensity generated by pump laser through electric field area is linearly related with the square of electric field intensity at the measured point.After the calibration of both, the electric field intensity of solid insulation material gas-solid interface in the process of needle-plate electrode charging and power-off can be measured, and the real-time charge distribution is obtained by charge inversion algorithm.The system can realize in-situ, real-time measurement, and the requirement of solid insulation material property, shape is lower, so application range is wide.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of high voltage and insulation, and particularly relates to a system and method for in-situ real-time measurement of charge distribution at a gas-solid interface of a solid insulation material. BACKGROUND

[0002] In high-voltage direct-current gas-insulated electrical equipment, such as gas-insulated transmission lines (GIL) and gas-insulated switchgears (GIS), insulators are subjected to monopolar direct-current voltage, and their surfaces are kept dry and clean for a long time, which makes it very easy to accumulate surface charges. These gas-solid interface charges can cause distortion of the surface electric field on one hand, and can cause a large drop in the surface flashover voltage when the electric field distortion is serious, thus inducing surface discharge. On the other hand, these charges also provide seed charges for the development of surface discharge, thus promoting the formation of the flashover process and greatly reducing the insulation level of the gas-insulated equipment. Therefore, in order to ensure the insulation safety of electrical and electronic equipment, it is necessary to conduct in-depth research on the accumulation and dissipation mechanism of the gas-solid interface charges of the solid insulation material and the influence of the gas-solid interface charges on the surface insulation performance of the solid insulation material. How to accurately measure the gas-solid interface charges of the solid insulation material has become a key technical problem in this field, and has important research value and significance.

[0003] At present, the measurement of gas-solid interface charge distribution is generally divided into qualitative measurement and quantitative measurement. Qualitative measurement can only be used to provide reference and evidence for quantitative measurement in most cases, and quantitative measurement is divided into invasive and in-situ. Common methods include dust map method (qualitative), Pockels effect method (invasive), and electrostatic probe method (invasive). The basic principle of dust map method is to spray charged colored solid dust onto the surface of solid insulating material, so that it is adsorbed with gas-solid interface charge, and the polarity and distribution characteristics of gas-solid interface charge are judged according to the distribution of adsorbed colored dust. Pockels effect, also known as linear electro-optic effect, refers to the linear relationship between the change of optical refractive index n of certain crystal and the applied electric field strength E. Pockels effect method includes Pockels effect transmission method and Pockels effect reflection method. Transmission method uses the light intensity transmitted through the dielectric to obtain the surface charge distribution, while reflection method calculates the surface charge distribution by recording the reflection light information, without the need for light to pass through the dielectric. Therefore, the reflection method has the advantages of non-contact and non-destructive dynamic measurement of the transmission method, and also makes it possible to measure the surface charge of some opaque media. The surface charge dynamic distribution measurement system and method in dielectric barrier discharge of Chinese patent application No. CN107991544A is based on Pockels effect reflection method, which can provide information on surface charge accumulation and dissipation by measuring the two-dimensional dynamic distribution of dielectric surface charge in dielectric barrier discharge under different atmospheres and different pressures, and can meet most requirements of measurement method for dielectric surface charge research. Electrostatic probe method is the most widely used method at present, and its basic principle is to paste the sensing metal probe close to the surface of the insulating material to be measured, and to obtain the distribution information of the material surface charge according to the probe sensing potential. This method includes active electrostatic probe method and passive electrostatic probe method, the difference between them is whether to apply voltage on the probe. Compared with passive electrostatic probe method, the advantage of active electrostatic probe method is that the requirement of distance between capacitive probe and insulating material is lower, and the influence on the gas-solid interface charge distribution of the insulating material to be measured is smaller, thereby improving the measurement accuracy. The surface charge measurement system of insulator of Chinese patent application No. CN212301702U is used to measure the surface charge of insulator to determine the charge accumulation characteristics of insulator, and then determine whether the insulator has the ability to withstand extra-high voltage direct current. However, the electrostatic probe method is invasive, and the measurement probe will affect the gas-solid interface charge distribution of the solid insulating material, and cannot realize real-time measurement. Although the measurement device and method of surface charge distribution before and after surface flashover of pot-type insulator of Chinese patent application No. CN111505463A solves the problem of difficult measurement of surface charge distribution before and after flashover and significantly improves the measurement accuracy, but there are still problems of invasive measurement and inability to realize real-time measurement.

[0004] With the rapid development of nonlinear optics, the in-situ electric field measurement technology based on the electric field induced second harmonic effect shows good advantages. The electric field induced second harmonic process is a third-order nonlinear process, including the interaction of electric field and laser field in the central symmetric atomic or molecular medium (such as air). In the central symmetric system, no second harmonic wave is generated, and in the presence of an external electric field, the atom or molecule generates a dipole moment, which destroys the symmetry of the medium, that is, the two incident photons produce oscillating polarization on the atom or molecule, generating a photon with a frequency twice that of the incident photon, forming a second harmonic wave. This method can reflect the spatial and temporal variation process of the electric field intensity and charge distribution of the gas-solid interface in real time without interfering with the original electric field distribution and environmental conditions, and has good spatial and temporal resolution. The electric field components in different directions can be obtained by measuring the signals in different polarization directions, and the charge distribution of the gas-solid interface of the solid insulating material can be obtained by inversion algorithm. Therefore, the in-situ electric field measurement technology based on the electric field induced second harmonic effect has great application prospect.

[0005] The dust mapping method cannot quantitatively characterize the density of surface charge, and spraying dust may change the original charge distribution. Although the Pockels effect transmission method can realize online real-time measurement, it can only be applied to transparent thin film insulating materials; and the Pockels effect reflection method can be used for opaque insulating materials, but there are still limitations for the thickness of solid insulating materials and electrode materials, and at present, it is mainly applied in dielectric barrier discharge, and the application range is limited. The electrostatic probe method is invasive, and the measurement probe will affect the charge distribution of the gas-solid interface of the solid insulating material, and cannot realize real-time measurement, and must be carried out after stopping pressurization, so that the whole change process of the charge of the gas-solid interface cannot be obtained. SUMMARY

[0006] In view of the above problems, the present application provides an in-situ real-time measurement system and method for the charge distribution of the gas-solid interface of a solid insulating material, which is based on the electric field induced second harmonic effect, can display the change process of the electric field and charge of the gas-solid interface during the charging and discharging process of the needle-plate electrode in real time, is helpful for studying the time and space evolution law of the charge of the gas-solid interface of the solid medium in the needle-plate electrode, and is not limited by factors such as the thickness of the medium and the electrode material, so it can be widely applied. At the same time, the present application can realize in-situ measurement, and compared with the invasive measurement method, can obtain more real and accurate charge distribution.

[0007] To achieve the above object, the technical scheme adopted by the present application is as follows:

[0008] An in-situ real-time measurement system for the charge distribution of the gas-solid interface of a solid insulating material, comprising a laser, a first optical path unit, a discharge unit, a second optical path unit, a laser receiving device and a data processing end;

[0009] The laser is composed of a Nd:YAG pump nanosecond pulse laser; the laser emits a laser beam with adjustable frequency and pulse width;

[0010] The first optical path unit is composed of a long-wave pass filter and a convex lens; the long-wave pass filter can filter out the stray second harmonic wave generated before the discharge part, and then the laser beam is focused in the electric field of the discharge unit through the focusing effect of the convex lens;

[0011] The discharge unit is composed of a sealed cavity, a needle electrode, a solid insulating medium and a flat plate electrode;

[0012] The second optical path unit is composed of two optical lenses with high transmittance at 1064nm and high reflectivity at 532nm, a dispersion prism, a narrow band pass filter and a convex lens;

[0013] The laser receiving device is composed of a photomultiplier tube and an infrared photodiode; the 532nm wavelength laser filtered by the second optical path unit enters the photomultiplier tube for signal collection, and the small part of 1064nm wavelength signal filtered by the second optical path unit enters the infrared photodiode;

[0014] The data processing end displays the laser signal collected by the photomultiplier tube as a voltage signal on an oscilloscope, and then transmits the data to a computer for processing.

[0015] Further, the needle electrode, flat plate electrode and solid insulating medium are all in the sealed cavity, which is provided with gas inlet and outlet, air pressure gauge and multiple observation windows; the laser passes through the inside of the sealed cavity from the left and right observation windows, and different pressure gases are filled in the sealed cavity.

[0016] Further, the flat plate electrode is grounded, and the needle electrode is connected to a positive or negative high-voltage DC power supply; the solid insulating medium is tightly attached to the upper surface of the flat plate electrode; the sealed cavity is placed on a three-dimensional displacement platform, the sealed cavity is moved to make the laser closely and parallel to the surface of the solid insulating medium, and the focal point of the laser beam is in the to-be-measured area; the three-dimensional displacement platform is adjusted to scan the focal point through the entire to-be-measured area, and a 532nm frequency-doubled light signal with varying intensity, i.e. a field-induced second harmonic signal, is generated; the charge distribution on the surface of the solid insulating medium is finally obtained according to this signal.

[0017] Further, the 1064nm wavelength pump laser and the 532nm frequency-doubled laser generated by the discharge unit are filtered twice, most of the 1064nm wavelength laser is transmitted out of the light path, only a small part of the 1064nm wavelength laser is deflected to the infrared photodiode by the dispersion prism, and the laser signal intensity is displayed in real time, and the 532nm wavelength frequency-doubled light is deflected by the dispersion prism, then passes through a narrow band filter and a convex lens to focus into a laser receiving device for signal collection.

[0018] Further, the intensity of the second harmonic signal is proportional to the square of the electric field intensity at the measurement point, and the coefficient is obtained from the calibration experiment in the uniform field; therefore, the 532nm frequency-doubled light signal collected by the photomultiplier is converted into the transient electric field intensity at the gas-solid interface measurement point, and the charge distribution of the gas-solid interface of the solid insulating material can be obtained by the charge inversion algorithm.

[0019] Further, the pulse power source in the laser selects a nanosecond pulse power source or a picosecond pulse power source, the pulse frequency and pulse width are adjusted according to experimental requirements, and the emitted pump laser wavelength is not limited to 1064nm.

[0020] Further, the gas is SF6, CF3I, C4F7N, C5F 10 O pure gas or mixed gas, and the gas pressure is adjusted according to experimental requirements.

[0021] Further, the solid insulating material is ceramic, epoxy resin, silicone rubber or glass.

[0022] The application also provides a measurement method of an in-situ real-time measurement system for charge distribution of a gas-solid interface of a solid insulating material, which comprises the following steps:

[0023] Step 1, building an in-situ real-time measurement system, including a laser, a first light path unit, a discharge unit, a second light path unit, a laser receiving device and a data processing end, adjusting the light path and setting the power supply parameters to power the discharge unit;

[0024] Step 2, performing a flat plate electrode calibration experiment: using a flat plate electrode for calibration, calculating the electric field intensity in the uniform field formed by the flat plate electrode, then collecting the corresponding second harmonic signals under different electric field intensities by a photomultiplier, obtaining the relationship curve between the square of the electric field intensity and the second harmonic signal intensity, and finally obtaining the calibration coefficient through curve fitting;

[0025] Step 3, moving the three-dimensional displacement platform to make the laser close to and parallel to the surface of the solid insulating medium, and making the laser beam focal point in the to-be-measured region;

[0026] Step 4, measuring the charge distribution: the flat electrodes of the discharge unit are replaced by needle-plate electrodes containing solid medium, the position of the discharge unit is adjusted so that the focal point of the laser beam is within the range of the electrodes and the laser is parallel to the surface of the medium to be measured. During the charging and discharging of the needle-plate electrodes, the experimental cavity in the discharge unit is constantly moved so that the laser focal point scans the entire area to be measured on the surface of the medium. Then the signal collected by the photomultiplier is read by the oscilloscope, the transient electric field at the gas-solid interface is calculated according to the calibration coefficient, and the real-time and accurate charge distribution is obtained through the charge inversion algorithm.

[0027] Advantages:

[0028] (1) The intensity of the second harmonic signal generated by the laser with a wavelength of 1064 nm after passing through the electric field region is proportional to the square of the electric field strength at the measurement point. Therefore, by calibrating the relationship between the two in the uniform field under the flat electrodes, the electric field strength at the gas-solid interface in the non-uniform field during the charging and discharging of the needle-plate electrodes can be measured, and the charge distribution can be obtained through the charge inversion algorithm;

[0029] (2) The gas-solid interface charge distribution of solid insulating materials is measured in situ and in real time, which not only ensures the accuracy and authenticity of the measurement, but also enables online monitoring and rapid response, reflecting the spatio-temporal evolution of the entire process;

[0030] (3) Wide application range: different types and pressures of gases can be filled in the sealed cavity during the experiment, and the material properties and thickness of the solid insulating materials have low requirements, and the needle-plate electrodes can also be powered by positive and negative direct current sources. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 The figure is a schematic diagram of the in-situ and real-time measurement system of the gas-solid interface charge distribution of solid insulating materials. In the figure, the reference signs are: 1. laser, 2. first optical unit, 3. discharge unit, 4. second optical unit, 5. laser receiving device, 6. data processing end. DETAILED DESCRIPTION

[0032] In order to make the purpose, technical scheme and advantages of the present application clearer, the following will further describe the present application in combination with the drawings and examples. It should be understood that the specific examples described herein are only used to explain the present application and do not limit the present application. In addition, the technical features involved in each embodiment of the present application described below can be combined with each other as long as they do not conflict with each other.

[0033] This invention proposes an in-situ real-time measurement system and method for the charge distribution at the gas-solid interface of solid insulating materials. This system enables accurate measurement of the charge at the gas-solid interface of solid insulating materials, facilitating further research on the accumulation and dissipation mechanisms of the charge at the gas-solid interface and its impact on the insulation performance of solid surfaces. It also provides a new method for future research on the surface flashover process and insulation characteristics of insulators in high-voltage DC gas-insulated electrical equipment.

[0034] like Figure 1 As shown, the in-situ real-time measurement system for the charge distribution at the gas-solid interface of solid insulating materials of the present invention consists of a laser 1, a first optical path unit 2, a discharge unit 3, a second optical path unit 4, a laser receiving device 5, and a data processing terminal 6.

[0035] The laser 1 is composed of an Nd:YAG-pumped nanosecond pulsed laser. The laser can emit a laser beam with a wavelength of 1064 nm and adjustable frequency and pulse width.

[0036] The first optical path unit 2 is composed of a long-pass filter and a convex lens. The long-pass filter can filter out the stray second harmonic generated before the discharge section. Then, the laser beam is focused into the electric field of the discharge unit 3 by the focusing effect of the convex lens.

[0037] The discharge unit 3 consists of a sealed cavity, a needle electrode, a solid insulating medium, and a flat electrode. The needle electrode, flat electrode, and solid insulating medium are all located within the sealed cavity, which is equipped with an inlet / outlet, a pressure gauge, and multiple observation windows. The laser can pass through the cavity from the observation windows at both ends. The sealed cavity can be filled with gases of different pressures, such as air, SF6, or an SF6 / N2 mixture. The flat electrode is grounded, and the needle electrode can be connected to a positive or negative high-voltage DC power supply. Both electrodes can be made of metal materials such as copper, aluminum, or stainless steel. The solid insulating medium is tightly bonded to the upper surface of the flat electrode; its length, width, and thickness can be selected according to experimental requirements, and common solid insulating materials such as epoxy resin, silicone rubber, and ceramics can be used. The sealed cavity is placed on a three-dimensional displacement platform. The cavity is moved so that the laser is close to and parallel to the surface of the solid insulating material, and the focal point of the laser beam is placed in the area to be measured. By adjusting the displacement platform, the focal point can scan the entire area to be measured, generating a 532nm frequency-doubled light signal with varying intensity, namely the field-induced second harmonic. Based on this signal, the charge distribution on the surface of the medium can be obtained.

[0038] The second light path unit 4 is composed of two optical lenses with high transmittance at 1064 nm and high reflectivity at 532 nm, one dispersive prism, one narrow band-pass filter and one convex lens. After being filtered twice, most of the 1064 nm laser transmits out of the light path, and only a small part of the 1064 nm laser is deflected to the infrared photodiode by the dispersive prism, so as to display the laser signal intensity in real time. The 532 nm frequency-doubled light is deflected by the dispersive prism, and then passes through the narrow band-pass filter and the convex lens to enter the laser receiving device 5 for signal collection.

[0039] The laser receiving device 5 is composed of a photomultiplier tube (PMT) and an infrared photodiode. The 532 nm laser filtered by the second light path unit 4 enters the photomultiplier tube for signal collection, and the small part of the 1064 nm signal filtered by the second light path unit 4 enters the infrared photodiode.

[0040] The data processing end 6 displays the laser signal collected by the photomultiplier tube as a voltage signal on an oscilloscope, and then transmits the data to a computer for processing. The intensity of the second harmonic signal is proportional to the square of the electric field strength at the measurement point, and the coefficient can be obtained by calibration experiment in a uniform field. Therefore, the 532 nm frequency-doubled light signal collected by the photomultiplier tube can be converted into the transient electric field strength at the measurement point on the surface of the medium, and the charge distribution of the gas-solid interface of the solid insulating material can be obtained by the charge inversion algorithm.

[0041] The flow steps of the use method of the application are as follows:

[0042] Step 1, build an in-situ real-time measurement system of the charge distribution of the gas-solid interface of the solid insulating material, including a laser 1, a first light path unit 2, a discharge unit 3, a second light path unit 4, a laser receiving device 5 and a data processing end 6, adjust the light path and set the power supply parameters to power the discharge unit 3. The discharge unit 3 first uses a flat electrode without solid medium.

[0043] Step 2, perform flat electrode calibration experiment. Use the flat electrode to calibrate, calculate the electric field strength in the uniform field formed by the flat electrode, collect the corresponding second harmonic signals under different electric field strengths by the photomultiplier tube, obtain the relationship curve between the square of the electric field strength and the second harmonic signal intensity, and finally obtain the calibration coefficient through curve fitting.

[0044] Step 3, move the three-dimensional displacement platform to make the laser close to and parallel to the surface of the solid insulating medium, and make the laser beam focal point in the to-be-measured region.

[0045] Step 4, the measurement of charge distribution is carried out. The flat plate electrode of the discharge unit 3 is replaced by a needle plate electrode containing solid medium, the position of the discharge unit 3 is adjusted, the focal point of the laser beam is located within the electrode range, and the laser is parallel to the surface of the medium to be measured. During the charging and discharging of the needle plate electrode, the experimental cavity in the discharge unit 3 is continuously moved, so that the laser focal point scans the entire measurement area of the surface of the medium. Then the signal collected by the photomultiplier is read by the oscilloscope, the transient electric field at the gas-solid interface is calculated according to the calibration coefficient, and the real-time and accurate charge distribution is obtained through the charge inversion algorithm.

[0046] The measurement system of the present application is not limited to the electric field formed by the flat plate electrode and the needle plate electrode, but can also be used for the measurement of other forms of electric field, such as the electric field formed by the finger electrode, the electric field form existing in GIL and GIS, etc., and can be used to study the influence law of the charge at the gas-solid interface on the flashover voltage.

[0047] The discharge unit of the measurement system of the present application is not limited to the sealed cavity, and can be directly measured in air at normal pressure.

[0048] Those skilled in the art will readily understand that the above description is only a preferred embodiment of the present application, and is not intended to limit the present application, and any modification, equivalent replacement and improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A system for in-situ real-time measurement of the charge distribution at the gas-solid interface of a solid insulating material, characterized by: The laser, the first optical path unit, the discharge unit, the second optical path unit, the laser receiving device and the data processing end are included. The laser is composed of a Nd:YAG pump nanosecond pulse laser; the laser emits a laser beam with adjustable frequency and pulse width. The first optical path unit is composed of a long-wave pass filter and a convex lens; the long-wave pass filter can filter out the stray second harmonic wave generated before the discharge part, and then the laser beam is focused in the electric field of the discharge unit through the focusing effect of the convex lens. The discharge unit is composed of a sealed cavity, a needle electrode, a solid insulating medium and a flat plate electrode. The second optical path unit is composed of two optical lenses with 1064nm high transmittance and 532nm high reflectivity, one dispersive prism, one narrow band pass filter and one convex lens. The laser receiving device is composed of a photomultiplier tube and an infrared photodiode; the 532nm wavelength laser filtered by the second optical path unit enters the photomultiplier tube for signal collection, and a small part of the 1064nm wavelength signal filtered by the second optical path unit enters the infrared photodiode. The data processing end displays the laser signal collected by the photomultiplier tube as a voltage signal on an oscilloscope, and then transmits the data to a computer for processing. The flat plate electrode is grounded, the needle electrode is connected to a positive or negative high-voltage direct current power supply; the solid insulating medium is tightly attached to the upper surface of the flat plate electrode; the sealed cavity is placed on a three-dimensional displacement platform, the sealed cavity is moved to make the laser closely and parallel to the surface of the solid insulating medium, and the laser beam focal point is in the measured area; the focal point is scanned through the adjustment of the three-dimensional displacement platform to generate a 532nm frequency-doubled light signal with varying intensity, i.e. a field-induced second harmonic signal, and the charge distribution on the surface of the solid insulating medium is finally obtained according to the signal. The intensity of the field-induced second harmonic signal is proportional to the square of the electric field intensity at the measurement point, and the coefficient is obtained from the calibration experiment in a uniform field; therefore, the 532nm frequency-doubled light signal collected by the photomultiplier tube is converted into the transient electric field intensity at the measurement point on the medium surface, and the charge distribution on the gas-solid interface of the solid insulating material can be obtained through the charge inversion algorithm.

2. The system for in-situ real-time measurement of the charge distribution on the gas-solid interface of a solid insulating material according to claim 1, characterized in that: The needle electrode, the flat plate electrode and the solid insulating medium are all in the sealed cavity, which is provided with gas inlet and outlet ports, a gas pressure gauge and multiple observation windows; the laser passes through the inside of the sealed cavity from the left and right observation windows, and different pressure gases are filled in the sealed cavity.

3. The system of claim 1, wherein: After being filtered twice, most of the 1064nm wavelength laser transmits out of the light path, only a small part of the 1064nm wavelength laser is deflected by the dispersive prism towards the infrared photodiode for real-time display of the laser signal intensity, and the 532nm frequency-doubled light is deflected by the dispersive prism, then passes through the narrow band pass filter and the convex lens to enter the laser receiving device for signal collection.

4. The system of claim 1, wherein: The pulse power source in the laser selects a nanosecond pulse power source or a picosecond pulse power source, the pulse frequency and the pulse width are adjusted according to experimental requirements, and the emitted pump laser wavelength is not limited to 1064 nm.

5. The system of claim 2, wherein: The gas is SF6, CF3I, C4F7N, C5F 10 O pure gas or mixed gas, gas pressure is adjusted according to experimental requirements.

6. The system of claim 1, wherein: The solid insulating material is ceramic, epoxy resin, silicone rubber or glass.

7. The measurement method of the system for in-situ real-time measurement of the charge distribution on the gas-solid interface of a solid insulating material according to any one of claims 1 to 6, characterized in that, The method comprises the following steps: Step 1, build an in-situ real-time measurement system, including a laser, a first optical path unit, a discharge unit, a second optical path unit, a laser receiving device and a data processing end, adjust the optical path and set the power supply parameters to power the discharge unit; Step 2, move the three-dimensional displacement platform to make the laser close to and parallel to the surface of the solid insulating medium, and make the laser beam focal point in the measured area; Step 3, perform a flat electrode calibration experiment: use a flat electrode for calibration, calculate the electric field strength in the uniform field formed by the flat electrode, then collect the corresponding second harmonic signals under different electric field strengths through a photomultiplier tube, obtain the relationship curve between the square of the electric field strength and the second harmonic signal strength, and finally obtain the calibration coefficient through curve fitting; Step 4, perform a measurement charge distribution experiment: replace the flat electrode of the discharge unit with a needle-plate electrode containing a solid medium, adjust the position of the discharge unit so that the focal point of the laser beam is within the electrode range and the laser is parallel to the surface of the medium to be measured; during the charging and discharging of the needle-plate electrode, constantly move the experimental cavity in the discharge unit so that the laser focal point scans the entire measured area of the medium surface; then read out the signals collected by the photomultiplier tube through an oscilloscope, calculate the transient electric field at the gas-solid interface according to the calibration coefficient, and then obtain the real-time and accurate charge distribution through the charge inversion algorithm.

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