Diagnostic system for obtaining plasma electron density

Abstract

The invention provides a diagnosis system for obtaining plasma electron density, which comprises a voltage probe, a current probe, an oscilloscope, a power supply, a computer, a spectrum diagnosis system and a plasma reactor, the reactor is composed of a high-voltage electrode and a low-voltage electrode, and the reactor is excited by the power supply to generate plasma. The voltage probe measures the change of excitation voltage between the high-voltage electrode and the low-voltage electrode along with time, the current probe measures the change of discharge current in the plasma along with time, and the computer automatically calls a program. And a curve of plasma electron density changing along with discharge time can be obtained. The spectrum diagnosis system comprises a spectrograph and an optical fiber probe and is used for diagnosing the gas temperature and the electron temperature in the plasma. The plasma electron density diagnosis method provided by the invention is suitable for measuring the plasma electron density of a reactor with a known plasma discharge channel size, solves the defect that a high-resolution spectrometer cannot measure the electron density of low-density plasma, and has the characteristics of simple measuring device system and method; the method is non-intrusive, and has no measurement lower limit and upper limit.

Description

Technical Field

[0001] The invention belongs to the field of low-temperature plasma discharge, and specifically designs a diagnostic system for obtaining plasma electron density. Background Art

[0002] Plasma is the fourth form of matter besides "solid", "liquid" and "gaseous". According to the temperature of plasma, it can be divided into high-temperature plasma and low-temperature plasma. Among them, high-temperature plasma is a fully ionized plasma, such as solar core fusion and magnetic confinement nuclear fusion plasma, etc. The gas temperature of low-temperature plasma is closer to room temperature. Gas discharge is an effective way to produce low-temperature plasma, which has been applied in biomedicine, material processing, environmental protection, flow control and other fields. As one of the most important parameters to describe the properties of plasma, electron density diagnosis plays a vital role in the research and application of low-temperature plasma. It not only helps to deeply understand the physical properties of plasma, but also guides the optimization of actual processes, thereby promoting the development and application of low-temperature plasma technology.

[0003] Methods for diagnosing the electron density of atmospheric pressure plasma include: Langmuir probe method, Stark broadening method, heterodyne interferometer method, Thompson scattering method, etc. Among them, the Langmuir probe method has a simple structure and obtains information such as electron density and electron temperature by measuring the current-voltage characteristic curve when the probe contacts the plasma. It is suitable for measuring the electron density of low-density plasma. However, the probe is an invasive method and has limited application for small-area plasmas in closed spaces. The Stark broadening method diagnoses the electron density by analyzing the spectral lines emitted or absorbed by the plasma. It is a non-invasive method. This method can obtain the chemical composition and temperature information of the plasma, but it requires complex spectral analysis and is not suitable for the diagnosis of lower plasma electron density. The heterodyne interferometer method requires the construction of an optical system, which is difficult to implement. It requires precise optical components and precise alignment, and has strict requirements on experimental conditions. The Thompson scattering method also has the disadvantages of expensive diagnostic equipment, high technical requirements, and complex data analysis. Therefore, it is necessary to propose a diagnostic system and method that is simple to operate, non-invasive, and suitable for small-area discharge areas and low plasma electron density. Summary of the invention

[0004] The purpose of the present invention is to provide a diagnostic system for obtaining plasma electron density, which can obtain plasma electron density in a non-invasive manner and can be applied to the collection and measurement of low plasma electron density diagnosis when the discharge channel size is known.

[0005] The method provided by the present invention is provided through the following diagnostic system and means, including: building a plasma reactor voltage and current acquisition platform, a computer substitutes the acquired voltage and current waveforms into an edited program, and obtains a curve of plasma electron density changing with discharge time.

[0006] The present invention provides a diagnostic system for obtaining plasma electron density, comprising a voltage probe, a current probe, an oscilloscope, a power supply, a plasma reactor, a computer, and a spectrum diagnostic system. The plasma reactor is composed of a high-voltage electrode and a low-voltage electrode. The high-voltage electrode of the plasma reactor is connected to a high-voltage output port of the power supply, and the low-voltage electrode of the reactor is connected to a low-voltage output port of the power supply. The voltage probe measures the voltage between the high-voltage electrode and the low-voltage electrode at both ends of the plasma reactor, the current probe measures the current of the discharge circuit, the oscilloscope collects voltage and current waveforms, and the computer obtains a curve of plasma electron density changing with discharge time by analyzing the voltage and current waveforms. The spectrum diagnostic system comprises a spectrometer and an optical fiber probe, and is used to obtain the gas temperature and electron temperature in the plasma.

[0007] The current-voltage method is used to calculate the plasma electron density versus discharge time curve as follows:

[0008] The plasma electron density can be calculated from the current density. If the size of the plasma discharge channel is known, the plasma electron density can be expressed as , where n e is the plasma electron density, J is the current density, E is the electric field strength, μ e is the electron mobility, e is the electron charge, the plasma discharge channel is known, the current density can be calculated using J = I / A, I is the collected current curve, A is the plasma cross-sectional area; the electric field strength can be calculated using E = V / L, V is the collected voltage waveform, and L is the length of the discharge channel. In addition, the electron mobility μ e equal . m e is the electron mass, is the collision frequency between electrons and neutral particles. The collision frequency between electrons and neutral particles in low-temperature plasma can be approximately regarded as the plasma collision frequency. The calculation formula of plasma electron density obtained by the current-voltage method is: , in addition to being related to the collected voltage and current waveforms, it is also related to the plasma collision frequency;

[0009] The plasma collision frequency is indirectly obtained by spectral diagnosis of the gas temperature and electron temperature in the plasma;

[0010] The plasma collision frequency can be expressed as , where N is the neutral molecule density in the discharge plasma, σ is the molecular cross-sectional area of ​​the discharge gas, and ve is the electron velocity; , P is the air pressure, k is the Boltzmann constant, T g is the gas temperature in the discharge, , T e is the electron temperature.

[0011] The beneficial effects of the present invention are:

[0012] The diagnostic system for obtaining plasma electron density proposed in the present invention is simple to operate and is suitable for plasma diagnosis in closed spaces, small discharge areas, and low electron density plasmas. By measuring the voltage waveform and current waveform at both ends of the plasma reactor, assisted by spectral diagnosis of electron temperature and gas temperature, a curve of plasma electron density changing with discharge time is obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 It is a structural diagram of a diagnostic system for obtaining plasma electron density;

[0014] Figure 2 It is the waveform of the plasma electron density obtained under DC power supply excitation changing with discharge time;

[0015] Figure 3 It is a comparison between the plasma electron density results measured by the current-voltage method and the electron density results measured by the microwave interferometer. DETAILED DESCRIPTION

[0016] In order to make the purpose, technical scheme and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in each embodiment of the present invention described below can be combined with each other as long as they do not conflict with each other. To achieve the above-mentioned purpose, the present invention adopts the following technical scheme.

[0017] Example 1

[0018] In order to make the above-mentioned objects, features and advantages of the present invention more clearly understood, the specific embodiments of the present invention are described in detail below with reference to the accompanying drawings. Figure 1 Shown is a diagnostic system for obtaining plasma electron density in real time.

[0019] The plasma reactor voltage and current acquisition platform constructed by the present invention includes a voltage probe, a current probe, an oscilloscope, a power supply, a plasma reactor, a computer, and a spectrum diagnosis system. The plasma reactor is composed of a high-voltage electrode and a low-voltage electrode. The high-voltage electrode of the plasma reactor is connected to the high-voltage output port of the power supply, and the low-voltage electrode of the plasma reactor is connected to the low-voltage output port of the power supply. The voltage probe measures the voltage between the high-voltage electrode and the low-voltage electrode at both ends of the plasma reactor, and the current probe measures the current of the discharge circuit. The oscilloscope collects the waveforms of voltage and current. The spectrum diagnosis system includes a spectrometer and an optical fiber probe, which are used to obtain the gas temperature and electron temperature in the plasma. The computer obtains the change curve of the plasma electron density with the discharge time by analyzing the voltage and current waveforms and the plasma parameters of the spectrum diagnosis.

[0020] The plasma reactor measured by the present invention has electrodes consisting of a high-voltage electrode and a low-voltage electrode. The plasma reactor adopts a sealed structure. The internal gas is a mixed inert gas with a gas pressure of 0.001 atmospheres. The cross-sectional area of ​​the discharge channel is A, and the length of the discharge channel is L.

[0021] Taking a DC power supply as an example, when the DC voltage applied between the two electrodes of a plasma reactor exceeds the breakdown voltage of the gas, plasma is generated. The voltage probe measures the voltage between the high-voltage electrode and the low-voltage electrode of the plasma reactor, and the current probe measures the current in the plasma discharge circuit. The waveform measured by the voltage probe is represented by V(t), and the waveform measured by the current probe is represented by I(t).

[0022] The current-voltage method is used to calculate the plasma electron density versus discharge time curve as follows:

[0023] The variation curve of plasma electron density with discharge time is expressed in N e (t) can be obtained by the variation curve of plasma discharge current and voltage across the reactor with discharge time. Plasma electron density can be expressed as , where J(t) is the variation curve of plasma current density with discharge time, E(t) is the variation curve of electric field intensity with discharge time, μ e is the electron mobility, e is the electron charge, and the plasma current density can be calculated using J(t) =I(t) / A, where I(t) is the collected current curve and A is the cross-sectional area of ​​the plasma discharge channel; the electric field strength can be obtained through the voltage curve collected by the voltage probe, E(t)=V(t) / L, where V(t) is the collected voltage waveform and L is the length of the discharge channel. In addition, the electron mobility μ e equal . m e is the electron mass, is the collision frequency between electrons and neutral particles. The collision frequency between electrons and neutral particles in low-temperature plasma can be regarded as the collision frequency of plasma. Therefore, the plasma electron density versus time curve obtained by the current-voltage method can be expressed by the plasma collision frequency, that is, .

[0024] The plasma collision frequency is indirectly obtained by spectral diagnosis of the gas temperature and electron temperature in the plasma; the plasma collision frequency can be expressed as , where N is the neutral molecule density in the discharge plasma, σ is the molecular cross-sectional area of ​​the discharge gas, and v e is the electron velocity. Among them, the neutral molecule density in the discharge plasma is N =P / kT g , which can be obtained indirectly through spectral diagnosis of gas temperature, P is the gas pressure, k is the Boltzmann constant, T g is the gas temperature in the discharge. The electron velocity in the discharge plasma , T e is the electron temperature, which can be obtained by spectral diagnosis of the electron temperature.

[0025] The gas temperature is mainly realized by spectral diagnosis of the rotation temperature of the plasma. In this example, the N2(CB) line (368~385 nm) in the plasma emission spectrum is selected to fit the rotation temperature. The plasma emission spectrum with a wavelength range of 300~400 nm is collected by a spectrometer, and the discharge spectrum is fitted using software to extract the rotation temperature T of the discharge gas. g .

[0026] Electron temperature diagnosis includes double spectrum method, multi-spectrum slope method, isoelectronic spectrum method, Saha-Boltzmann method, spectrum line absolute intensity method and other emission spectroscopy methods to obtain electron temperature T. e In this example, the double-line method is used to diagnose the electron temperature. Two different spectral lines are selected from the emission spectrum of the same atom or ion. The wavelengths are recorded as λ1 and λ2, and the corresponding spectral line intensities are recorded as I1 and I2. The electron temperature T e for: , A i represents the transition probability, g i represents the statistical weight, E i represents the excited state energy, k is the Boltzmann constant, and the plasma electron temperature T is obtained according to the above formula e .

[0027] In the present invention, automatic acquisition of oscilloscope waveforms by a computer can be achieved through a variety of methods, such as using the oscilloscope's own software, third-party software or a programming interface to write a script or program to control the acquisition. By writing a program according to the method introduced above, a curve of the plasma electron density changing with the discharge time can be directly obtained.

[0028] According to the obtained T e、 T g、 , and the curve N of plasma electron density changing with discharge time is obtained e (t), such as Figure 2 As shown, a commercial microwave interferometer is used to compare the measured results, such as Figure 3 As shown, the measurement results have good consistency, which proves the correctness of the plasma electron density measurement system and method.

Claims

1. A diagnostic system for obtaining plasma electron density, characterized in that: The invention comprises a voltage probe, a current probe, an oscilloscope, a power supply, a plasma reactor, a computer and a spectrum diagnosis system. The plasma reactor is composed of a high-voltage electrode and a low-voltage electrode. The high-voltage electrode of the plasma reactor is connected to the high-voltage output port of the power supply, and the low-voltage electrode of the plasma reactor is connected to the low-voltage output port of the power supply. The voltage probe measures the voltage between the high-voltage electrode and the low-voltage electrode at both ends of the plasma reactor, the current probe measures the current of the discharge circuit, the oscilloscope collects the waveforms of the voltage and the current, and the computer obtains the curve of the change of the plasma electron density with the discharge time by analyzing the waveforms of the voltage and the current; the spectrum diagnosis system comprises a spectrometer and an optical fiber probe, which are used to obtain the gas temperature and the electron temperature in the plasma reactor.

2. The diagnostic system for obtaining plasma electron density according to claim 1, characterized in that: The plasma reactor is sealed inside and filled with inert gas, and the high-voltage electrode and the low-voltage electrode are blade electrodes.

3. The diagnostic system for obtaining plasma electron density according to claim 2, characterized in that: The size of the discharge channel of the plasma reactor is known or can be measured by other means.

4. The diagnostic system for obtaining plasma electron density according to claim 1, characterized in that: The power supply adopts one of a variety of excitation power supplies, including a direct current power supply or other types of low-frequency excitation power supplies.

5. The diagnostic system for obtaining plasma electron density according to claim 1, characterized in that: The voltage and current waveforms collected by the oscilloscope are used as two measurement curves reflecting the electron density of the discharge plasma. Through the corresponding algorithm, a curve showing the change of plasma electron density with discharge time is obtained.

6. The diagnostic system for obtaining plasma electron density in real time according to claim 1, characterized in that: The voltage probe, current probe and oscilloscope meet a certain resolution to ensure that the resolution can truly and accurately reflect the instantaneous changes of plasma discharge voltage and discharge current over time.

7. The diagnostic system for obtaining plasma electron density according to claim 5, characterized in that: The specific algorithm is: establish a relationship between the plasma electron density and the discharge plasma discharge current density and electric field strength to satisfy the following equation: ； J is the plasma discharge current density, and the current density is calculated by J=I / A, where I represents the instantaneous value of the current displayed by the oscilloscope, A represents the cross-sectional area of ​​the plasma discharge channel, E represents the electric field intensity, and μ e represents electron mobility and e is the electron charge.

8. The diagnostic system for obtaining plasma electron density according to claim 7, characterized in that: The electron mobility μ e Satisfies the following equation: ； Represents the collision frequency between electrons and neutral particles. The electric field strength is calculated using E=V / L, where V is the voltage waveform displayed by the oscilloscope and L is the length of the plasma discharge channel, m e is the electron mass, and the electron mobility equation is substituted into the plasma electron density equation. The plasma electron density equation is as follows: 。 9. The diagnostic system for obtaining plasma electron density according to claim 8, characterized in that: The relationship between the plasma electron density and the voltage and current waveforms obtained by the oscilloscope and the collision frequency between electrons and neutral particles is established. satisfy ; , where N is the density of neutral molecules in the plasma, P represents the gas pressure, k represents the Boltzmann constant, and T g is the gas temperature of the reaction gas in the plasma discharge, σ is the molecular cross section of the reaction gas, and v e is the electron speed.

10. The diagnostic system for obtaining plasma electron density according to claim 9, characterized in that: Electron speed v e The electron temperature is obtained The electron temperature and gas temperature are obtained through the spectral diagnostic system. The collision frequency between plasma electrons and neutral particles is obtained based on the electron velocity and neutral molecule density obtained by the spectral diagnostic system. Based on the equation between plasma electron density and collision frequency, the plasma electron density is finally obtained as follows: 。 11. The diagnostic system for obtaining plasma electron density according to claim 10, characterized in that: The gas temperature is diagnosed by analyzing the particle number distribution of the rotational energy level of the excited diatomic molecules, and the radiation spectrum of the diatomic molecules is fitted to obtain the rotational temperature of the gas, which is regarded as the gas temperature T of the plasma. g .

12. The diagnostic system for obtaining plasma electron density according to claim 10, characterized in that: The electron temperature T is obtained by using one of the emission spectroscopy methods, including the double-line method, multi-line slope method, isoelectronic line method, Saha-Boltzmann method, and line absolute intensity method. e .

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