Microwave measurement device and method for plasma parameters of dynamic flow field of arc wind tunnel

Abstract

The invention provides an arc wind tunnel dynamic flow field plasma parameter microwave measurement device and method, the device comprises an arc wind tunnel, a microwave system, a vector analyzer and a data processing module, a transmitting antenna and a receiving antenna of the microwave system are coaxially and horizontally arranged on two sides of a plasma flow field, and a beam waist is accurately aligned with the center of the flow field. According to the method, static and dynamic S parameters are measured through system self-calibration and wide-frequency-band frequency sweeping, a net transmission coefficient is calculated, and the electron density and the collision frequency are synchronously obtained by combining a forward calculation model, a residual objective function, a genetic algorithm and a Levenberg-Marquardt combined inversion algorithm. According to the invention, non-contact measurement is adopted, and a time domain gating filtering technology is matched, so that the problems of clutter interference and equipment ablation are avoided, online real-time accurate measurement of plasma parameters is realized, and reliable data support is provided for research on related characteristics of aircrafts.

Description

Technical Field

[0001] This invention relates to the field of aerodynamic and thermal testing and measurement technology for aircraft, and in particular to a microwave measurement device and method for plasma parameters of dynamic flow field in an electric arc wind tunnel. Background Technology

[0002] Continuous high-enthalpy wind tunnels, such as arc wind tunnels and high-frequency induction plasma wind tunnels, are key equipment for simulating aerodynamic heating of aircraft. During the aerodynamic heating process, these wind tunnels cause violent dissociation and ionization reactions in the gas, generating high-temperature plasma flow. Based on the high-temperature plasma flow field generated by arc wind tunnels and high-frequency induction plasma wind tunnels, research can be conducted on aircraft communication interruptions and plasma sheath characteristics. The plasma flow parameters (electron density...) and collision frequency Measurement of enthalpy plasma flow field is crucial for evaluating high enthalpy plasma flow field.

[0003] Currently, conventional plasma parameter measurement methods are represented by the Langmuir probe. However, this contact-based measurement method is easily ablated and damaged by high-temperature, high-pressure plasma flow fields, and the processing of the probe's volt-ampere characteristics under high-pressure conditions requires theoretical correction, making it quite complex. Traditional microwave measurement methods are based on single-frequency microwave measurements, measuring only the phase and attenuation of a single frequency, which can lead to multiple... and In the case of combined solutions, for on-site measurements of wind tunnel equipment such as electric arc wind tunnels and high-frequency induction plasma wind tunnels, multiple reflections and diffractions from the metal walls of the test chamber, the limitation of the wind tunnel observation window, and background noise severely affect the application and measurement accuracy of the microwave method in wind tunnel electron density measurement. Summary of the Invention

[0004] The purpose of this invention is to provide a microwave measurement device and method for plasma parameters of dynamic flow field in electric arc wind tunnels. Based on broadband microwave measurement and algorithm processing, it realizes online real-time measurement of electron density and collision frequency of plasma flow in high enthalpy wind tunnels, thereby improving the determinism, reliability and accuracy of the measurement.

[0005] According to one objective of the present invention, a microwave measurement device for plasma parameters of a dynamic flow field in an arc wind tunnel is provided, comprising an arc wind tunnel, a microwave system, a vector analyzer, and a data processing module; the arc wind tunnel is used to generate a plasma flow field, and the microwave system is installed inside the test chamber of the arc wind tunnel; the microwave system includes a transmitting antenna and a receiving antenna, which are coaxially and horizontally arranged on both sides of the plasma flow, and the microwave beam waist is located at the center of the plasma flow field and passes perpendicularly through the flow field; the vector analyzer is connected to the microwave system and is used to measure S-parameters under different operating conditions; the data processing module is signal-connected to the vector analyzer and is used to calculate the net transmission coefficient, construct a model, and obtain the electron density and collision frequency through an inversion algorithm.

[0006] Furthermore, both the transmitting antenna and the receiving antenna are horn-focused horn antennas with a focal length ≥2L, where L is the plasma flow field thickness and the beam waist diameter is 15-25mm.

[0007] Furthermore, the vector analyzer uses a continuous wave sweep mode and time-domain gated filtering technology to measure S-parameters. The sweep frequency range is 26.5-40GHz in the Ka band, and the number of sweep points is 201 or 1601.

[0008] Furthermore, the data processing module constructs a forward calculation model and a residual objective function. The forward calculation model is constructed based on elementary charge, electron mass, vacuum dielectric constant, speed of light, and plasma flow field thickness.

[0009] Furthermore, the residual objective function includes terms related to the number of sweep points, group delay, group delay residual relaxation factor, regularization coefficient, prior initial value, and weight coefficient, with the regularization coefficient taking values ​​in the range

[10] . -4 10 -1 The typical value is 0.01.

[0010] According to another objective of the present invention, the present invention provides a microwave measurement method for plasma parameters of dynamic flow field in an electric arc wind tunnel, employing the above-mentioned measuring device, and comprising the following steps: S1: The electric arc wind tunnel generates a plasma flow field; S2: Arrange the transmitting and receiving antennas of the microwave system in the electric arc wind tunnel test chamber, placing them on both sides of the plasma flow, keeping the antennas coaxial and horizontal, adjusting the distance between the antennas and the center of the plasma flow field so that the waist of the microwave beam is located at the center of the plasma flow field and passes through the plasma flow field vertically, and then perform self-calibration on the microwave system. S3: Under the vacuum and cold, non-operational conditions of the wind tunnel, the S-parameters of the microwave system under static conditions are measured using a vector analyzer. ; S4: Under wind tunnel vacuum and thermally stable operating conditions, a vector analyzer is used to measure the microwave system's performance over time under dynamic heating conditions. t Continuously changing S-parameters ; S5: Using what was obtained in S3 And obtained in S4 The net transmission coefficient caused by plasma was obtained. ; S6: Construct a forward calculation model to obtain the theoretical transmission coefficient calculation model. ; S7: Constructing the residual objective function , where x = [ e， c 】 T The parameter vector to be inverted; S8: Execute the inversion algorithm and simultaneously calculate the electron density. and collision frequency .

[0011] Furthermore, in step S2, the microwave system calibration includes the following steps: S201: Keep the vector network analyzer of microwave system 2 powered on for at least 30 minutes to ensure that the equipment is in a stable working state; S202: Use electronic calibration kits and RF cables to connect to ports 1 and 2 of the vector network analyzer, and calibrate according to the equipment operating specifications to eliminate system errors; S203: Measure the S-parameters under two sets of operating conditions respectively: (a) The space between the transmitting and receiving antennas of the microwave antenna is empty. The wind tunnel test chamber is evacuated to the required vacuum level for the test, and the vector network analyzer measures the vacuum. ; (b) A metal plate of a certain thickness and width is placed and fixed between the transmitting and receiving antennas of the microwave antenna. The wind tunnel test chamber is evacuated to the required vacuum level for the test, and the vector network analyzer measures the vacuum. ; S204. Adjust the microwave system and antenna to ensure... .

[0012] Furthermore, in step S5, the net transmittance coefficient satisfy: = - .

[0013] Furthermore, in step S7, the residual objective function : ; in: For the number of sweep points, N is preferably 201 or 1601; For group delay, The phase of the S-parameters; The group delay residual relaxation factor; The regularization coefficient has a value range of

[10] . -4 , 10 -1 The typical value is 0.01. This is the inversion result from the previous time step or the initial a priori value from the first step; These are the weighting coefficients; Weighting coefficient satisfy: ; in, The system noise floor threshold is defined, with values ​​ranging from -70 dB to -60 dB. Values ​​range from [0.5, 1.0].

[0014] Furthermore, in step S8, the inversion algorithm includes: S801, using a genetic algorithm within a preset plasma parameter range ( Search within Minimum approximate solution ; S802, with As initial values, the Levenberg-Marquardt algorithm is used to... Perform iterative calculations; S803, when Stop the iteration and output the electron density. and collision frequency .

[0015] This invention's technical solution effectively avoids clutter interference from the test chamber by optimizing antenna layout and parameter configuration, combined with wideband frequency sweep measurement and "time-domain gating" filtering technology, ensuring the purity and effectiveness of the measurement signal. The method integrates system self-calibration, net transmission coefficient calculation, and combined inversion algorithms to achieve synchronous and accurate measurement of electron density and collision frequency. This solves the problems of multiple solutions, susceptibility to damage, and low accuracy in traditional methods, enabling online real-time measurement of high-enthalpy wind tunnel plasma parameters and providing reliable data support for research on aircraft-related characteristics. Attached Figure Description

[0016] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0017] Figure 1 This is a schematic diagram of the overall structure of the measuring device according to an embodiment of the present invention; Figure 2 This is a schematic flowchart of the measurement method according to an embodiment of the present invention; In the diagram: 1. Arc wind tunnel, 11. Test chamber, 12. Plasma flow field, 2. Microwave system, 21. Transmitting antenna, 22. Receiving antenna, 3. Vector analyzer, 4. Data processing module. Detailed Implementation

[0018] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," and "counterclockwise," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting this invention.

[0020] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the stated features. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified. Furthermore, the terms "installed," "connected," and "linked" should be interpreted broadly; for example, they may refer to a fixed connection, a detachable connection, or an integral connection; a mechanical connection or an electrical connection; a direct connection or an indirect connection through an intermediate medium; or a connection within two components. Those skilled in the art will understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0021] Example 1 like Figure 1 As shown, the microwave measurement device for dynamic flow field plasma parameters of an electric arc wind tunnel provided in this embodiment includes an electric arc wind tunnel 1, a microwave system 2, a vector analyzer 3, and a data processing module 4, wherein: The electric arc wind tunnel 1 is used to generate a plasma flow field 12, providing an experimental object for parameter measurement.

[0022] The microwave system 2 includes a transmitting antenna 21 and a receiving antenna 22, both arranged inside the test chamber 11 of the arc wind tunnel 1, located on opposite sides of the plasma flow, and coaxially horizontally positioned to ensure that the microwave beam waist is located at the center of the plasma flow field 12 and passes perpendicularly through the flow field. Both the transmitting antenna 21 and the receiving antenna 22 are horn-focusing horn antennas with a focal length ≥2L (L is the thickness of the plasma flow field 12) and a beam waist diameter of 15-25mm to ensure beam focusing effect and measurement coverage.

[0023] Vector analyzer 3 is connected to microwave system 2 to measure the S-parameters of microwave system 2 under different operating conditions. The measurement adopts continuous wave frequency sweep mode, with the frequency range preferably in the Ka band (26.5-40GHz), and the number of sweep points N preferably 201 or 1601; at the same time, "time-domain gating" filtering technology is used to filter out the clutter reflected multiple times from the inner wall of the test chamber 11, thereby improving the purity of the measurement signal.

[0024] The data processing module 4 is connected to the vector analyzer 3 by signal and is used to calculate the net transmission coefficient based on the measured S-parameters, construct the forward calculation model and residual objective function, and simultaneously obtain the electron density and collision frequency through the inversion algorithm.

[0025] like Figure 2 As shown, the microwave measurement method for dynamic flow field plasma parameters in an electric arc wind tunnel provided by the present invention, using the aforementioned measurement device, includes the following steps: S1: Control the start of the electric arc wind tunnel 1 to generate a plasma flow field 12 that meets the test requirements; S2: Arrange the transmitting antenna 21 and receiving antenna 22 of microwave system 2 in the test chamber 11 of arc wind tunnel 1 according to the preset requirements, adjust the distance between the antenna and the center of plasma flow field 12 so that the waist of microwave beam is located at the center of plasma flow field 12 and passes through the flow field perpendicularly, and then perform self-calibration on microwave system 2. The specific steps of microwave system 2 self-calibration in step S2 include: S201: Keep the vector network analyzer of microwave system 2 powered on for at least 30 minutes to ensure that the equipment is in a stable working state; S202: Use electronic calibration kits and RF cables to connect to ports 1 and 2 of the vector network analyzer, and calibrate according to the equipment operating specifications to eliminate system errors; S203: Measure the S-parameters under two sets of operating conditions respectively: (a) The space between the transmitting and receiving antennas of the microwave antenna is empty. The wind tunnel test chamber is evacuated to the required vacuum level for the test, and the vector network analyzer measures the vacuum. ; (b) A metal plate of a certain thickness and width is placed and fixed between the transmitting and receiving antennas of the microwave antenna. The wind tunnel test chamber is evacuated to the required vacuum level for the test, and the vector network analyzer measures the vacuum. ; S204. Adjust the microwave system and antenna to ensure... .

[0026] S3: Under the vacuum and cold, non-operational conditions of the wind tunnel, the S-parameters of the microwave system 2 under static conditions are measured using vector analyzer 3. ; S4: Under the vacuum and thermally stable operation conditions of the wind tunnel, the S-parameters of the microwave system 2 under dynamic heating conditions are continuously measured with time t using vector analyzer 3. ; In S3 and S4, the frequency range f for measuring S-parameters by vector analyzer 3 is selected as Ka band (26.5-40GHz), the number of sweep points N is selected as 201, and the "time domain gating" filtering function is continuously enabled during the measurement process.

[0027] S5: Using what was obtained in S3 And obtained in S4 The net transmission coefficient caused by plasma was obtained. ; S6: Construct a forward calculation model to obtain the theoretical transmission coefficient calculation model. ; The forward computation model includes: ; in, e It is the elementary charge; m e For electronic quality; It is the vacuum permittivity; c The speed of light; L The thickness of plasma flow field 12 (determined by the test conditions); S7: Constructing the residual objective function , where x = [ e， c 】 T The parameter vector to be inverted; Where: residual objective function : ; in: For the number of sweep points, N is preferably 201 or 1601; For group delay, The phase of the S-parameters; The group delay residual relaxation factor; The regularization coefficient has a value range of

[10] . -4 , 10 -1 The typical value is 0.01. This is the inversion result from the previous time step or the initial a priori value from the first step; These are the weighting coefficients.

[0028] Specifically, weighting coefficients satisfy: ; in, The system noise floor threshold is defined, with values ​​ranging from -70 dB to -60 dB. Values ​​range from [0.5, 1.0].

[0029] Specifically, in step S7: e =log 10 ( ); c =log 10 ( ); , The electron density and collision frequency can be estimated based on the actual conditions of the plasma flow field.

[0030] S8: Execute the inversion algorithm and simultaneously calculate the electron density. and collision frequency .

[0031] In step S8, the specific process of the inversion algorithm includes: S801, using a genetic algorithm within a preset plasma parameter range ( Search within Minimum approximate solution ; S802, with As initial values, the Levenberg-Marquardt algorithm is used to... Perform iterative calculations; S803, when Stop the iteration and output the electron density. and collision frequency .

[0032] This invention effectively solves many shortcomings of traditional measurement methods by using wide-band frequency sweep measurement, precise filtering technology and optimized inversion algorithm, and realizes high-precision, real-time online measurement of plasma parameters in dynamic flow field of electric arc wind tunnel, providing reliable data support for the study of aircraft-related characteristics.

[0033] Example 2 like Figure 2 As shown, a microwave measurement method for plasma parameters of dynamic flow field in an electric arc wind tunnel includes the following steps: S1: The electric arc wind tunnel generates a plasma flow field; S2: Arrange the transmitting and receiving antennas of the microwave system in the electric arc wind tunnel test chamber, placing them on both sides of the plasma flow, keeping the antennas coaxial and horizontal, adjusting the distance between the antennas and the center of the plasma flow field so that the waist of the microwave beam is located at the center of the plasma flow field and passes through the plasma flow field vertically, and then perform self-calibration on the microwave system. S3: Under the vacuum and cold, non-operational conditions of the wind tunnel, the S-parameters of the microwave system under static conditions are measured using a vector analyzer. ; S4: Under wind tunnel vacuum and thermally stable operating conditions, a vector analyzer is used to measure the microwave system's performance over time under dynamic heating conditions. t Continuously changing S-parameters ; S5: Using what was obtained in S3 And obtained in S4 The net transmission coefficient caused by plasma was obtained. ; S6: Construct a forward calculation model to obtain the theoretical transmission coefficient calculation model. ; S7: Constructing the residual objective function , where x = [ e， c 】 T The parameter vector to be inverted; S8: Execute the inversion algorithm and simultaneously calculate the electron density. and collision frequency .

[0034] Specifically, in steps S3 and S4, the S-parameters are measured using a continuous wave frequency sweep mode to obtain a certain frequency range. f S-parameters, f Ka band (26.5-40 GHz) is preferred; Specifically, in step S2, the self-calibration steps of the microwave system include: S201. Keep the vector network analyzer of the microwave system powered on for at least 30 minutes. S202. Use the electronic calibration kit and RF cable to connect to ports 1 and 2 of the vector network analyzer for calibration; S203. Measure the S-parameters under the two sets of operating conditions respectively: (a) The space between the transmitting and receiving antennas of the microwave antenna is empty. The wind tunnel test chamber is evacuated to the required vacuum level for the test, and the vector network analyzer measures the vacuum. ; (b) A metal plate of a certain thickness and width is placed and fixed between the transmitting and receiving antennas of the microwave antenna. The wind tunnel test chamber is evacuated to the required vacuum level for the test, and the vector network analyzer measures the vacuum. ; S204. Adjust the microwave system and antenna to ensure... .

[0035] Specifically, in this embodiment, the vector network analyzer must use "time-domain gating" filtering to filter out clutter reflected multiple times from the inner wall of the test chamber when measuring S-parameters; Specifically, in step S5, satisfy: = - .

[0036] Specifically, in step S6, the forward calculation model includes: ; in, e It is the elementary charge; m e For electronic quality; It is the vacuum permittivity; c The speed of light; L The thickness of the plasma flow field is denoted as .

[0037] Specifically, the residual objective function in step S7 : ; in: For the number of sweep points, N is preferably 201 or 1601; For group delay, The phase of the S-parameters; The group delay residual relaxation factor; The regularization coefficient has a value range of

[10] . -4 , 10 -1 The typical value is 0.01. This is the inversion result from the previous time step or the initial a priori value from the first step; These are the weighting coefficients.

[0038] Specifically, weighting coefficients satisfy: ; in, The system noise floor threshold is defined, with values ​​ranging from -70 dB to -60 dB. Values ​​range from [0.5, 1.0].

[0039] Specifically, in step S7: e =log 10 ( ); c =log 10 ( ); , The electron density and collision frequency can be estimated based on the actual conditions of the plasma flow field.

[0040] Specifically, the inversion algorithm in step S8 includes: S801, using a genetic algorithm within a preset plasma parameter range ( Search within Minimum approximate solution ; S802, with As initial values, the Levenberg-Marquardt algorithm is used to... Perform iterative calculations; S803, when Stop the iteration and output the electron density. and collision frequency .

[0041] In this embodiment, the transmitting antenna and receiving antenna are horn-focused horn antennas with a focal length ≥2. L The beam waist diameter is 15-25mm.

[0042] In summary, this invention employs wideband continuous wave sweep frequency measurement combined with algorithm inversion, avoiding the problem of multiple solutions that easily occur in traditional single-frequency microwave measurements, achieving the identification of unique solutions for plasma parameters, and improving the determinism and reliability of the measurement. The microwave system of this invention employs "time-domain gating" filtering technology to effectively filter out clutter reflected multiple times from the inner wall of the test chamber, reducing the interference of the wind tunnel equipment's on-site environment on the measurement and improving the measurement accuracy; The present invention uses a horn-focusing horn antenna with specific parameters for both the transmitting and receiving antennas to ensure that the microwave beam accurately passes through the center of the plasma flow field, thus guaranteeing the effectiveness of the measurement signal. This invention achieves simultaneous and accurate inversion of electron density and collision frequency by constructing a reasonable forward calculation model and residual objective function, and combining a combined inversion strategy of genetic algorithm and Levenberg-Marquardt algorithm, taking into account both global search capability and local convergence speed. The measurement process of this invention is non-contact, which avoids the problem of Langmuir probes being easily ablated and damaged. It also does not require complex theoretical corrections, is easy to operate, and can realize online real-time measurement of plasma flow fields in high enthalpy wind tunnels.

[0043] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A microwave measurement device for plasma parameters of dynamic flow field in an electric arc wind tunnel, characterized in that, The system includes an arc wind tunnel, a microwave system, a vector analyzer, and a data processing module. The arc wind tunnel is used to generate a plasma flow field, and the microwave system is installed inside the test chamber of the arc wind tunnel. The microwave system includes a transmitting antenna and a receiving antenna, which are coaxially and horizontally arranged on both sides of the plasma flow. The microwave beam waist is located at the center of the plasma flow field and passes perpendicularly through the flow field. The vector analyzer is connected to the microwave system and is used to measure S-parameters under different operating conditions. The data processing module is connected to the vector analyzer and is used to calculate the net transmission coefficient, construct a model, and obtain the electron density and collision frequency through an inversion algorithm.

2. The microwave measurement device for plasma parameters of dynamic flow field in an electric arc wind tunnel according to claim 1, characterized in that, Both the transmitting antenna and the receiving antenna are horn-focused horn antennas with a focal length ≥2L, where L is the plasma flow field thickness and the beam waist diameter is 15-25mm.

3. The microwave measurement device for dynamic flow field plasma parameters of an electric arc wind tunnel according to claim 1, characterized in that, The vector analyzer uses continuous wave sweep mode and time-domain gated filtering technology to measure S-parameters. The sweep frequency range is 26.5-40GHz in the Ka band, and the number of sweep points is 201 or 1601.

4. The microwave measurement device for plasma parameters of dynamic flow field in an electric arc wind tunnel according to claim 1, characterized in that, The data processing module constructs a forward calculation model and a residual objective function. The forward calculation model is based on elementary charge, electron mass, vacuum dielectric constant, speed of light, and plasma flow field thickness.

5. The microwave measurement device for plasma parameters of dynamic flow field in an electric arc wind tunnel according to claim 4, characterized in that, The residual objective function includes terms related to the number of sweep points, group delay, group delay residual relaxation factor, regularization coefficient, prior initial value, and weight coefficient. The regularization coefficient takes values ​​in the range [10]. -4 10 -1 The typical value is 0.

01.

6. A microwave measurement method for plasma parameters of dynamic flow field in an electric arc wind tunnel, characterized in that, The measuring device according to any one of claims 1-5 includes the following steps: S1: The electric arc wind tunnel generates a plasma flow field; S2: Arrange the transmitting and receiving antennas of the microwave system in the electric arc wind tunnel test chamber, placing them on both sides of the plasma flow, keeping the antennas coaxial and horizontal, adjusting the distance between the antennas and the center of the plasma flow field so that the waist of the microwave beam is located at the center of the plasma flow field and passes through the plasma flow field vertically, and then perform self-calibration on the microwave system. S3: Under the vacuum and cold, non-operational conditions of the wind tunnel, the S-parameters of the microwave system under static conditions are measured using a vector analyzer. ; S4: Under wind tunnel vacuum and thermally stable operating conditions, a vector analyzer is used to measure the microwave system's performance over time under dynamic heating conditions. t Continuously changing S-parameters ; S5: Using what was obtained in S3 And obtained in S4 The net transmission coefficient caused by plasma was obtained. ; S6: Construct a forward calculation model to obtain the theoretical transmission coefficient calculation model. ; S7: Constructing the residual objective function , where x = [ e， c 】 T The parameter vector to be inverted; S8: Execute the inversion algorithm and simultaneously calculate the electron density. and collision frequency .

7. The measurement method according to claim 6, characterized in that, In step S2, the microwave system calibration includes the following steps: S201: Keep the vector network analyzer of microwave system 2 powered on for at least 30 minutes to ensure that the equipment is in a stable working state; S202: Use electronic calibration kits and RF cables to connect to ports 1 and 2 of the vector network analyzer, and calibrate according to the equipment operating specifications to eliminate system errors; S203: Measure the S-parameters under two sets of operating conditions respectively: (a) The space between the transmitting and receiving antennas of the microwave antenna is empty. The wind tunnel test chamber is evacuated to the required vacuum level for the test, and the vector network analyzer measures the vacuum. ; (b) A metal plate of a certain thickness and width is placed and fixed between the transmitting and receiving antennas of the microwave antenna. The wind tunnel test chamber is evacuated to the required vacuum level for the test, and the vector network analyzer measures the vacuum. ; S204. Adjust the microwave system and antenna to ensure... .

8. The measurement method according to claim 6, characterized in that, Net transmittance in step S5 satisfy: = - 。 9. The measurement method according to claim 6, characterized in that, In step S7, the residual objective function : ； in: For the number of sweep points, N is preferably 201 or 1601; For group delay, The phase of the S-parameters; The group delay residual relaxation factor; The regularization coefficient has a value range of [10]. -4 , 10 -1 The typical value is 0.

01. This is the inversion result from the previous time step or the initial a priori value from the first step; These are the weighting coefficients; Weighting coefficient satisfy: ； in, The system noise floor threshold is defined, with values ​​ranging from -70 dB to -60 dB. Values ​​range from [0.5, 1.0].

10. The measurement method according to claim 6, characterized in that, In step S8, the inversion algorithm includes: S801, using a genetic algorithm within a preset plasma parameter range ( Search within Minimum approximate solution ; S802, with As initial values, the Levenberg-Marquardt algorithm is used to... Perform iterative calculations; S803, when Stop the iteration and output the electron density. and collision frequency .

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