A microwave measuring device for diagnosing electron density of an atmospheric pressure plasma jet

Abstract

The application discloses a microwave measuring device for diagnosing atmospheric pressure plasma jet electron density, which comprises a waveguide-coaxial converter (10), characterized in that the left and right ends of a slotted rectangular waveguide (20) are respectively connected with the waveguide-coaxial converter (10) through flanges; the slotted rectangular waveguide (20) is connected in parallel with a parallel resonant cavity (30); and the slotted rectangular waveguide (20) comprises a rectangular slot (21), and the rectangular slot (21) is located at the center position of the slotted rectangular waveguide (20). The technical problem that the prior art cannot accurately measure atmospheric pressure plasma jet electron density is solved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of microwave measurement, and particularly relates to a microwave measurement device for diagnosing electron density of an atmospheric pressure plasma jet. BACKGROUND

[0002] Compared with low-pressure plasma, atmospheric pressure plasma has the advantages of high electron density, high electron temperature and high gas temperature, and the excitation and maintenance power of the atmospheric pressure plasma jet is low, and the atmospheric pressure plasma jet can generate abundant active particles, such as excited state atoms and molecules, ions, active groups and the like. Therefore, the atmospheric pressure plasma jet is widely applied to the fields of new material synthesis, material surface treatment, medical surgical treatment and the like. The electron density of the atmospheric pressure plasma jet is one of key parameters affecting the performance of the atmospheric pressure plasma jet, and therefore needs to be accurately measured.

[0003] Common plasma diagnostic techniques include a spectrum measurement technique and a microwave measurement technique. The spectrum measurement technique needs to build a complex optical measurement system, and optical equipment is relatively expensive, and is not suitable for large-scale engineering application; the microwave measurement technique includes a microwave interference method, a microwave reflection method and a microwave perturbation method, the first two methods are to place the plasma in an open space for measurement, and are susceptible to environmental interference, and the last method is to place the plasma in a microwave cavity, and has high robustness.

[0004] The microwave perturbation method cannot accurately measure the electron density of the atmospheric pressure plasma jet, and the reasons include the following two points:

[0005] (1) The electric field intensity of a measurement region of a traditional microwave cavity is not high, so that a large plasma electron density is needed to cause a shift of a cavity resonance frequency, and therefore the sensitivity is low;

[0006] (2) The atmospheric pressure plasma jet is a non-uniform lossy medium, and an existing model is difficult to accurately measure the plasma electron density according to the shift of the cavity resonance frequency; SUMMARY

[0007] The technical problem to be solved by the application is to provide a microwave measurement device for diagnosing electron density of an atmospheric pressure plasma jet, so as to solve the technical problem that the microwave perturbation method cannot accurately measure the electron density of the atmospheric pressure plasma jet.

[0008] The technical scheme of the application is:

[0009] The microwave measurement device for diagnosing electron density of an atmospheric pressure plasma jet comprises a waveguide-coaxial converter, and left and right ends of a slotted rectangular waveguide are respectively connected with the waveguide-coaxial converter through flanges; the slotted rectangular waveguide is connected in parallel with a parallel resonant cavity; the slotted rectangular waveguide comprises a rectangular slot, and the position of the rectangular slot is at a central position of the slotted rectangular waveguide.

[0010] The waveguide-coaxial converter comprises a radio frequency coaxial cable and a waveguide excitation cavity; the waveguide excitation cavity has the same cross-sectional dimension as a standard BJ-22 rectangular waveguide and a length of 66 mm; and the maximum working frequency of the radio frequency coaxial cable is 40 GHz.

[0011] The depth and width of the rectangular slit are determined as 71 mm and 10 mm respectively.

[0012] An observation hole is arranged on the parallel resonant cavity, and the size of the observation hole is consistent with the size of the atmospheric pressure plasma jet; the length of the parallel resonant cavity is 140 mm, so that the working mode is TE 101 mode, the depth is the same as that of a standard BJ-22 rectangular waveguide, and the width is 29 mm.

[0013] The length of the slotted rectangular waveguide is adjusted so that the coupling coefficient of electromagnetic waves from the slotted rectangular waveguide to the parallel resonant cavity reaches 90, the depth and width of the rectangular slit are adjusted so that the resonant frequency of the parallel resonant cavity is equal to 2.1221 GHz, and the frequency of the atmospheric pressure plasma jet is 2.45 GHz, which does not interfere with each other; and the length of the parallel resonant cavity is adjusted so that the working mode of the electromagnetic waves is adjusted to TE 101 mode.

[0014] The length of the slotted rectangular waveguide, the depth and width of the rectangular slit, and the length and width of the parallel resonant cavity are determined by simulation optimization through a finite element algorithm.

[0015] The simulation optimization method comprises:

[0016] Step 1, defining variables: the length of the slotted rectangular waveguide, the depth and width of the rectangular slit, and the length and width of the parallel resonant cavity are defined as variables with different symbols, and the electric field in the parallel resonant cavity is calculated by changing the values of these variables;

[0017] Step 2, geometric modeling: first, a geometric model of the waveguide-coaxial converter on one side is constructed, and then a geometric model on the other side is constructed through symmetrical operation; then, according to the variables defined in step 1, the slotted rectangular waveguide, the rectangular slit and the parallel resonant cavity are constructed to complete the parameterized modeling;

[0018] Step 3, creating a boundary: through the geometric model constructed in step 2, a lumped port excitation boundary is given to the coaxial joint in the waveguide-coaxial converter, and a finite electrical conductor boundary condition is given to the four sides of the waveguide-coaxial converter, the slotted rectangular waveguide and the parallel resonant cavity, and the electrical conductivity is set to 5.7×10 7 S / m;

[0019] Step 4, define material: the inside of waveguide-coaxial converter, slotted rectangular waveguide, parallel resonant cavity is set to air material, the four sides are finite electric conductor boundary conditions; the inner conductor of waveguide-coaxial converter is set to copper material, the outer conductor is set to finite electric conductor boundary conditions, and the intermediate filling medium is polytetrafluoroethylene, the dielectric constant is 2.1, and the loss tangent is 0.001;

[0020] Step 5, set physical field: the calculation domain of all geometric models is set to electromagnetic wave propagation physical field, and the solver is set to electromagnetic wave frequency solver;

[0021] Step 6, divide the grid: all geometric models of the calculation domain are divided into grids, and the grid shape is tetrahedral grid, wherein the grid size of waveguide-coaxial converter and slotted rectangular waveguide is one-twentieth of the wavelength, and the grid size of parallel resonant cavity is one-thirtieth of the wavelength;

[0022] Step 7, scanning calculation: the value range of the variable defined in step 1 is determined, and scanning is carried out in the determined value range, and the electric field distribution results under all variable arrangement combinations are calculated, and the variable combination with the maximum electric field intensity in the parallel resonant cavity is taken as the result.

[0023] The use method of the measuring device is that the left waveguide-coaxial converter is a microwave incident port, and the right waveguide-coaxial converter is a microwave emission port; first, the microwave enters the slotted rectangular waveguide through the incident port, and then is coupled into the parallel resonant cavity through the rectangular slot, the coupling coefficient near the resonant frequency is adjusted to 90 by adjusting the length of the slotted rectangular waveguide and the depth and width of the rectangular slot; the resonant frequency is adjusted to be equal to 2.1221GHz by adjusting the depth and width of the rectangular slot; the working mode of electromagnetic wave in the structure is single mode, i.e. TE 101 mode, by adjusting the width of the parallel resonant cavity, the electric field intensity in the cavity is increased, and the sensitivity of measurement is improved.

[0024] The beneficial effects of the application are:

[0025] The present application optimizes the length of the BJ-22 rectangular waveguide, adjusts the electric field peak at the center position of the waveguide, and opens a rectangular slot at the center position, the depth and width of the slot are optimized and designed, so that the microwave can be fully coupled into the resonant cavity from the slot, and adjusting the depth and width of the slot can change the resonant frequency; secondly, the narrow side of the BJ-22 rectangular waveguide is compressed as a resonant cavity, so as to enhance the electric field intensity of the measurement area and improve the measurement sensitivity; then, the non-uniform electron density distribution of the atmospheric pressure plasma jet is described by using the Gaussian model, and the resonant frequency shift of the resonant cavity is calculated; finally, the atmospheric pressure plasma jet is placed at the measurement area, and the resonant frequency shift of the resonant cavity is measured to determine the electron density of the plasma.

[0026] The present application solves the technical problem that the prior art cannot accurately measure the electron density of the atmospheric pressure plasma jet by improving the electric field intensity of the measurement area and proposing to use the Gaussian model to describe the electron density distribution of the atmospheric pressure plasma jet. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 It is a structural schematic diagram of the present application;

[0028] Figure 2 It is a resonant frequency curve diagram of the present application;

[0029] Figure 3 It is a Gaussian model schematic diagram of the present application. DETAILED DESCRIPTION

[0030] As shown in Figure 1 The present application discloses a microwave measurement device for diagnosing the electron density of atmospheric pressure plasma jet, which comprises a waveguide-coaxial converter 10, a slotted rectangular waveguide 20 and a parallel resonant cavity 30.

[0031] The waveguide-coaxial converter 10 further comprises a radio frequency coaxial cable 11 and a waveguide excitation cavity 12; the coaxial filling medium in the waveguide-coaxial converter is polytetrafluoroethylene material, the dielectric constant is 2.08, and the loss tangent is 0. The cross-sectional size of the waveguide excitation cavity 12 is the same as that of the standard BJ-22 rectangular waveguide, and the length is 66mm;

[0032] The slit rectangular waveguide is a standard BJ-22 rectangular waveguide with a rectangular slit in the middle; the depth and width of the rectangular slit of the slit rectangular waveguide are optimized by a finite element algorithm, the thickness is consistent with the thickness of the waveguide; the length of the slit rectangular waveguide is determined as 275mm through optimization; the position of the rectangular slit 21 is at the center position of the slit rectangular waveguide 20, and the depth and width are respectively determined as 71mm and 10mm through optimization, so as to determine the resonant frequency; the two sides of the slit rectangular waveguide 20 are respectively connected with the waveguide-coaxial converter 10 through flanges, and the waveguide-coaxial converter is connected with the slit rectangular waveguide in series through flanges; one side is used for transmitting microwaves, and the other side is used for receiving microwaves, so as to measure the scattering parameter S 21 .

[0033] The parallel resonant cavity 30 also comprises an observation hole 31; the maximum working frequency of the radio frequency coaxial cable 11 is 40GHz; the length of the parallel resonant cavity 30 is determined as 140mm through optimization, so as to make the working mode TE 101 mode, the depth is the same as that of the standard BJ-22 rectangular waveguide, and the width is compressed to 29mm, so as to enhance the electric field intensity in the cavity; the parallel resonant cavity is a BJ-22 rectangular waveguide with a narrow side compressed and a terminal short-circuited, and the working mode is TE 101 mode, and the resonant cavity is connected with the slit rectangular waveguide in parallel; the observation hole 31 is a circular observation hole with a size consistent with that of the atmospheric pressure plasma jet, and is used for placing the atmospheric pressure plasma jet to be measured.

[0034] By adjusting the length of the slit rectangular waveguide 20, the coupling coefficient of electromagnetic waves from the slit rectangular waveguide 20 to the parallel resonant cavity 30 can reach 90; by adjusting the depth and width of the rectangular slit 21, the resonant frequency of the parallel resonant cavity 30 can be equal to 2.1221GHz, which does not interfere with the frequency 2.45GHz of the atmospheric pressure plasma jet; by adjusting the length of the parallel resonant cavity 30, the working mode of electromagnetic waves can be adjusted as TE 101 mode;

[0035] The length of the slit rectangular waveguide is optimized and designed, so that the electric field reaches the maximum at the slit in the center of the waveguide, thereby effectively increasing the coupling of electromagnetic waves from the slit rectangular waveguide to the parallel resonant cavity.

[0036] The length of the parallel resonant cavity is optimized and designed, so that the electric field mode is TE 101 mode, the narrow side of the parallel resonant cavity is compressed, the electric field intensity of the plasma measurement area is increased, and the measurement sensitivity is effectively improved.

[0037] The application has the characteristics that the electron density distribution of the atmospheric pressure plasma jet is described by using a Gaussian model, so that the atmospheric pressure plasma jet is described as a non-uniform lossy medium, and the dielectric constant distribution of the plasma jet can be accurately calculated.

[0038] The length of the slotted rectangular waveguide 20, the depth and width of the rectangular slot 21, and the length and width of the parallel resonant cavity 30 are simulated and optimized by a finite element algorithm.

[0039] Simulation process:

[0040] a. Define variables: define the length of the slotted rectangular waveguide 20, the depth and width of the rectangular slot 21, and the length and width of the parallel resonant cavity 30 as different variables, and perform numerical simulation calculation on the electric field in the parallel resonant cavity 30 by changing the values of these variables;

[0041] b. Geometric modeling: first, construct a geometric model of the waveguide-coaxial converter 10 on one side, and then construct a geometric model on the other side by symmetry operation; then, according to the variables defined in a, construct the slotted rectangular waveguide 20, the rectangular slot 21 and the parallel resonant cavity 30, so as to complete the parametric modeling of the structure;

[0042] c. Create boundary: from the geometric model constructed in b, give the coaxial joint in the waveguide-coaxial converter 10 a lumped port excitation boundary, and give the waveguide-coaxial converter 10, the slotted rectangular waveguide 20 and the parallel resonant cavity 30 a finite electric conductor boundary condition, and the electric conductivity is set to 5.7x10 7 S / m;

[0043] d. Define materials: set the inside of the waveguide-coaxial converter 10, the slotted rectangular waveguide 20 and the parallel resonant cavity 30 as air material, and the four sides as finite electric conductor boundary condition; set the inner conductor of the waveguide-coaxial converter 10 as copper material, the outer conductor as finite electric conductor boundary condition, and the middle filling medium as polytetrafluoroethylene, with a dielectric constant of 2.1 and a loss tangent of 0.001;

[0044] e. Set physical field: set electromagnetic wave propagation physical field for all geometric models in the calculation domain, and set the solver as electromagnetic wave frequency solver;

[0045] f. Divide the grid: further, divide the grid of all geometric models in the calculation domain, and the grid shape is tetrahedral grid, wherein the grid size of the waveguide-coaxial converter 10 and the slotted rectangular waveguide 20 is one-twentieth of the wavelength, and the grid size of the parallel resonant cavity 30 is one-thirtieth of the wavelength;

[0046] g. Scan calculation: finally, the value range of the variable defined in a is determined, and the electric field distribution results under all variable arrangement combinations are calculated by scanning in the determined value range, and the variable combination with the maximum electric field intensity in the parallel resonant cavity 30 is taken as the optimal result.

[0047] A circular observation hole with a diameter of 30mm is opened on the side of the parallel resonant cavity 30 for placing the atmospheric pressure plasma jet.

[0048] The present application can also measure the electron density of low-pressure plasma, at which time only the Gaussian model needs to be replaced by a uniform distribution model.

[0049] The operation method of the present application:

[0050] The left waveguide-coaxial converter 10 is a microwave incident port, and the right waveguide-coaxial converter 10 is a microwave outgoing port; first, microwaves enter the slotted rectangular waveguide 20 through the incident port, and then are coupled into the parallel resonant cavity 30 through the rectangular slot 21; almost all microwaves are coupled at the resonant frequency, and almost all microwaves are transmitted to outside the structure through the outgoing port at other frequencies; the coupling coefficient at the resonant frequency is made to be 90 by adjusting the length of the slotted rectangular waveguide 20 and the depth and width of the rectangular slot 21; the resonant frequency is made to be equal to 2.1221GHz by adjusting the depth and width of the rectangular slot 21, and if the atmospheric pressure plasma jet is excited by microwaves, the resonant frequency must not interfere with the microwave excitation frequency; the working mode of electromagnetic waves in the structure is made to be single mode, i.e. TE 101 mode, by adjusting the length of the parallel resonant cavity 30; the electric field intensity in the cavity can be increased by compressing the width of the parallel resonant cavity 30, so as to improve the measurement sensitivity, i.e. a small change in the electron density of the measured plasma jet can cause a large shift in the resonant frequency.

[0051] As Figure 2 As shown in the resonant frequency curve of the present application, the resonant frequency of the present application is 2.1221GHz, and the 3-dB bandwidth is 1.25MHz, so the loaded quality factor of the present application can be calculated to be 1705, and the present application has good resonant characteristics.

[0052] It can be known from the simulation comparison of the electric field distribution of the present application and the traditional microwave resonant cavity that under the condition of the same microwave input power, the maximum electric field intensity in the parallel resonant cavity 30 of the present application can reach 4751V / m, which is 1.4 times of the maximum electric field intensity in the traditional microwave resonant cavity, so the measurement sensitivity of the present application is improved.

[0053] As Figure 3The axial distribution and radial distribution of the electron density of the atmospheric pressure plasma jet in the application both satisfy the Gaussian model, wherein the maximum of the radial distribution is at the center position, and the maximum of the axial distribution is at the starting position.

[0054] The application discloses a high-sensitivity microwave measuring device for diagnosing the electron density of an atmospheric pressure plasma jet. The length of a slotted rectangular waveguide 20 and the depth and width of a rectangular slot 21 are optimized to make electromagnetic waves fully coupled into a parallel resonant cavity 30. The narrow side of the parallel resonant cavity 30 is compressed to increase the electric field intensity by 1.4 times, thereby improving the measurement sensitivity of the plasma electron density. The Gaussian model is used to describe the electron density distribution of the atmospheric pressure plasma jet, and the value of the electron density is determined by calculating the resonant frequency shift.

Claims

1. A method of using a microwave diagnostic device for atmospheric pressure plasma jet electron density, said device comprising a waveguide-to-coaxial converter (10), characterized in that: The left and right ends of the slotted rectangular waveguide (20) are respectively connected with the waveguide-coaxial converter (10) through flanges; the slotted rectangular waveguide (20) is connected in parallel with the parallel resonant cavity (30); the slotted rectangular waveguide (20) comprises a rectangular slot (21), and the position of the rectangular slot (21) is at the center position of the slotted rectangular waveguide (20); the use method of the measuring device is that the left waveguide-coaxial converter is a microwave incident port, and the right waveguide-coaxial converter is a microwave emission port; first, the microwave enters the slotted rectangular waveguide through the incident port, and then is coupled into the parallel resonant cavity through the rectangular slot, the length of the slotted rectangular waveguide and the depth and width of the rectangular slot are adjusted so that the coupling coefficient near the resonant frequency reaches 90; the depth and width of the rectangular slot are adjusted so that the resonant frequency is equal to 2.1221 GHz; the length of the parallel resonant cavity is adjusted so that the working mode of the electromagnetic wave in the device is single mode, i.e. TE 101 mode, the width of the parallel resonant cavity is compressed to increase the electric field intensity in the cavity, thereby improving the sensitivity of the measurement; the electron density distribution of the atmospheric pressure plasma jet is described by using the Gaussian model, so that the atmospheric pressure plasma jet is described as a non-uniform lossy medium, and the dielectric constant distribution of the plasma jet is calculated; the axial distribution and the radial distribution of the electron density of the atmospheric pressure plasma jet both satisfy the Gaussian model, wherein the maximum value of the radial distribution is at the center position, and the maximum value of the axial distribution is at the starting position; the length of the slotted rectangular waveguide (20) and the depth and width of the rectangular slot (21) are optimized to make the electromagnetic wave fully coupled into the parallel resonant cavity (30); the narrow side of the parallel resonant cavity (30) is compressed to increase the electric field intensity by 1.4 times, thereby improving the measurement sensitivity of the plasma electron density; the Gaussian model is used to describe the electron density distribution of the atmospheric pressure plasma jet, so that the value of the electron density is determined by calculating the resonant frequency shift; the length of the slotted rectangular waveguide (20), the depth and width of the rectangular slot (21), and the length and width of the parallel resonant cavity (30) are all determined by simulation optimization through the finite element algorithm. The simulation optimization method comprises: Step 1, defining variables: the length of the slotted rectangular waveguide (20), the depth and width of the rectangular slot (21), and the length and width of the parallel resonant cavity (30) are defined as different variables, respectively, and the electric field in the parallel resonant cavity (30) is calculated by changing the values of these variables; Step 2, geometric modeling: first, a geometric model of the waveguide-coaxial converter (10) on one side is constructed, and then the geometric model on the other side is constructed through symmetry operation; then, according to the variables defined in step 1, the slotted rectangular waveguide (20), the rectangular slot (21) and the parallel resonant cavity (30) are constructed to complete the parameterized modeling; Step 3, Create Boundaries: Impose lumped port excitation boundary to the coaxial junction in the waveguide-coaxial converter (10) through the geometry model built in Step 2, impose finite conductivity boundary condition to the four sides of the waveguide-coaxial converter (10), the slotted rectangular waveguide (20), and the parallel resonant cavity (30), with the conductivity set to 5.7 x 10 7 S / m; Step 4, defining materials: the inside of the waveguide-coaxial converter (10), the slotted rectangular waveguide (20) and the parallel resonant cavity (30) is set as air material, and the four sides are set as finite electric conductor boundary conditions; the inner conductor of the waveguide-coaxial converter (10) is set as copper material, the outer conductor is set as finite electric conductor boundary condition, and the middle filling medium is polytetrafluoroethylene with dielectric constant of 2.1 and loss tangent of 0.001; Step 5, setting physical field: the calculation domain of all geometric models is set as electromagnetic wave propagation physical field, and the solver is set as electromagnetic wave frequency solver; Step 6, mesh division: all geometric models of the calculation domain are meshed, and the mesh shape is tetrahedral mesh, wherein the mesh size of the waveguide-coaxial converter (10) and the slotted rectangular waveguide (20) is one-twentieth of the wavelength, and the mesh size of the parallel resonant cavity (30) is one-thirtieth of the wavelength; Step 7, scanning calculation: the value range of the variables defined in step 1 is determined, and scanning is performed in the determined value range, and the electric field distribution results under all variable arrangement combinations are calculated, and the variable combination with the maximum electric field intensity in the parallel resonant cavity (30) is taken as the result.

2. The method of claim 1, wherein the microwave diagnostic device is used to measure the electron density of an atmospheric pressure plasma jet. The waveguide-coaxial converter (10) comprises a radio frequency coaxial cable (11) and a waveguide excitation cavity (12); the cross-sectional size of the waveguide excitation cavity (12) is the same as that of the standard BJ-22 rectangular waveguide, and the length is 66 mm; the maximum working frequency of the radio frequency coaxial cable (11) is 40 GHz.

3. The method of claim 1, wherein the microwave diagnostic device is used to measure the electron density of an atmospheric pressure plasma jet. The depth and width of the rectangular slot (21) are determined as 71 mm and 10 mm, respectively.

4. The method of claim 1, wherein the microwave diagnostic device is used to measure the electron density of an atmospheric pressure plasma jet. The parallel resonant cavity (30) is provided with an observation hole (31), the size of the observation hole (31) is consistent with the size of the atmospheric pressure plasma jet; the length of the parallel resonant cavity (30) is 140 mm, so that the working mode is TE 101 The mode is a rectangular waveguide, the depth is the same as that of a standard BJ-22 rectangular waveguide, and the width is 29 mm.

5. The method of claim 1, wherein the microwave diagnostic device is used to measure the electron density of an atmospheric pressure plasma jet. The length of the slotted rectangular waveguide (20) is adjusted to make the coupling coefficient of electromagnetic wave from the slotted rectangular waveguide (20) to the parallel resonant cavity (30) reach 90, and the depth and width of the rectangular slot (21) are adjusted to make the resonant frequency of the parallel resonant cavity (30) equal to 2.1221 GHz, which is not interfered with the frequency 2.45 GHz of the atmospheric pressure plasma jet; the length of the parallel resonant cavity (30) is adjusted to adjust the working mode of the electromagnetic wave to TE 101 mode.

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