Microwave plasma jet device, system and design method

Abstract

The invention relates to the field of microwave plasma jet devices, in particular to a microwave plasma jet device, a system and a design method. A cylindrical resonant cavity is arranged in the resonant structure; one end of the quartz tube extends into the cylindrical resonant cavity to the first side cavity wall, and the quartz tube is provided with a hollow inner cavity; a linear coil is arranged in the cylindrical resonant cavity body, the linear coil and the quartz tube are arranged at an interval, one end of the linear coil is insulated from the first side cavity wall, and the other end of the linear coil is connected with the second side cavity wall; and the central inner conductor is positioned in the hollow inner cavity, and the central inner conductor is positioned on one side, close to the first side cavity wall, in the quartz tube. The microwave plasma jet device has a wide distribution characteristic and low-amplitude concentration in an electric field space domain, the size of the microwave plasma jet device can be designed to be smaller, the microwave energy efficiency is higher, the jet temperature is lower, the adjustable range is larger, the microwave plasma jet device can be suitable for application needing a larger machining area, and the application scene is wider.

Description

Technical Field

[0001] This invention relates to the field of microwave plasma jet devices, and particularly to a microwave plasma jet device, system, and design method. Background Technology

[0002] Microwave plasma technology is a method of generating and maintaining plasma using electromagnetic waves in the microwave frequency band (300MHz-300GHz). It is widely used in industry due to its high ionization rate and active particle density. Microwave plasma jet generators can be classified into waveguide type, coaxial type, and microstrip type according to the microwave transmission method, each suitable for different industrial applications.

[0003] As plasma technology continues to advance in industrial applications, new uses have emerged, placing new demands on equipment. Examples include whole-machine paint removal, large-aperture optical processing, and solar cell surface texturing. These applications require large processing areas, thus necessitating large plasma jet diameters. Furthermore, when processing complex curved surfaces, the jet device must be compact to ensure flexibility. Moreover, given the limited thermal tolerance of the substrate, the jet temperature must be controlled within a safe range. Current equipment cannot simultaneously meet all these conditions. Waveguide generators, due to their large structural volume and high microwave power capacity, are suitable for generating large-diameter thermal plasma jets and are primarily used in hazardous waste treatment and materials preparation. Coaxial generators, with their structural flexibility and low microwave power requirements, are commonly used in materials synthesis and surface treatment. Microstrip generators, due to their compact structure and suitability for planar manufacturing, are primarily used for materials modification.

[0004] Existing waveguide-type plasma generators have a large structural volume and high microwave power, capable of generating hot plasma jets with diameters exceeding 60 mm. They are mainly used in hazardous waste pollution control, materials preparation, and material decomposition. Coaxial-type generators have a flexible structure and lower microwave power, capable of generating room-temperature plasma jets with diameters of 1 to 3 mm. They are commonly used in materials synthesis, sterilization, surface modification, etching, and polishing. Microstrip-type generators have a compact structure and can generate plasma using mW-level microwave power, primarily used in materials modification.

[0005] Although existing technologies perform well in specific areas, the jet devices in these technologies directly feed the current to the center of the quartz tube, where the peak electric field is very high. However, the electric field diffuses very rapidly to the surrounding area, resulting in a large gradient in the electric field intensity distribution. Therefore, they can only satisfy a certain electric field intensity [≥10]. 6 The effective excitation area of ​​the above is very small. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of existing technologies where the jet device directly feeds the middle of the quartz tube, where the peak electric field is very high but diffuses very quickly to the surrounding area, resulting in a large electric field intensity distribution gradient. This invention provides a microwave plasma jet device, system, and design method.

[0007] Firstly, a microwave plasma jet device, comprising: A resonant structure having an internal cylindrical resonant cavity, the resonant structure having a first side cavity wall and a second side cavity wall arranged opposite to each other, the first side cavity wall and the second side cavity wall surrounding the cylindrical resonant cavity; A quartz tube extends from one end of the second side cavity wall into the cylindrical resonant cavity and up to the first side cavity wall. The quartz tube has a hollow inner cavity, which is separated from the cylindrical resonant cavity by the quartz tube. A straight coil is disposed inside the cylindrical resonant cavity. The straight coil is spaced apart from the quartz tube. One end of the straight coil is insulated from the first side cavity wall, and the other end of the straight coil is connected to the second side cavity wall. The central inner conductor is located inside the hollow inner cavity, and the central inner conductor is located on the side of the quartz tube near the first side cavity wall.

[0008] This application describes a microwave plasma jet device with a resonant structure. It includes a cylindrical resonant cavity that induces electromagnetic field resonance within the cavity, concentrating microwave energy within a quartz tube. This energy is then used to ionize the gas through a strong electric field. A straight coil within the cylindrical resonant cavity is insulated from the first side wall and connected to the far-end second side wall. After entering the straight coil, the microwave first contacts the far end of the resonant structure, then forms a closed loop with the entire cylindrical resonant cavity, creating a closed loop and inducing a magnetic field. A central inner conductor induces an electric field within the quartz tube, adjusting the intensity and distribution of the induced electric field in the internal region of the quartz tube and increasing the electric field strength within the tube. This microwave plasma jet device achieves a certain electric field strength [≥10]. 6 The effective excitation area of ​​the device described above is larger, and the electric field intensity distribution gradient in this area is smaller. Compared with the prior art, under the same input power, the microwave plasma jet device described in this application has a wider distribution characteristic and lower amplitude concentration in the electric field spatial domain. Its volume can be designed to be smaller, the microwave energy efficiency is higher, the jet temperature is lower, and the adjustable range is larger. It can be applied to applications that require a larger processing area, thus making the microwave plasma jet device described in this application applicable to a wider range of scenarios.

[0009] Preferably, a cylindrical inner conductor is further provided inside the cylindrical resonant cavity. The cylindrical inner conductor is sleeved on the outside of the quartz tube and is located on the side of the cylindrical resonant cavity near the first side wall.

[0010] Preferably, the cylindrical inner conductor and the central inner conductor are arranged correspondingly along the radial direction of the quartz tube.

[0011] The cylindrical inner conductor works in conjunction with the central inner conductor to better regulate the intensity and distribution of the induced electric field in the internal region of the quartz tube, and the diameter of the cylindrical resonant cavity can be designed to be smaller based on the perturbation principle.

[0012] Preferably, the microwave plasma jet device of this application further includes a cavity annular sidewall, one end of which is integrally formed with the second sidewall, and the other end of which is connected to the first sidewall. The first sidewall, the second sidewall, and the cavity annular sidewall form the cylindrical resonant cavity.

[0013] Preferably, the straight coil is spaced apart from the inner conductor of the cylinder.

[0014] Preferably, the outer wall of the cylindrical inner conductor has a rounded corner at the end near the second side cavity wall. This not only enhances the electric field strength inside the quartz tube but also adjusts the electric field distribution, making the electric field strength distribution more uniform.

[0015] The second side cavity wall includes a bottom side wall and a cavity annular side wall. The cylindrical resonant cavity is located between the first side cavity wall and the bottom side wall. One end of the cavity annular side wall is integrally formed with the bottom side wall, and the other end of the cavity annular side wall is connected to the first side cavity wall. The first side cavity wall, the bottom side wall, and the cavity annular side wall form the cylindrical resonant cavity.

[0016] The first and second side cavity walls are conductive structures.

[0017] Secondly, this application also discloses a microwave plasma jet system, including the microwave plasma jet device described in this application, and further including a microwave power supply and an N-type connector. The N-type connector is insulated from the first side cavity wall, the microwave power supply is connected to the N-type connector via a coaxial cable, and the N-type connector is connected to the end of the straight coil away from the second side cavity wall.

[0018] The microwave plasma jet system described in this application uses a microwave power supply connected to the N-type connector via a coaxial cable. The use of a coaxial cable as a transmission line ensures the flexibility of the microwave plasma jet device described in this application and further expands the applicable scenarios of the microwave plasma jet system described in this application.

[0019] Preferably, a first inlet and a second inlet are provided through the first side cavity wall, and both the first inlet and the second inlet are connected to the cylindrical resonant cavity.

[0020] Preferably, the resonant structure is provided with a second inlet for the second gas to enter, and the second inlet is connected to the cylindrical resonant cavity.

[0021] Preferably, the microwave plasma jet system described in this application further includes a gas source, the output end of which has an output gas path.

[0022] Preferably, the output gas path is connected to the first inlet via a working gas path, and the output gas path is connected to the second inlet via a cooling gas path. A first mass flow meter is provided on the working gas path to control the gas flow rate through the working gas path, and a second mass flow meter is provided on the cooling gas path to control the gas flow rate through the cooling gas path.

[0023] Thirdly, this application also discloses a design method for designing the microwave plasma jet device described in this application, the design method comprising the following steps: S1: Propose the structure of the microwave plasma jet device; S2: Determine the basic dimensions of the cylindrical resonant cavity by calculating its natural frequency and optimize the tuning capability of the microwave plasma jet device; S3: Optimize the structure and dimensions of the cylindrical inner conductor and the central inner conductor by calculating the electric field intensity and distribution before the excitation of the microwave plasma jet device; S4: Estimate the microwave energy efficiency by calculating the microwave reflection after excitation, and optimize the structural dimensions of the microwave plasma jet device based on the microwave energy efficiency.

[0024] The design method described in this application is used to design a microwave plasma jet device, which optimizes the tuning capability of the microwave plasma jet device, the structure and dimensions of the cylindrical inner conductor and the central inner conductor, and the structural dimensions of the microwave plasma jet device, thereby enabling the designed microwave plasma jet device to achieve a certain electric field strength [≥10]. 6 The effective excitation area of ​​the device described above is larger, and the electric field intensity distribution gradient in this area is smaller. Compared with the prior art, under the same input power, the microwave plasma jet device described in this application has a wider distribution characteristic and lower amplitude concentration in the electric field spatial domain. Its volume can be designed to be smaller, the microwave energy efficiency is higher, the jet temperature is lower, and the adjustable range is larger. It can be applied to applications that require a larger processing area, thus making the microwave plasma jet device described in this application applicable to a wider range of scenarios.

[0025] Preferably, in the calculation of microwave energy efficiency, a microwave absorber is added to the discharge space, the microwave absorber being used to simulate the continuous energy consumption under plasma excitation conditions in the hollow cavity of the quartz tube.

[0026] Currently, in the electromagnetic field calculations of microwave plasma jet devices, static field strength calculations do not consider the load effect generated after plasma excitation. However, in actual plasma excitation, the hollow cavity of the quartz tube continuously consumes energy due to energy absorption. Dynamic field strength calculations are difficult to model and require high computing power. Existing models have limited simulation scale and cannot meet the design requirements of large-diameter plasma excitation devices. Therefore, the method described in this application… establishes a microwave absorber in the hollow cavity of the quartz tube during design calculations to simulate the load consumption of plasma. The electromagnetic field strength and distribution obtained through static field strength calculations are closer to the actual continuous operating conditions of the microwave plasma jet device.

[0027] Preferably, the size of the microwave absorber depends on whether it can absorb the microwaves that reach the discharge space, and the boundary condition of the absorber model is a scattering boundary.

[0028] Compared with the prior art, the present invention has the following beneficial effects: This application describes a microwave plasma jet device with a resonant structure. It includes a cylindrical resonant cavity that induces electromagnetic field resonance within the cavity, concentrating microwave energy within a quartz tube. This energy is then ionized by a strong electric field. A central inner conductor induces an electric field within the quartz tube, regulating the intensity and distribution of this induced electric field and increasing its strength. The core objective of this microwave plasma jet device is to achieve a certain electric field strength [≥10]. 6 The effective excitation area of ​​the above-mentioned device is larger, and the electric field intensity distribution gradient in this area is smaller. Compared with the prior art, under the same input power, the microwave plasma jet device described in this application has a wider distribution characteristic and lower amplitude concentration in the electric field spatial domain. Its volume can be designed to be smaller, the microwave energy efficiency is higher, the jet temperature is lower, and the adjustable range is larger. Attached Figure Description

[0029] Figure 1 This is a longitudinal sectional view of the structure of a microwave plasma jet device according to this application.

[0030] Figure 2 This is a schematic diagram of the structure of a microwave plasma jet system according to this application.

[0031] Figure 3 This is a schematic diagram of the design process for a microwave plasma jet device according to this application.

[0032] Figure 4 This is a schematic diagram of the basic structure of a microwave plasma jet device in a design method for a microwave plasma jet device according to this application.

[0033] Figure 5 This is a schematic diagram illustrating the relationship between coil depth and microwave energy efficiency in a design method for a microwave plasma jet device according to this application.

[0034] Figure 6 This is a schematic diagram illustrating the relationship between coil wire diameter and microwave energy efficiency in a design method for a microwave plasma jet device according to this application.

[0035] Figure 7 This is a schematic diagram illustrating the relationship between microwave power, microwave energy efficiency, and jet temperature in a design method for a microwave plasma jet device according to this application.

[0036] Figure 8 This is a schematic diagram of the magnetic field intensity distribution in a design method for a microwave plasma jet device according to this application, wherein, Figure 8 a is a front view of the electromagnetic field structure of the resonant cavity; Figure 8 b is a top view of the electromagnetic field structure of the resonant cavity; Figure 8 c. Magnetic field intensity distribution at the intercept line location; Figure 8 The curve showing the change of resonant frequency as a function of the depth of the tuning screw.

[0037] Figure 9 This is a schematic diagram of the electric field distribution in a design method for a microwave plasma jet device according to this application, wherein, Figure 9 a is the electric field distribution cloud map; Figure 9 b is a schematic diagram of the electric field distribution at the location of the intercept line.

[0038] Figure 10 This is a schematic diagram illustrating the microwave energy efficiency calculation in a design method for a microwave plasma jet device according to this application, wherein... Figure 10 a is a schematic diagram of the microwave energy efficiency calculation model; Figure 10 b is a graph showing the change in microwave energy efficiency with coil depth; Figure 10 c is a graph showing the change in microwave energy efficiency as a function of coil wire diameter.

[0039] Figure 11 This is a schematic diagram of a VNA for measuring the resonant frequency of a cavity, as described in this application. Figure 11 a is a physical diagram of the cavity resonant frequency measured by the VNA; Figure 11 b is a graph showing the VNA measurement results.

[0040] Figure 12 This is a schematic diagram of the plasma jet patterns of helium, argon, and nitrogen in this application, wherein, Figure 12 a is a radial view of a typical gas plasma jet; Figure 12 b is an axial view of a typical gas plasma jet; Figure 12 c is a schematic diagram of the gas temperature of a typical gas plasma; Figure 12 d is a schematic diagram of the microwave energy efficiency of a typical gas plasma; Figure 12 e is a schematic diagram of the minimum sustaining power of a typical gas plasma.

[0041] Figure 13 This is a schematic diagram of the experimental results in the design method of this application, wherein, Figure 13 a. Variation of air plasma jet morphology with microwave power; 13b. Curve of microwave energy efficiency as a function of microwave power; Figure 13 c. Curve showing the change in gas temperature with microwave power.

[0042] Figure 14 This describes the microwave energy transfer process of the microwave plasma jet device described in this application. Detailed Implementation

[0043] The present invention will now be described in further detail with reference to specific embodiments. However, this should not be construed as limiting the scope of the present invention to the following embodiments; all technologies implemented based on the content of the present invention fall within the scope of the present invention.

[0044] Unless otherwise specified, the use of terms such as "upper," "lower," "left," "right," "center," "inner," and "outer" to indicate orientation or positional relationships in the description of specific embodiments of the present invention is based on the orientation or positional relationships shown in the accompanying drawings, or the orientation or positional relationship in which the product / equipment / device is typically placed during use. These terms are merely for the purpose of facilitating the description of the present invention or simplifying the description in specific embodiments, enabling those skilled in the art to quickly understand the solution, and do not indicate or imply that a particular device / component / element must have a specific orientation, or be constructed and operated in a specific positional relationship. Therefore, they should not be construed as limitations on the present invention.

[0045] Furthermore, the use of terms such as "horizontal," "vertical," "suspended," and "parallel" does not imply that the corresponding device / component / element must be absolutely horizontal, vertical, suspended, or parallel, but rather that it can be slightly tilted or have a deviation. For example, "horizontal" merely means that its direction is more horizontal relative to "vertical," not that the structure must be completely horizontal, but that it can be slightly tilted. Alternatively, it can be simplified to mean that the corresponding device / component / element, when set in a "horizontal," "vertical," "suspended," or "parallel" direction, can have an error / deviation of ±10% relative to the corresponding direction, more preferably within ±8%, more preferably within ±6%, more preferably within ±5%, and more preferably within ±4%. As long as the corresponding device / component / element is within the error / deviation range, it can still achieve its function in the present invention.

[0046] Furthermore, the use of terms such as "first," "second," and "third" in terminology is merely for distinguishing between identical or similar components and should not be interpreted as emphasizing or implying the relative importance of a particular component.

[0047] Furthermore, in the description of the embodiments of the present invention, "several", "more than", and "a number of" represent at least two. The number can be any number, such as two, three, four, five, six, seven, eight, or nine, and can even exceed nine.

[0048] Furthermore, in the description of the technical solution of this invention, unless otherwise explicitly specified / limited / restricted, the terms "set up," "install," "connect," "link," "provided with," "laid out," and "arranged" should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to common connection methods in the art, such as welding, riveting, bolting, and threaded connections. Such connections can be mechanical, electrical, or communication connections; they can be direct connections or indirect connections through an intermediate medium; and they can refer to the internal communication between two components.

[0049] Example 1 like Figure 1 and 2 As shown, the microwave plasma jet device described in this embodiment includes a resonant structure 5, a quartz tube 9, a central inner conductor 8, and a cylindrical inner conductor 7, wherein: The resonant structure 5 has a cylindrical resonant cavity 15 inside, and the resonant structure 5 has a first side cavity wall 13 and a second side cavity wall 14 arranged opposite to each other.

[0050] One end of the quartz tube 9 extends from the second side cavity wall 14 into the cylindrical resonant cavity 15 and reaches the first side cavity wall 13. The quartz tube 9 has a hollow inner cavity 17, which is separated from the cylindrical resonant cavity 15 by the quartz tube 9. The central inner conductor 8 is located inside the hollow inner cavity 17, and the central inner conductor 8 is located on the side of the quartz tube 9 near the first side cavity wall 13.

[0051] The cylindrical inner conductor 7 is sleeved on the outside of the quartz tube 9, and the cylindrical inner conductor 7 is located inside the cylindrical resonant cavity 15 on the side close to the first side cavity wall 13.

[0052] The microwave power supply 1 is connected to the N-type connector 22 of the cylindrical resonant cavity 15 via a coaxial cable 3. The other end of the N-type connector 22 is connected to one end of the straight coil 6 inside the cylindrical resonant cavity 15, and the other end of the straight coil 6 is connected to the inner wall of the cylindrical resonant cavity 15.

[0053] In a preferred embodiment, the N-type connector 22 is mounted on the first side cavity wall 13, and the other end of the straight coil 6 is connected to the second side cavity wall 14.

[0054] The gas source is connected to a mass flow meter via an output gas path 19, and then connected to the cylindrical resonant cavity 15 via the mass flow meter. The gas from the gas source is divided into two paths: a working gas path 10 and a cooling gas path 11. Both the working gas path 10 and the cooling gas path 11 are equipped with mass flow meters, namely a first mass flow meter 20 and a second mass flow meter 4. That is, the first mass flow meter 20 is installed on the working gas path 10 to control the gas flow rate through the working gas path 10, and the second mass flow meter 4 is installed on the cooling gas path 11 to control the gas flow rate through the cooling gas path 11.

[0055] The working air passage 10 is connected to the quartz tube 9 of the cylindrical resonant cavity 15, and the cooling air passage 11 is connected to the internal space of the cylindrical resonant cavity 15.

[0056] Gas source 2 is preferably a gas cylinder. Output gas path 19 is preferably a gas pipe.

[0057] In a preferred embodiment, the resonant structure 5 is provided with a first inlet 16 for the entry of a first gas, and the first inlet 16 is connected to the hollow inner cavity 17.

[0058] In a preferred embodiment, the resonant structure 5 is provided with a second inlet 18 for the entry of a second gas, and the second inlet 18 is connected to the cylindrical resonant cavity 15.

[0059] In a preferred embodiment, both the cylindrical inner conductor 7 and the central inner conductor 8 are fixed to the inner wall of the cylindrical resonant cavity 15 by threaded connection.

[0060] Microwave power supply 1 provides microwave energy.

[0061] Gas cylinders provide the gas supply.

[0062] Coaxial cable 3 transmits microwave energy.

[0063] Mass flow meters control gas flow rate, providing more accurate control.

[0064] The cylindrical resonant cavity 15 can induce electromagnetic field resonance within the cavity, concentrating microwave energy within the quartz tube 9, and then ionizing the gas through a strong electric field. The rotating annular cavity structure formed by the cylindrical resonant cavity 15 and the quartz tube 9 ensures that the resulting strong electric field distribution is also a centrally symmetrical rotating body, resulting in a more uniform electric field distribution.

[0065] The straight coil 6 feeds the microwave energy transmitted from the coaxial cable 3 into the cylindrical resonant cavity 15.

[0066] The cylindrical inner conductor 7 compresses the space of the strong electric field. According to the perturbation principle, the diameter of the cylindrical resonant cavity 15 can be designed to be smaller, and the electric field strength inside the quartz tube 9 can be increased. The outer rounded corner at the bottom of the cylindrical inner conductor 7 can also increase the electric field strength inside the quartz tube 9. The outer rounded corner at the bottom of the cylindrical inner conductor 7, namely rounded corner 21, can not only enhance the electric field strength inside the quartz tube 9, but also adjust the distribution of the electric field, making the electric field strength distribution more uniform.

[0067] The central inner conductor 8 induces an electric field within the quartz tube 9, thereby increasing the electric field strength within the quartz tube 9.

[0068] Quartz tube 9 is the discharge space that contains the ionized gas.

[0069] The function of the working gas path 10 is to deliver the working gas into the quartz tube 9.

[0070] The function of the cooling gas path 11 is to deliver cooling gas into the cylindrical resonant cavity 15.

[0071] The function of cooling gas outlet 12 is to discharge cooling gas to remove heat from the cylindrical resonant cavity 15.

[0072] The straight coil 6 is connected to the cavity in a closed loop to form a magnetic field excitation. The upper end of the straight coil 6 is insulated from the first side cavity wall 13, and the lower end of the straight coil 6 and the far end of the second side cavity wall 14 form a closed loop induced magnetic field.

[0073] The preferred workflow of the microwave plasma jet device described in this application is as follows: Open the gas valve of the gas cylinder, control the gas flow rate in the working gas path 10 and the cooling gas path 11 through the mass flow meter, and then deliver the working gas and the cooling gas to the quartz tube 9 and the cylindrical resonant cavity 15 through the working gas path 10 and the cooling gas path 11 respectively.

[0074] When the microwave power supply 1 is switched on, microwave energy is transmitted through the coaxial cable 3 and then fed into the cylindrical resonant cavity 15 via the straight coil 6. The feeding of microwave energy causes electromagnetic field resonance within the cylindrical resonant cavity 15, with a resonance mode of TM010. Therefore, the microwave energy is concentrated within the quartz tube 9, thereby generating a strong electric field region within the quartz tube 9. The working gas passing through the quartz tube 9 is ionized under the influence of the strong electric field to form plasma, which flows out of the quartz tube 9 outlet as a plasma jet under the action of gas expansion and electric field acceleration.

[0075] In a preferred embodiment: a cylindrical resonant cavity 15 is provided inside the resonant structure 5. The resonant structure 5 has a first side cavity wall 13 and a second side cavity wall 14 arranged opposite to each other. The straight coil 6 inside the cylindrical resonant cavity 15 is insulated from the first side cavity wall 13 and is connected to the far end of the second side cavity wall 14. After the microwave enters the straight coil 6 through the coaxial cable, it first contacts the far end of the resonant structure 5, and then forms a closed loop with the entire cylindrical resonant cavity 15. After the straight coil 6 and the cylindrical resonant cavity 15 form a closed loop, a magnetic field is induced.

[0076] Based on the magnetic field input, a central inner conductor 8 is set in the middle of the quartz tube 9 at the center of the cylindrical resonant cavity 15, which further strengthens and enhances the locally induced electromagnetic field inside the quartz tube 9.

[0077] The purpose of the cylindrical inner conductor 7 is similar to that of the central inner conductor 8, which is to adjust the intensity and distribution of the induced electric field in the internal region of the quartz tube 9. The core purpose of the microwave plasma jet device described in this application is to achieve a certain electric field intensity [≥10]. 6 The effective excitation region of the device described above is larger, and the electric field intensity distribution gradient in this region is smaller. That is, compared with existing technologies, under the same input power, the microwave plasma jet device described in this application has a wider distribution characteristic and lower amplitude concentration in the electric field spatial domain. Existing jet devices are directly fed to the middle of the quartz tube, where the peak electric field is very high, but it diffuses very quickly to the surrounding area, resulting in a large electric field intensity distribution gradient. Therefore, it can only satisfy a certain electric field intensity [≥10]. 6 The effective excitation area of ​​the above is very small.

[0078] The inner conductor 7 of the cylinder can be used to compress a space with a strong electric field.

[0079] The microwave plasma jet device described in this application has a higher energy coupling efficiency for the same size compared to existing technologies. The core of the structure of the microwave plasma jet device described in this application is the control of the electromagnetic field. For example, when all the electrons in the middle section of the quartz tube 9 meet the excitation requirements, the excitation efficiency is much higher when the same working gas enters the hollow inner cavity 17 of the quartz tube 9. However, the temperature inside the hollow inner cavity 17 will not be very high because the electric field strength of the hollow inner cavity 17 is much lower than that of the prior art.

[0080] The following example illustrates this: For instance, if 10 ml of working gas is introduced into the hollow inner cavity 17, 7-8 ml of working gas will be excited. In this case, the excitation efficiency is considered very high. Improving the excitation efficiency allows the microwave plasma jet device of this application to achieve larger diameter jets. Currently, existing products cannot achieve larger diameter jets because they do not meet the 10 ml requirement. 6 The electric field distribution range is small. Even if the diameter of the quartz tube is increased, it is still very difficult to meet the usage requirements. This is because even if the diameter of the quartz tube is made very large, the area inside that can excite the working gas cannot be increased with the diameter of the quartz tube. The jet diameter is still very small. The jet diameter and the area that can excite the working gas must be matched. In the existing jet devices, even if the diameter of the quartz tube is increased, if the area where the working gas can be excited is very small, the subsequent jet will still be very thin, or even worse, the jet may not be emitted at all.

[0081] The lower part of the quartz tube 9 is sealed to the first side cavity wall 13. This design increases the stability of the lower structure of the quartz tube 9. On the other hand, it seals the cylindrical resonant cavity 15, thus avoiding the risk of rapid leakage of cooling gas when the cylindrical resonant cavity 15 is filled with cooling gas.

[0082] Along the length of the quartz tube 9, the central inner conductor 8 and the cylindrical inner conductor 7 are arranged correspondingly with matching heights to jointly adjust the electric or magnetic field. If the lower parts of the central inner conductor 8 and the lower parts of the cylindrical inner conductor 7 are at different heights, the adjustment effect on the magnetic or electric field will be different, which will increase the difficulty of using the device.

[0083] Since electromagnetic fields are unaffected by a medium, the medium here is the quartz tube 9. The purpose of the quartz tube 9 is to separate the gas in the hollow inner cavity 17 from the gas in the cylindrical resonant cavity 15. For example, some inert gas is filled into the cylindrical resonant cavity 15 as a protective gas, and some reactive gas is filled into the hollow inner cavity 17 of the quartz tube 9.

[0084] Meanwhile, existing microwave plasma jet generators also have the following drawbacks: Size and flexibility limitations: Waveguide generators are large and unsuitable for processing complex curved surfaces or applications requiring compact dimensions. While coaxial and microstrip generators are compact, they produce smaller plasma jet diameters, making them unsuitable for applications requiring large processing areas.

[0085] Temperature control issue: When processing substrates with limited heat resistance, the jet temperature must be controlled within a safe range, which existing equipment cannot meet.

[0086] Limitations of design methods: Circuit theory-based design methods are only applicable to simple coaxial structures, while electromagnetic field theory-based design methods, although widely applicable, are time-consuming and rely on empirical data and assumptions about plasma characteristic parameters.

[0087] Microwave energy efficiency calculation problem: Existing design methods have difficulty in accurately assessing the ability of microwave energy to be transferred to the discharge space, which leads to difficulties in the structural optimization process.

[0088] The microwave plasma jet device described in this invention targets emerging applications such as paint removal from entire machines, large-aperture optical processing, and surface texturing of solar cells. The designed device overcomes the shortcomings of existing devices. This patent has the following advantages: (1) The designed microwave plasma jet device is small in size and highly flexible: such as Figure 6 As shown, the use of coaxial cable 3 as the transmission line instead of waveguide ensures the flexibility of the device. The designed cylindrical inner conductor reduces the cavity diameter, resulting in an inner diameter of 64 mm and a cavity height of 30.5 mm for the cylindrical resonant cavity 15, a diameter of 24 mm for the cylindrical inner conductor 7, and a length of 15 mm for the cylindrical inner conductor 7.

[0089] (2) Large jet diameter and high microwave energy efficiency: The designed device can generate an air plasma jet filled with quartz tube 9. When the microwave power is 200W, the jet length exceeds 20mm and the diameter exceeds 9mm. When the microwave power does not exceed 100W, the microwave energy efficiency can exceed 90%.

[0090] (3) Low jet temperature with a wide adjustable range: such as Figure 5 As shown, the rounded transition design of the central inner conductor 8 and the cylindrical inner conductor 7 can increase the electric field strength within the quartz tube 9, thereby reducing the power required to excite the plasma. Since the plasma jet temperature is positively correlated with the power, a lower excitation power can produce a plasma jet with a lower temperature. Within the power range of 70 to 200 W, the plasma nozzle temperature varies linearly from 352°C to 535°C.

[0091] In industrial applications such as large-aperture optical processing, overall machine surface cleaning, and solar cell surface texturing, there is a need for flexible and portable generators to produce large-size, low-temperature plasma jets to meet the demands of efficient manufacturing. This paper designs a cylindrical resonant cavity generator based on a coaxial cable transmission system, fully considering issues such as resonant frequency, electric field strength and distribution, and microwave energy efficiency. Initially, microwaves are transmitted via a coaxial cable and fed into the cavity through a direct-connected coil. Inside the cavity, a cylindrical inner conductor compresses the microwaves, enhancing the electric field strength while reducing the cavity diameter. By analyzing the electric field strength and distribution within the resonant cavity, it was found that the outer rounded corners of the cylindrical inner conductor and the addition of a central inner conductor 8 can enhance the electric field strength within the quartz tube 9. By assuming a microwave absorber, different excitation coil structures can be quickly analyzed, and it was calculated that the microwave energy efficiency is proportional to the coil depth and diameter. Finally, an air plasma jet with a diameter exceeding 9 mm was obtained at a microwave power of 200 W. At microwave powers of 100 W or lower, the microwave energy efficiency can exceed 90%. Within a power range of 70 to 200 watts, the plasma nozzle temperature varies almost linearly, from 352°C to 535°C.

[0092] Example 2 This embodiment discloses a design method for designing the microwave plasma jet device described in Embodiment 1. The preferred design process is as follows: Figure 3 As shown, once the basic structure is determined, it needs to be optimized, typically in three steps. First, the basic dimensions are determined and the tuning capability is optimized by calculating the natural frequency. Second, the electric field strength and distribution are analyzed to optimize the inner conductor structure. Next, microwave reflection after excitation is calculated to estimate microwave energy efficiency and optimize the structural dimensions. Finally, experiments are conducted to verify whether the device meets the design specifications. The aforementioned basic structure refers to the selected model and the proposed approximate dimensions; the dimensions obtained through natural frequency calculation are more optimized dimensions.

[0093] like Figure 4 As shown, in the calculation of microwave energy efficiency, a microwave absorber is added to the discharge space. The boundary condition of the microwave absorber is a scattering boundary, which makes the design method described in this embodiment widely applicable, fast in calculation, and independent of empirical data and assumptions about plasma characteristic parameters. Therefore, it can more accurately evaluate the microwave energy transmission capability to the discharge space to optimize the structure. Since the microwave absorber is added to calculate the device's ability to transmit microwaves to the discharge space, the size of the microwave absorber depends on whether it can absorb the microwaves reaching the discharge space. The microwave energy efficiency is calculated based on the port reflection at resonance.

[0094] like Figure 5 and Figure 6As shown, the microwave energy efficiency of different coil sizes was analyzed using the design method of this embodiment. It was found that the microwave energy efficiency gradually increases with increasing coil depth. When the coil depth reaches its maximum, the coil is directly connected to the bottom surface of the cavity, forming a straight coil 6. The straight coil 6 is preferably a straight wire. In this case, by adjusting the wire diameter, it was found that the microwave energy efficiency continues to improve with increasing wire diameter, eventually reaching 99% when the wire diameter is 2mm.

[0095] Currently, in the electromagnetic field calculation of microwave plasma jet devices, the static field strength calculation does not consider the load effect generated after plasma excitation. However, after actual plasma excitation, the hollow inner cavity 17 of the quartz tube 9 continuously consumes energy due to energy absorption. Dynamic field strength calculation modeling is difficult and requires high computing power. Existing models have limited simulation scale and cannot meet the design requirements of large-diameter plasma excitation devices. Therefore, in the design calculation of the microwave plasma jet device described in this application, a microwave absorber is established in the hollow inner cavity 17 of the quartz tube 9 to simulate the load consumption of plasma. The electromagnetic field strength and distribution obtained through static field strength calculation are closer to the actual continuous operating conditions of the microwave plasma jet device.

[0096] The microwave plasma jet device designed using the above method can be applied to emerging applications such as paint removal from entire machines, large-aperture optical processing, and surface texturing of solar cells. The designed microwave plasma jet device overcomes the shortcomings of existing devices, giving it the following advantages: A. The microwave plasma jet device designed using the method described in this embodiment is small in size and highly flexible: replacing the waveguide and using coaxial cable 3 as the transmission line ensures the flexibility of the device. The designed cylindrical inner conductor reduces the cavity diameter. The inner diameter of the cylindrical resonant cavity 15 is preferably 60-70 mm, the cavity height is preferably 28-32 mm, the diameter of the cylindrical inner conductor 7 is preferably 22-26 mm, and the length of the cylindrical inner conductor 7 is preferably 13-17 mm.

[0097] (2) Large jet diameter and high microwave energy efficiency: The microwave plasma jet device designed using the design method described in this embodiment can generate an air plasma jet filled with quartz tube 9. When the microwave power is 200W, the jet length exceeds 20mm and the diameter exceeds 9mm. When the microwave power does not exceed 100W, the microwave energy efficiency can exceed 90%.

[0098] (3) Low jet temperature with a wide adjustable range: such as Figure 7As shown, the arc transition design between the central inner conductor 8 and the cylindrical inner conductor 7 increases the electric field strength within the quartz tube 9, thereby reducing the power required to excite the plasma. Since the plasma jet temperature is positively correlated with power, a lower excitation power can produce a lower-temperature plasma jet. Within a power range of 70 to 200 W, the plasma nozzle temperature varies linearly from 352°C to 535°C.

[0099] In industrial applications such as large-aperture optical processing, overall machine surface cleaning, and solar cell surface texturing, there is a need for flexible and portable generators to produce large-size, low-temperature plasma jets to meet the demands of efficient manufacturing. This paper designs a cylindrical resonant cavity generator based on a coaxial cable transmission system, fully considering issues such as resonant frequency, electric field strength and distribution, and microwave energy efficiency. Initially, microwaves are transmitted via a coaxial cable and fed into the cavity through a direct-connected coil. Inside the cavity, a cylindrical inner conductor compresses the microwaves, enhancing the electric field strength while reducing the cavity diameter. By analyzing the electric field strength and distribution within the resonant cavity, it was found that the outer rounded corners of the cylindrical inner conductor and the addition of a central inner conductor 8 can enhance the electric field strength within the quartz tube 9. By assuming a microwave absorber, different excitation coil structures can be quickly analyzed, and it was calculated that the microwave energy efficiency is proportional to the coil depth and diameter. Finally, an air plasma jet with a diameter exceeding 9 mm was obtained at a microwave power of 200 W. At microwave powers of 100 W or lower, the microwave energy efficiency can exceed 90%. Within a power range of 70 to 200 watts, the plasma nozzle temperature varies almost linearly, from 352°C to 535°C.

[0100] Existing waveguide-type plasma jet generators typically require microwave power exceeding 1 kW to excite the plasma. Furthermore, the jet temperature is very high; even with Ar plasma, the jet temperature is usually above 2000℃. They are also very large in volume, while the resulting jet size is relatively small.

[0101] The microwave plasma jet device designed using the method described in this embodiment achieves the same jet size but with a smaller volume, and its jet temperature also has a significant advantage over conventional waveguide-type plasma jet generators. The table below shows the jet temperatures of the microwave plasma jet device designed using the method described in this embodiment under different combinations of microwave power, gas flow rate, and gas type:

[0102] As can be seen from the table above, the microwave plasma jet device described in this application has a jet temperature in the range of 200~900℃, depending on the gas type and gas flow rate, which is much lower than the jet temperature of existing waveguide plasma jet generators.

[0103] The microwave plasma jet device described in this application has a lower jet temperature, which means that the required temperature of the heat source (i.e., the temperature requirement of the microwave power supply 1) is also lower, and the temperature of the object being processed is also lower. This makes the microwave plasma jet device described in this application more widely applicable and suitable for more scenarios.

[0104] A preferred embodiment of the design method described in this example is as follows: Microwave plasma is generated and maintained by electromagnetic waves in the microwave frequency band of 300MHz-300GHz, possessing advantages such as high ionization rate and high active particle density. Microwave plasma jet generators can be classified into three types based on the microwave transmission method: waveguide type, coaxial type, and microstrip type. Waveguide type generators typically have a large structural volume and high microwave power capacity, capable of generating thermal plasma jets with diameters exceeding 60mm. They are mainly used in hazardous waste pollution control, material preparation, and substance decomposition. Coaxial type generators have a flexible structure and lower microwave power, capable of generating room-temperature plasma jets with diameters of 1 to 3mm. They are commonly used in material synthesis, sterilization, surface modification, etching, and polishing. Microstrip type generators have a compact structure, are suitable for planar manufacturing, and can generate plasma using mW-level microwave power. The main characteristic of this type is its integrable manufacturing capability, and it is currently mainly used in material modification.

[0105] With the continuous advancement of plasma technology applications in industry, many new uses have emerged, placing new demands on equipment. Examples include whole-machine paint removal, large-aperture optical processing, and solar cell surface texturing. These applications require large processing areas, thus necessitating large plasma jet diameters. Furthermore, when processing complex curved surfaces, the jet device must remain compact to ensure flexibility. Moreover, given the limited thermal tolerance of the substrate, the jet temperature must be controlled within a safe range. Current equipment cannot simultaneously meet all these criteria. Therefore, by integrating the structural features of coaxial and waveguide generators, a large-scale microwave plasma jet device based on coaxial cable transmission needs to be developed. To minimize the temperature of the plasma jet, the electric field strength within the discharge space must be maximized. Optimization can reduce the power required for plasma excitation. To maximize the electric field strength within the discharge space and prevent overheating of the generator's inner walls due to the skin effect, the microwave energy transfer efficiency into the discharge space must be maximized. Therefore, the design of the microwave plasma generator must prioritize two key indicators: maximizing the electric field strength within the discharge space and microwave energy efficiency.

[0106] Typically, the design methods for microwave plasma generators are primarily based on circuit theory and electromagnetic field theory. The core of the circuit theory-based design method lies in achieving impedance matching through input impedance calculation. However, usually only simple coaxial structures can be solved analytically, making this method unsuitable for precise generator design requirements. The electromagnetic field theory-based design method is applicable to all types of microwave plasma generators because it determines the structure and distribution of the electromagnetic field by calculating the propagation and reflection of electromagnetic waves—the most fundamental operating principle in all microwave devices. Numerous microwave plasma generators have been designed based on electromagnetic field theory. For example, Leins et al. designed a self-igniting microwave plasma generator with two resonant modes by calculating the eigenfrequency of the resonant cavity. Liu et al. optimized the conductor structure within the cavity by calculating the electric field strength. Xiao et al. simulated the excitation of argon plasma using a fluid approximation method and calculated the scattering parameter S11. Furthermore, they compared the microwave energy efficiency of tapered and ridged waveguides. However, this design method is extremely time-consuming because it requires multiphysics calculations, and therefore is rarely used for structural optimization in industrial applications. Furthermore, recent studies have optimized cavity structure dimensions by calculating microwave energy efficiency under given plasma characteristic parameters and assuming uniform plasma distribution. This method relies not only on empirical data but also on the assumption of constant plasma characteristic parameters, introducing errors into structural optimization. Moreover, microwave energy efficiency is affected by plasma characteristic parameters and therefore cannot fully reflect the generator's ability to transmit microwaves into the discharge space. It is difficult to determine the source of microwave reflection during generator structure optimization. Therefore, Zhong et al. used the nozzle of a coaxial microwave plasma generator as a waveport to calculate microwave energy efficiency, which reflects the efficiency of microwave energy transmission to the generator nozzle. Even so, this method is not applicable to generators where the nozzle blocks electromagnetic waves. Therefore, microwave energy efficiency is calculated by assuming the plasma is a complete microwave absorber. By avoiding calculations on complex plasma models, the ability to transmit microwave energy into the discharge space can be quickly assessed. Optimizing structural dimensions based on this method allows the plasma jet generator to meet design requirements.

[0107] In this embodiment, a cylindrical resonant cavity generator was designed based on a coaxial cable transmission system, taking into full account issues such as resonant frequency, electric field strength and distribution, and microwave energy efficiency. Microwaves are transmitted via the coaxial cable and fed into the cavity through a direct-connected coil. Inside the cavity, a cylindrical inner conductor compresses the microwaves, enhancing the electric field while reducing the cavity diameter. The inner conductor structure was optimized by calculating the electric field strength distribution. When calculating the microwave reflectivity, it was assumed that the plasma was a complete microwave absorber, thus enabling rapid estimation of microwave energy efficiency to optimize the size of the excitation coil. Finally, an air plasma jet with a diameter exceeding 9 mm was obtained at a microwave power of 200 W. The plasma discharge characteristics were studied using a camera and thermometer, confirming the temperature and excitation characteristics of the large-size plasma jet.

[0108] Structural design of microwave plasma generator: The basic structure of microwave plasma jet system and generator is as follows Figure 2 As shown. The generator is driven by a 2.45 GHz solid-state microwave source with an output power range of 0–300 W. Gas is supplied via a gas cylinder or pump and precisely controlled by a mass flow meter. The system is designed with two gas channels, one for the working gas and the other for the cooling gas. The cooling gas passes through the cavity, carrying away heat dissipated from the outer wall of the quartz tube 9. When the working gas passes through the resonant cavity, it is excited by a strong electric field, generating plasma within the quartz tube 9. The microwave energy transfer process is as follows: Figure 2 As shown. The power supply transmits microwave energy to the resonant cavity via a transmission line, and the resonant cavity couples the microwave energy to the working gas. The propagation of microwaves between individual components is an excitation and coupling process. Therefore, impedance matching and microwave transmission efficiency need to be considered. However, the commercial power supply [CHN, WattsineWSPS-2450-300-MCFA] used in this paper has integrated an impedance matcher, so there is no need to consider the transmission process between the power supply and the transmission line. To ensure the flexibility of equipment operation and minimize power loss in the transmission line, a coaxial transmission line [RG401] with a power capacity of 650W and an operating frequency of 2.6GHz was selected. The inner conductor of the coaxial cable 3 extends into the cavity and is connected to the inner wall of the cavity to form an excitation coil. This excitation method avoids positional conflicts with the quartz tube 9 because coil excitation can achieve high microwave coupling efficiency without causing the excitation element to overlap with the strong electric field region. The dimensions of the resonant cavity and the excitation element were designed according to the transverse magnetic mode [TM010], which makes the strong electric field energy more concentrated in the discharge quartz tube 9 region. The diameter of a cylindrical resonant cavity can be calculated using the following formula: (1) In the formula, f r Represents the resonant frequency; D Represents the diameter of the cavity; μ Represents the magnetic permeability of the medium; ɛ The dielectric constant of the medium is represented by the resonant frequency. It can be seen that the resonant frequency is inversely proportional to the cavity diameter. At an operating frequency of 2.45 GHz, the cavity diameter required to ensure resonance can be calculated to be 93.7 mm. Such a cavity diameter is too large and cannot meet the flexibility requirements. However, reducing the cavity diameter will increase the cavity's resonant frequency, causing it to be inconsistent with the power supply frequency. According to perturbation theory, the cavity's resonant frequency is related to the volume change of the strong electric field region, as shown in the following formula: (2) In the formula, ω This represents the resonant angular frequency after perturbation; ω r This represents the resonant angular frequency before the perturbation; This represents the average energy density before the perturbation; Represents the magnetic energy density before the perturbation; △ v For volume change; This represents the total average electromagnetic energy after the perturbation. In the region of strong electric fields, Because when compressing a region with a strong electric field, Δ v <0, therefore ω < ω r This means the resonant frequency decreases. This indicates that the change in resonant frequency due to the reduced cavity diameter can be compensated for by inserting a metal conductor to compress the strong electric field region. Therefore, a cylindrical inner conductor was added inside the cavity to reduce its diameter. Furthermore, this structure compresses the propagation space of electromagnetic waves, thereby enhancing the electric field strength. Additionally, an inner conductor was placed at the center of the quartz tube 9 to utilize the tip effect, which reduces the difficulty of plasma excitation and allows the plasma to be excited at the center of the quartz tube 9 rather than at the tube wall.

[0109] Natural Frequency Analysis: One of the most critical aspects of resonant cavity design is ensuring that the operating frequency matches the cavity's resonant frequency. Therefore, the cavity dimensions and internal components must be designed based on the resonant frequency. While analytical formulas can be used to calculate the resonant frequency and diameter of a cylindrical resonant cavity, the influence of internal components such as the excitation element, inner conductor, and quartz tube on the resonant frequency is difficult to assess. Therefore, numerical calculations based on waveguide equations are necessary to address this issue.

[0110]

[0111]

[0112] The above equation is the wave equation in complex vector form. Substituting the boundary conditions n·E=0 and n·H=0 of the electromagnetic field on an ideal conductor into the wave equation and solving it, we can obtain a series of eigenvalues ​​K. Different K values ​​correspond to different resonance modes and resonance frequencies.

[0113] All numerical calculation results in this embodiment were obtained using COMSOL based on the finite element algorithm. In the resonant frequency analysis, the calculation model did not consider the excitation element. Since electromagnetic waves can escape from the model boundary (wave port) of the excitation element, the accurate calculation of the resonant mode is affected. However, the influence of the excitation element on the resonant frequency cannot be ignored. Therefore, a certain margin must be left when setting the target resonant frequency to compensate for the influence of the excitation element. Since inserting the excitation coil into the cavity effectively reduces the equivalent diameter of the cavity, the resonant frequency will increase according to formula (1). Therefore, the target resonant frequency is preferably set to 2.43 GHz. The frequency analysis results of the resonant cavity are as follows: Figure 3 As shown. Based on the set target resonant frequency, the inner diameter of the cavity is preferably 64 mm, the height of the cavity is preferably 30.5 mm, the diameter of the cylindrical inner conductor is preferably 24 mm, and the length of the cylindrical inner conductor is preferably 15 mm. Below the cylindrical inner conductor, the electric field is distributed along the axial direction, and the magnetic field is distributed along the circumference, which is a typical TM010 mode. Figure 8 The magnetic field strength distribution at the location of the gray dashed line in a is as follows: Figure 8 As shown in Figure c, the magnetic field is strongest at a distance of 20 mm from the center. To maximize excitation efficiency, the distance between the excitation coil and the center should be less than 20 mm to ensure that the coil surrounds the region with the strongest magnetic field. Due to manufacturing errors and model simplification, there will always be a difference between the calculated resonant frequency and the actual resonant frequency; therefore, a tuning element is essential. Based on perturbation theory, screw tuning is used within the cavity, and its tuning capability is as follows: Figure 8 As shown in Figure d, when the screw depth is between 1 and 15 mm, the screw's frequency adjustment capability is only 20 MHz. When the screw depth is between 15 and 20 mm, the screw's frequency adjustment capability can reach 89 MHz. This is because when the screw depth exceeds the length of the cylindrical inner conductor, the screw enters a region with higher electromagnetic field energy, making the change in cavity volume have a more significant impact on the resonant frequency.

[0114] Electric Field Intensity and Distribution: Within the cylindrical resonant cavity 15, the electric field intensity and distribution significantly influence the excitation and maintenance of plasma. Under specific resonant modes, the structural shapes of the inner cylindrical conductor 7 and the central inner conductor 8 are the main factors affecting the electric field characteristics. Therefore, by comparing and analyzing the electric field intensity and distribution of three inner conductor structures, the inner conductor structure within the cavity was optimized. The electric field calculation is still based on the wave equation, where the key parameter K is determined according to the operating frequency and the permeability and permittivity of the medium. In setting the boundary conditions, impedance boundary conditions are adopted for the conductor boundaries, and the conductor surface roughness is considered to be Ra3.2 to simulate actual physical conditions. Simultaneously, the excitation coil is included in the calculation model, and the excitation power of the electromagnetic wave excitation port is set to 100W. The operating frequency is selected in the range of 2.43GHz to 2.46GHz, and frequency sweep calculations are performed to determine the resonant frequency. The calculated results of the electric field intensity and distribution of the resonant cavity with three inner conductor structures before plasma excitation are as follows: Figure 9 As shown, all comparison results were selected at their respective resonant frequencies to ensure accuracy and comparability. The red areas in the contour plot represent electric field strength amplitudes greater than 8 × 10⁵ V / m (higher than the excitation electric field threshold of Ar plasma, 2.7 × 10⁵ V / m), meaning that Ar plasma can be excited in these regions. Type A structure represents the most basic configuration of the conductor within the resonant cavity. Type B structure is a cylindrical inner conductor with rounded outer corners. Type C structure adds a central inner conductor 8 to the Type B structure. From... Figure 9 As can be seen from the electric field cloud diagrams of the three structures in diagram a, the presence of the central inner conductor 8 increases the red and light blue regions inside the quartz tube 9. From... Figure 9 The electric field distribution curves at the dashed line in section b show that, compared to the right-angled cylindrical inner conductor structure (Type A), the cylindrical inner conductor with rounded outer corners causes the electric field distribution to concentrate towards the outer wall of the quartz tube 9, resulting in higher electric field strengths at the outer wall (point B) and inside the quartz tube 9 (point C). For Type C, the electric field distribution further concentrates towards the inside of the quartz tube 9, leading to a decrease in the electric field strength at point B, but reaching a peak at point C. This is because the conductor induces an electric field inside the quartz tube 9, increasing the electric field strength and the region of strong electric field within the quartz tube 9. Since the electromagnetic energy within the resonant cavity is the integral of the square of the electric field amplitude in space, the increase in electric field strength and the region of strong electric field implies an increase in energy. The electric field strengths at points A, B, and C for these three internal conductor structures are shown in Table 1. Compared to Type A, the electric field strength at the center of the quartz tube 9 increases by 10% in Type B. Compared to Type B, the electric field strength at the center of the quartz tube 9 increases by 500% in Type C. Therefore, the outer rounded corners of the cylindrical inner conductor and the addition of a central inner conductor help increase the electric field strength inside the quartz tube 9. A higher electric field strength is beneficial for exciting plasma.

[0115] Table 1 Electric field strength at marked points

[0116] Microwave energy efficiency: The plasma sustaining process is the process of transferring microwave energy to the gas, and the energy transfer capability can be expressed by microwave energy efficiency. Microwave energy efficiency is defined as the ratio of plasma absorbed power to microwave power, which is a key parameter in the design process of microwave plasma generators. This parameter affects the state of the plasma jet and the design of the device. Plasma absorbed power can be calculated from reflected power, and reflected power is also easy to measure. Therefore, microwave energy efficiency is calculated by the following formula: η=1-Pr / Pi(5) In the formula, Pr represents the reflected microwave power, and Pi represents the input microwave power. To quickly calculate microwave energy efficiency and provide guidance for device design, plasma is considered as an object that completely absorbs microwaves. In reality, some microwave energy is reflected by the plasma, and the reflection depends on the plasma characteristics, electromagnetic wave intensity, and frequency. The microwave energy efficiency calculated based on this assumption is not equal to the actual microwave energy efficiency after plasma excitation, but it can indicate whether the designed device effectively propagates microwaves to the discharge region. Therefore, the microwave energy efficiency calculated using this method can serve as a guide for optimizing the structural dimensions of the plasma generator. The geometric model used to calculate microwave energy efficiency is as follows: Figure 10 As shown in Figure a, the microwave absorber is located at the center of the quartz tube 9, with its upper end face adjacent to the central inner conductor 8 and its lower end face flush with the nozzle, having a diameter of 2 mm. The surface of the microwave absorber is set to a first-order scattering boundary condition, while the nozzle is set to an ideal magnetic conductor boundary condition to simulate a natural boundary. Since the first-order scattering boundary condition has a reflectivity exceeding 10% for electromagnetic waves with an incident angle greater than 60°, the surface of the microwave absorber should be as perpendicular as possible to the electromagnetic wave propagation direction. This means that the diameter of the microwave absorber depends on whether most of the microwaves entering the quartz tube 9 can be absorbed, rather than the diameter of the plasma. The microwave energy on the transmission line is transmitted to the cavity through the excitation coil; therefore, the structural dimensions and position of the excitation coil are key factors affecting the transmission efficiency. The calculation results for excitation coils with different depths and wire diameters are shown in Figure a. Figure 10 As shown in b and 10c. With the increase of the excitation coil depth, η Gradually increase. When the coil depth exceeds the cavity height, the coil is directly connected to the bottom of the cavity to form a direct-connected coil. In this case, η The efficiency increases continuously with increasing wire diameter. Ultimately, a direct-drive coil with a 2mm wire diameter achieves a microwave energy efficiency of 99.8%. This is because the transmission efficiency of the excitation coil is related to the magnetic flux excited within the coil. According to the definition of magnetic flux: (6) It can be seen that the magnetic flux is related to the area enclosed by the coil and the magnetic induction intensity. BThis is relevant. Therefore, as the coil depth increases, the area enclosed by the coil increases, thus increasing the magnetic flux within the coil and consequently improving energy transfer efficiency. According to Ampere's circuital law, the following formula applies: (7) The integral on the right side of the equation represents the current and the magnetic field strength. H=B / μ This formula indicates the magnetic flux density. B It is directly proportional to the current. Therefore, as the diameter of the coil wire increases, the coil resistance decreases, the current in the coil increases, the magnetic induction intensity increases, and thus the magnetic flux and energy transfer efficiency are improved.

[0117] Resonant Frequency Detection: Although the structural dimensions of the resonant cavity are designed using numerical calculations of the resonant frequency, manufacturing errors, model simplification, and experimental environmental conditions can all affect the actual resonant frequency. Therefore, it is necessary to measure the actual resonant frequency using a vector network analyzer (VNA) and adjust it with tuning elements to match the power supply frequency. The microwave plasma generator is a single-port network device; therefore, it is only necessary to directly connect the VNA to the microwave input port of the cavity, such as... Figure 11 As shown in figure a. The resonant frequency is usually determined by measuring the amplitude-frequency response of the resonant cavity, i.e. S 11 - Obtained from the frequency curve. Due to power reflectivity. P r / P i= S 11 2 ,therefore S 11 Proportional to power reflectivity. Considering the resonant cavity as an equivalent circuit, at resonance, the cavity's equivalent susceptance is 0, and its equivalent impedance is a real number. At this point, the cavity's equivalent impedance is closest to the transmission line's characteristic impedance. Therefore, at resonance... S 11 To reach the minimum value. However, for cavities with high excitation efficiency, the reflectivity of the unexcited resonant cavity is almost 1, making it difficult to pass through. S 11 - The frequency response curve determines the resonant frequency. The phase response describes the phase shift of the reflected wave relative to the incident wave, and its calculation equation is as follows: (8) Where, X ( jω )and Y ( jω) are the frequency domain functions of the input wave and the reflected wave, respectively. At resonance, the phase response is equal to an integer multiple of 2π. Since the range of equation (8) is [-π, π], the phase response will have a discontinuity point every 2π, i.e., the resonance point. The group delay is the derivative of the phase response with respect to the angular frequency; therefore, the maximum value of the group delay corresponds to the resonance frequency. Measured by VNA S 11 - Frequency curve and group delay, such as Figure 11 As shown in b, the yellow curve represents S 11 The purple curve represents group delay. When the tuning screw depth is adjusted to 18mm, S 11 The frequency curve shows only a slight arc, while the group delay curve exhibits a significant peak at 2.45 GHz, indicating that the resonant frequency of the plasma generator has been tuned to 2.45 GHz. The resonant frequency conforming to design specifications confirms that the actual device's structural dimensions are consistent with the design.

[0118] Excitation characteristics of typical elemental gas plasmas: Different gases typically have different discharge plasma excitation characteristics. To clarify the applicability of this generator to various gas types, we studied the plasma jet morphology of three typical gases and analyzed their minimum sustaining power, jet temperature, and microwave energy efficiency.

[0119] Figure 12 The plasma jet patterns of helium, argon, and nitrogen were demonstrated. Figure 12 a and 12b are radial and axial views of the plasma jets, respectively. When the microwave power is set to 180W and the gas flow rate to 10 slm, the helium plasma jet is the shortest, so short that no visible plasma flows from the nozzle. The argon plasma jet exceeds 10 mm but exhibits a filamentary discharge. In contrast, the nitrogen plasma jet exceeds 20 mm in length and exhibits a relatively uniform glow. From the axial view, the helium and nitrogen plasma jets can be divided into a core region and a diffusion region. The core region is smaller and brighter than the diffusion region. The diffusion region exhibits a uniform glow and completely fills the quartz tube 9. In the argon plasma, only a small amount of glow is visible around the discharge current; its dynamics are unstable and extend along the tube wall towards the nozzle. These differences are believed to be primarily related to the atomic mass of the gases. The plasma motion is mainly formed by the superposition of migration and diffusion processes. For the designed generator, the electric field always points towards the axis of the discharge quartz tube 9, which determines the direction of charged particle migration. This means that the radial motion of charged particles is mainly controlled by the diffusion process. The diameter of a plasma jet is determined by the radial motion of charged particles. Therefore, different diffusion rates are the reason for the differences in diameter of the three typical gas plasma jets. (Diffusion rate of charged particles) ,in D a The bipolar diffusion coefficient, n Let be the density of charged particles. Therefore, given the same charged particle density, the diffusion rate depends on the bipolar diffusion coefficient. D a = K ( T i + T e ) / ( Mv (9) In the formula T e For electron temperature, T i The ion temperature. K Boltzmann's constant, v Let be the collision frequency. From the above formula, the theoretical diffusion rate can be deduced. In practical applications, the jet temperature usually needs to be controlled within a fixed range. When the jet temperature is constant, the ion temperature... T i Approaching jet temperature T And electron temperature T e The diffusion rate remains almost constant. Therefore, the diffusion rate depends primarily on the atomic mass of the gas. Given that the relative atomic masses of helium-4 and nitrogen-14 are less than those of argon-40, helium and nitrogen diffuse faster than argon. Consequently, the jet diameters of helium and nitrogen are larger.

[0120] The minimum sustaining power refers to the lowest power required to ensure the plasma state persists without extinguishing. This parameter is crucial for determining the applicable power range of a plasma generator. By adjusting the microwave power while keeping the gas flow rate constant, we can excite the plasma and determine the minimum sustaining power for different gases. Specifically, such as... Figure 12 As shown in Figure c, the minimum sustaining power for helium, argon, and nitrogen, three typical gases, is 50W, 15W, and 180W, respectively. For microwave plasma at atmospheric pressure, the collision frequency between electrons and atoms is much higher than the power frequency (2.45GHz). Therefore, the condition for sustaining the plasma is that the energy gained by electrons from the electric field before each collision must be equal to or greater than the energy lost in each collision. Assume the average electron energy is 0.5... mv 2 Then the average electron energy loss in each elastic collision is ( m 2 v 2 ) / M .in m For electronic quality,M The atomic mass is the energy lost during elastic collisions between electrons and atoms. This means that the energy loss during elastic collisions between electrons and atoms is inversely proportional to the atomic mass. Argon (Ar) has a relative atomic mass of 40, which is greater than that of helium (He) of 4. Therefore, less energy is lost during electron collisions in argon. Consequently, less energy is required to maintain an argon plasma, and the breakdown electric field strength of argon is lower than that of helium. Because the electron collision process in nitrogen (N2) also involves additional energy losses such as vibrational level excitation, rotational level excitation, and molecular decomposition, more energy is required to maintain a nitrogen plasma than a helium plasma. Therefore, nitrogen has the highest minimum sustaining power.

[0121] The temperature of the plasma jet determines its application range; therefore, it is necessary to detect the plasma jet temperature of three typical gases. The plasma jet temperature is detected by placing a thermocouple at the nozzle of the generator. Figure 12 As shown in d, the plasma jet temperatures for helium, argon, and nitrogen are 514°C, 218°C, and 671°C, respectively. The microwave energy efficiency is calculated using the reflected power measured from the power source, according to equation (5). Figure 12 As shown in Figure e, the microwave energy efficiencies of helium, argon, and nitrogen plasmas are 77%, 44%, and 86%, respectively. This means that these three typical gases do not have the same microwave absorption characteristics. Based on the principles of plasma maintenance described earlier, the differences in plasma jet temperature and microwave energy efficiency among these three typical gases are still related to atomic mass and gas type. The energy loss process of electrons during collisions is also a process of electron energy transfer. The greater the energy loss of electrons, the greater the corresponding increase in atomic energy; therefore, the jet temperature of helium plasma is higher than that of argon plasma. In microwave plasma, electrons oscillate within the discharge space. If electrons do not transfer energy through collisions, they cannot continue to obtain energy from the electric field. Therefore, the more energy electrons transfer to atoms, the more microwave energy is absorbed, meaning that helium plasma has a higher microwave energy efficiency. Similarly, in nitrogen plasma, due to additional energy loss during collisions with molecules, electrons transfer more energy to molecules, resulting in a higher jet temperature and microwave energy efficiency than helium and argon plasmas.

[0122] Excitation characteristics of air plasma: N2 plasma jets are the longest and have the highest microwave energy efficiency, making them the ideal carrier gas under suitable temperature conditions. Air, as a readily available gaseous medium primarily composed of N2, has significant economic value in engineering applications. Therefore, air was chosen as the working gas to further evaluate the performance of the plasma jet generator. The experimental design included adjusting the power and then characterizing the jet morphology, temperature, and microwave energy efficiency. Detailed experimental parameters are listed in Table 2.

[0123] Table 2 Experimental parameters

[0124] Experimental results are as follows Figure 13 As shown, the jet length and diameter gradually increase with increasing microwave power. At 200W microwave power, the jet length exceeds 20mm and the jet diameter exceeds 9mm. The state of the air plasma jet is similar to that of the nitrogen plasma jet, mainly because nitrogen is the main component of air. However, the sustaining power of the air plasma jet is only 70W, which can be attributed to the small amount of argon in the air. Once excited, the argon helps reduce the power required to maintain the plasma state through the Penning ionization process. At the same time, the microwave energy efficiency gradually decreases. When the microwave power is below 100W, the microwave energy efficiency can exceed 90%. When the microwave power reaches 200W, the microwave energy efficiency has dropped to 78%. Although the microwave plasma generator can only achieve efficient microwave energy conversion within a specific power range, its microwave energy efficiency is still at an acceptable level in the power range below 200W. In the power range of 70 to 200W, the temperature at the plasma nozzle changes almost linearly, from 352℃ to 535℃. Because the power required to maintain the plasma is relatively low, the jet temperature of air plasma has a wider adjustment range, which can better meet the needs of different application scenarios.

[0125] In summary, to address emerging applications such as paint stripping of entire machines, large-aperture optical processing, and surface texturing of solar cells, we designed a microwave plasma jet device with a large jet size, low temperature, and relatively small generator volume. The basic structural dimensions of the generator were determined through natural frequency analysis. The inner conductor structure was optimized by calculating the electric field intensity distribution. Specifically, the microwave energy efficiency was rapidly estimated by treating the plasma as a complete microwave absorber. Finally, the generator's adaptability to different gases was analyzed by comparing it with three typical gas plasmas. Air was chosen as the working gas to further evaluate the generator's performance. The specific conclusions are as follows: Based on natural frequency analysis, at the target resonant frequency, the calculated inner diameter of the cavity is 64 mm, the cavity height is 30.5 mm, the diameter of the cylindrical inner conductor is 24 mm, and the length of the cylindrical inner conductor is 15 mm. Furthermore, when the tuning screw depth is between 1 and 20 mm, the resonant frequency can be adjusted to 109 MHz.

[0126] By analyzing the electric field intensity and distribution inside the resonant cavity, it was found that the rounded corners outside the cylindrical inner conductor and the addition of the central inner conductor 8 can enhance the electric field intensity inside the quartz tube 9.

[0127] By assuming a microwave absorber, different excitation coil structures can be quickly analyzed, and the calculated microwave energy efficiency is proportional to the coil depth and wire diameter.

[0128] Under the same discharge parameters, the diameter of the plasma jet depends on the type of gas. When using molecular gases, the plasma generator exhibits an ideal jet state.

[0129] The designed device can generate an air plasma jet filling a quartz tube 9. At a microwave power of 200W, the jet length exceeds 20mm and the diameter exceeds 9mm. At microwave powers of 100W and below, the microwave energy efficiency can exceed 90%. In the power range of 70 to 200W, the plasma nozzle temperature changes almost linearly from 352℃ to 535℃.

[0130] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A microwave plasma jet device, characterized in that, include: The resonant structure (5) has a cylindrical resonant cavity (15) inside. The resonant structure (5) includes a first side cavity wall (13) and a second side cavity wall (14), which together form the cylindrical resonant cavity (15). A quartz tube (9) extends from one end of the second side cavity wall (14) into the cylindrical resonant cavity (15) and up to the first side cavity wall (13). The quartz tube (9) has a hollow inner cavity (17), which is separated from the cylindrical resonant cavity (15) by the quartz tube (9). A straight coil (6) is provided inside the cylindrical resonant cavity (15). The straight coil (6) is spaced apart from the quartz tube (9). One end of the straight coil (6) is insulated from the first side cavity wall (13), and the other end of the straight coil (6) is connected to the second side cavity wall (14). The central inner conductor (8) is located inside the hollow inner cavity (17), and the central inner conductor (8) is located inside the quartz tube (9) on the side close to the first side cavity wall (13).

2. The microwave plasma jet device according to claim 1, characterized in that, A cylindrical inner conductor (7) is also provided inside the cylindrical resonant cavity (15). The cylindrical inner conductor (7) is sleeved on the outside of the quartz tube (9). The cylindrical inner conductor (7) is located inside the cylindrical resonant cavity (15) on the side close to the first side cavity wall (13). Along the radial direction of the quartz tube (9), the cylindrical inner conductor (7) is arranged correspondingly to the central inner conductor (8).

3. The microwave plasma jet device according to claim 2, characterized in that, The straight coil (6) is spaced apart from the cylindrical inner conductor (7).

4. A microwave plasma jet device according to claim 2, characterized in that, The end of the outer wall of the cylindrical inner conductor (7) near the second side cavity wall (14) is provided with a rounded corner (21).

5. A microwave plasma jet device according to any one of claims 1-4, characterized in that, The second side cavity wall (14) includes a bottom side wall (24) and a cavity annular side wall (23). The cylindrical resonant cavity (15) is located between the first side cavity wall (13) and the bottom side wall (24). One end of the cavity annular side wall (23) is integrally formed with the bottom side wall (24), and the other end of the cavity annular side wall (23) is connected to the first side cavity wall (13). The first side cavity wall (13), the bottom side wall (24) and the cavity annular side wall (23) form the cylindrical resonant cavity (15). One end of the straight coil (6) is connected to the bottom side wall (24).

6. A microwave plasma jet system, characterized in that, The device includes a microwave plasma jet apparatus as described in any one of claims 1-5, and further includes a microwave power supply (1) and an N-type connector (22), wherein the N-type connector (22) is insulated from the first side cavity wall (13), the microwave power supply (1) is connected to the N-type connector (22) via a coaxial cable (3), and the N-type connector (22) is connected to the end of the straight coil (6) away from the second side cavity wall (14).

7. A microwave plasma jet system according to claim 6, characterized in that, It also includes a gas source (2), the output end of which has an output gas path (19); A first inlet (16) and a second inlet (18) are provided through the first side cavity wall (13), and both the first inlet (16) and the second inlet (18) are connected to the cylindrical resonant cavity (15); The output gas path (19) is connected to the first inlet (16) through the working gas path (10), and the output gas path (19) is connected to the second inlet (18) through the cooling gas path (11). A first mass flow meter (20) is provided on the working gas path (10), and the first mass flow meter (20) is used to control the gas flow rate through the working gas path (10). A second mass flow meter (4) is provided on the cooling gas path (11), and the second mass flow meter (4) is used to control the gas flow rate through the cooling gas path (11).

8. A design method, characterized in that, The method for designing a microwave plasma jet device according to any one of claims 2-5 includes the following steps: S1: Propose the structure of the microwave plasma jet device; S2: The size of the cylindrical resonant cavity (15) is determined by calculating the natural frequency of the cylindrical resonant cavity (15), and the tuning capability of the microwave plasma jet device is optimized. S3: Optimize the structure and size of the cylindrical inner conductor (7) and the central inner conductor (8) by calculating the electric field intensity and distribution before the excitation of the microwave plasma jet device; S4: Estimate the microwave energy efficiency by calculating the microwave reflection after excitation, and optimize the structural dimensions of the microwave plasma jet device based on the microwave energy efficiency.

9. The design method according to claim 8, characterized in that, In the calculation of microwave energy efficiency, a microwave absorber is added to the discharge space to simulate the continuous energy consumption under plasma excitation conditions in the hollow cavity (17) of the quartz tube (9).

10. A design method according to claim 9, characterized in that, The size of the microwave absorber depends on whether it can absorb the microwaves that reach the discharge space, and the boundary condition of the absorber model is a scattering boundary.

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