Multi-frequency microwave excited plasma electrothermal thruster

Abstract

The application provides a multi-frequency microwave excited plasma electric heat thruster. It aims to solve the problem that the microwave cannot be effectively absorbed after the critical plasma electron density under the excitation of a single frequency microwave source. The thruster adopts a multi-port excitation system and a frequency band separation design, and is excited by a discontinuous microwave power source with different duty cycles, so that the output can be flexibly adjusted, the plasma density and uniformity in the time dimension of the discharge process can be accurately controlled, the waveguide system adopts a waveguide slot antenna to form a waveguide slot array, the energy distribution is adjusted by accurately controlling the slot position, inclination and size, and the microwave energy in the spatial dimension of the thruster is finely adjusted. The design effectively breaks through the microwave absorption efficiency bottleneck of single frequency excitation, reduces the interference between microwave sources, improves the coupling efficiency, ensures the effective transmission of microwave energy, further improves the plasma density and uniformity, and optimizes the overall performance of the thruster.

Description

Technical Field

[0001] This invention relates to the field of aerospace electric propulsion technology, specifically to a multi-frequency microwave-excited plasma electrothermal thruster. More particularly, it relates to a dual-frequency microwave-excited plasma electrothermal thruster. Background Technology

[0002] In electrothermal propulsion systems, electrical energy is used to heat the propellant and convert it into thermal energy, causing the propellant to expand in the thrust chamber. The thermal energy is then converted into directional kinetic energy by the thermally neutral or ionized gas discharged through nozzles, thus generating thrust. Common electrothermal thrusters include resistance type, arc type, and microwave type, each with its own advantages and disadvantages: resistance heating is limited by the heating wire material, its operating temperature cannot exceed the temperature of the heating wire itself, and its heat transfer and conversion efficiency is low, resulting in poor thruster performance; arc thrusters have stronger performance, but their lifespan is shorter due to the characteristics of electrode discharge.

[0003] A microwave electro-thermal thruster (MET) is an electric propulsion device that deposits microwave energy into a cylindrical resonant cavity. Employing an electrodeless discharge mode and high-energy-density heating method, it outperforms resistance-heated thrusters and has a longer service life than arc-heated thrusters. Its working principle is as follows: plasma is generated within the resonant cavity through electrodeless microwave discharge, heating the feedstock and increasing the cavity's operating pressure. Finally, the working fluid is expelled into a vacuum environment through a convergent-divergent nozzle to generate thrust. Compared to the widely used ion thrusters and Hall thrusters, the MET also boasts a higher thrust-to-power ratio (100 mN / kW) and a wider range of adaptable working fluids, including argon, helium, nitrogen, and air.

[0004] Previously, due to the low specific impulse of electrothermal thrusters, the application and research scope of MET (Mechanical Electric Thruster) was limited compared to other electromagnetic electric propulsion devices. However, with the new demands for high-thrust electric propulsion technology from new space missions such as large satellite space orbital maneuvers and on-orbit servicing platforms, MET systems have regained industry attention. At the same time, in the field of very low Earth orbit air-breathing technology, MET has also shown good development potential due to its ability to discharge in an air environment and its high thrust-to-power ratio.

[0005] Patent document CN112333911B discloses a microwave power source and plasma generation system for a dual-frequency driven plasma generator. It discloses that during the excitation and ignition phase of the plasma generator, a first microwave signal generator operates; during the transition from the excitation and ignition phase to the maintenance phase of the plasma generator, the first microwave signal generator and the second microwave signal generator operate simultaneously; and during the maintenance phase of the plasma generator, at least the second microwave signal generator operates.

[0006] Although the technical solution of patent document CN112333911B discloses a method for controlling microwave power sources of different frequencies to complete the transition from excitation and ignition to stable maintenance of an ion generator, it does not propose a specific structure for a multi-frequency microwave-excited plasma electrothermal thruster; it fails to propose the use of discontinuous wave power sources with different duty cycles to achieve precise time-dimensional control of the plasma transient process; nor does it propose the use of a waveguide slot antenna structure to replace a single rectangular waveguide to achieve precise control of the microwave energy distribution within the resonant cavity.

[0007] In summary, microwave electrothermal thrusters face several core technical challenges in current engineering applications, for which existing technologies lack effective solutions. These challenges are as follows:

[0008] 1. During the ignition and steady-state operation phases of the MET (Mechanical Thruster), the plasma density varies, and the corresponding cutoff frequencies also differ. However, existing microwave sources feed in microwaves at a single frequency, which cannot achieve efficient absorption in plasma environments with different densities, directly limiting the improvement of the overall thruster efficiency.

[0009] 2. Currently, the microwave sources used in MET are all in steady-state operation mode, which cannot provide precise time-dimensional control over the transient processes of plasma. During the switching between ignition and steady-state maintenance, the microwave frequency needs to be adjusted to match changes in plasma load. However, the adjustment flexibility of steady-state microwave sources is insufficient, which can easily lead to plasma instability or even interruption.

[0010] 3. Existing MET uses a single rectangular waveguide to transmit microwaves, which makes it difficult to achieve flexible control in the spatial dimension. This results in the distribution and transmission of microwave energy not being able to be adjusted as needed, limiting the uniformity and stability of plasma in the spatial dimension, and thus affecting the overall performance of the system. Summary of the Invention

[0011] To address the shortcomings of existing technologies, the purpose of this invention is to provide a multi-frequency microwave-excited plasma electrothermal thruster.

[0012] A multi-frequency microwave-excited plasma electrothermal thruster according to the present invention includes: a resonant cavity, multiple microwave sources, and a waveguide system; The resonant cavity is provided with multiple independent microwave input ports, and the waveguide system includes multiple independent waveguide subsystems. The multiple microwave sources correspond one-to-one with the multiple microwave input ports and waveguide subsystems. Different microwave sources output microwave signals of different frequency bands and are connected to the corresponding microwave input ports through the corresponding waveguide subsystems to feed microwave signals of different frequency bands into the resonant cavity. The resonant cavity is used to receive the energy of the microwave signals and generate plasma.

[0013] Preferably, the microwave source includes a first microwave source and a second microwave source, wherein the frequency of the first microwave source is lower than that of the second microwave source; The resonant cavity has multiple natural frequencies. The frequency of the first microwave source is within a preset range centered on any one of the natural resonant frequencies of the resonant cavity, while the frequency of the second microwave source is outside the preset range centered on any one of the natural resonant frequencies of the resonant cavity.

[0014] Preferably, the width of the preset interval is set to be within ±10% of the inherent resonant frequency.

[0015] Preferably, the microwave output directions of the first microwave source and the second microwave source are perpendicular to each other, the first microwave source and the second microwave source use discontinuous wave power supply as the feeding method, and the duty cycles of the power supply used by the first microwave source and the second microwave source are different.

[0016] Preferably, the output direction of the first microwave source is parallel to the axis of the resonant cavity, and the output direction of the second microwave source is perpendicular to the axis of the resonant cavity, or The output direction of the first microwave source is perpendicular to the axis of the resonant cavity, and the output direction of the second microwave source is parallel to the axis of the resonant cavity.

[0017] Preferably, the waveguide system includes a first waveguide subsystem and a second waveguide subsystem, and the microwave input port includes a first microwave port and a second microwave port; The first microwave source is connected to the first microwave port through the first waveguide subsystem, and the second microwave source is connected to the second microwave port through the second waveguide subsystem.

[0018] Preferably, both the first waveguide subsystem and the second waveguide subsystem adopt a structure combining a rectangular waveguide and a ring waveguide slot antenna to form a waveguide slot array; The characteristic frequency of the slot antenna in the waveguide subsystem is the same as the characteristic frequency of the corresponding microwave source of the waveguide subsystem.

[0019] Preferably, each microwave input port is provided with a corresponding coupler and matching network, and the coupler and matching network have the same frequency as the corresponding microwave input port.

[0020] Preferably, the control logic of the first microwave source and the second microwave source is as follows: during the ignition and start-up phase of the plasma electrothermal thruster, only the first microwave source works; during the transition phase from ignition to steady-state maintenance, both the first and second microwave sources work simultaneously; and during the steady-state maintenance phase, the second microwave source works.

[0021] Preferably, the system further includes a nozzle connected to the resonant cavity, wherein the plasma generated by the resonant cavity flows directly into the nozzle and exits from the nozzle outlet.

[0022] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention adopts a multi-port excitation system and frequency band separation design to adapt to the density differences and corresponding cutoff frequencies of plasma at different working stages, solves the problem of inefficient absorption of single-frequency microwaves, achieves a breakthrough in microwave absorption efficiency, and flexibly controls plasma density and uniformity to improve the overall operating efficiency of the thruster.

[0023] 2. This invention uses discontinuous wave power supplies with different duty cycles as the feeding method to replace the traditional steady-state microwave source. The output parameters of the microwave source can be flexibly adjusted as needed to achieve precise control of the time dimension of the plasma transient process, avoid the phenomenon of plasma instability or interruption during the switching phase, and ensure a smooth connection between the thruster start-up and steady-state operation.

[0024] 3. This invention employs a waveguide slot antenna structure to precisely control the position, tilt angle, and size of each slot and form a waveguide slot array, replacing a single rectangular waveguide. This optimizes the spatial distribution of microwave energy, achieves fine control over the microwave energy distribution within the resonant cavity, improves the uniformity and stability of the plasma in the spatial dimension, and further optimizes system performance. Attached Figure Description

[0025] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings: Figure 1 This is a schematic diagram of the structure of the multi-frequency microwave-excited plasma electrothermal thruster, which is the main feature of this invention.

[0026] Reference numerals: resonant cavity 1, first microwave port 21, second microwave port 22, first microwave source 31, second microwave source 32, first waveguide subsystem 41, second waveguide subsystem 42, nozzle 5. Detailed Implementation

[0027] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several changes and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0028] This embodiment provides a multi-frequency microwave-excited plasma electrothermal thruster. Through a multi-port excitation system and frequency band separation design, it solves the problem of microwave plasma electrothermal thrusters under single-frequency microwave source excitation failing to effectively absorb microwaves after reaching the electron density criticality. Simultaneously, by combining different frequencies, the density and uniformity of the plasma can be flexibly controlled, improving overall efficiency and effectively overcoming the bottleneck of microwave absorption efficiency under single-frequency excitation. The core design of this embodiment lies in achieving efficient adaptation of microwave energy and plasma through multi-dimensional control methods. This ensures a smooth transition between ignition and steady-state operation, optimizes energy utilization efficiency, and adapts to various working fluid characteristics, broadening the application scenarios of the thruster.

[0029] The multi-port excitation system of this embodiment uses a resonant cavity 1 as its core structure. The main function of this component is to receive microwave energy and generate plasma. The resonant cavity 1 has multiple natural frequencies. Two independent microwave input ports, a first microwave port 21 and a second microwave port 22, are configured around the resonant cavity 1. The waveguide system includes multiple independent waveguide subsystems. Multiple microwave sources correspond one-to-one with multiple microwave input ports and waveguide subsystems. In this embodiment, two microwave sources are correspondingly provided: a first microwave source 31 and a second microwave source 32. The two microwave sources output microwave signals of different frequency bands and are connected to the corresponding microwave input ports through the corresponding waveguide subsystems, feeding the microwave signals of different frequency bands into the resonant cavity 1. The first microwave port 21 is used to inject a specific frequency to achieve high-density ionization, while the second microwave port 22 is used to inject a specific frequency to achieve uniformity control. Each microwave input port is equipped with a corresponding coupler and matching network. The coupler and matching network have the same frequency as the corresponding microwave input port. The multi-port excitation system allows for independent optimization of the coupler and matching network of each port, achieving the effect of non-interference and flexible control of each port.

[0030] The first microwave source 31, which powers the first microwave port 21, is a low-frequency microwave source, while the second microwave source 32, which powers the second microwave port 22, is a high-frequency microwave source. The frequency of the first microwave source 31 is lower than that of the second microwave source 32. The low-frequency microwave is used to generate a large-area, low-density initial discharge, which is beneficial to improving the reliability of the thruster's start-up. The high-frequency microwave is used to induce a local high-density discharge after the initial discharge, which is beneficial to increasing the plasma density of the thruster. The local critical plasma density can be increased by one order of magnitude.

[0031] The center frequency of the first microwave source 31 is close to the characteristic frequency corresponding to the critical plasma density at which the microwave electrothermal thruster is excited and ignited. The frequency of the first microwave source 31 is within a preset range centered on any one of the inherent resonant frequencies of the resonant cavity 1, enabling the resonant cavity 1 to generate a strong standing wave mode. Therefore, using the first microwave source 31 as the main amplifier power supply, its low-frequency characteristics help generate a large-area, low-density plasma, which is beneficial for improving the thruster's start-up reliability. The center frequency of the second microwave source 32 is close to the characteristic frequency corresponding to the critical plasma density at which the microwave electrothermal thruster operates in steady state. The frequency of the second microwave source 32 is outside the preset range centered on any one of the inherent resonant frequencies of the resonant cavity 1, using high-frequency microwaves to excite and generate localized high-density plasma. The width of the preset range can be set to within ±10% of the rated point, which is the frequency value corresponding to the microwave port. For example, for the low-frequency inlet center frequency of 2.45 GHz, the preset range can be set to 2.2 GHz to 2.7 GHz. Other frequencies can be determined based on the upper and lower limits of the ±10% range. The reason why the preset range exists is that the rated point is an ideal value. In reality, due to the deviation of the actual device, the actual operating frequency will have a certain deviation, which is generally around ±10%.

[0032] In one optional embodiment, the frequency of the first microwave port 21 is set to 2.45 GHz, and the frequency of the second microwave port 22 is set to be greater than 7.5 GHz. The term "independent optimization" mentioned above means that the couplers and matching networks corresponding to the first microwave port 21 and the second microwave port 22 must be consistent with the frequencies set for the respective ports. The above-mentioned different frequency ports and their corresponding couplers and matching networks are designed to utilize microwaves of different frequency bands to achieve different excitations of the plasma. For example, during the thruster discharge start-up process, a low-frequency port and coupler and matching network (2.45 GHz) are used to initially achieve a plasma density of 10-1. 15 m -3 The magnitude is on the order of magnitude, and after startup, high-frequency ports, couplers, and matching networks (greater than 7.5 GHz) are used to further increase the critical plasma density to enhance the discharge, for example, increasing the plasma density to 10. 16 m -3 Magnitude.

[0033] The critical plasma densities corresponding to different frequencies are shown in the table below.

[0034] Table 1 Microwave Frequency and Corresponding Critical Plasma Density

[0035] As shown in Table 1, under single-frequency microwave source feeding conditions, there is a critical plasma density upper limit. Once the plasma density exceeds the critical value, the microwave frequency falls below the characteristic frequency corresponding to the critical plasma density, and the microwave can no longer interact with the plasma, thus producing a shielding effect. In this embodiment, the frequencies of the first microwave source 31 and the second microwave source 32 are selected based on the different critical plasma densities corresponding to the ignition excitation and maintenance states. Typical frequencies and corresponding critical densities are shown in the table above, thereby overcoming the absorption efficiency bottleneck of a single frequency.

[0036] This dual-frequency design allows two frequencies to operate simultaneously or in a time-sharing manner, thereby further improving the uniformity and density of the plasma without increasing the cavity size. At the same time, it accurately adapts to the density differences and corresponding cutoff frequencies of the plasma in different operating stages (ignition, steady state), enabling rapid and precise switching of the frequency points required to excite and maintain the plasma, and solving the instability problem caused by load changes.

[0037] Both the first microwave source 31 and the second microwave source 32 employ discontinuous wave power supplies as their feeding method, replacing traditional steady-state microwave sources. They do not operate in continuous wave mode, and the duty cycles of the discontinuous wave power supplies used in the first microwave source 31 and the second microwave source 32 are different. This design allows for flexible adjustment of the microwave source output signal according to different experimental requirements, enabling precise control of plasma density and uniformity, optimizing microwave energy absorption efficiency, and simultaneously achieving fine-grained time-dimensional control of plasma transient processes. This avoids plasma instability or interruption during switching phases, ensuring a smooth transition between thruster startup and steady-state operation.

[0038] In this embodiment, the microwave output directions of the first microwave source 31 and the second microwave source 32 are perpendicular to each other, thereby reducing mutual interference between the two microwave sources, improving coupling efficiency, ensuring effective transmission of microwave energy, and reducing energy loss. Two specific arrangement schemes can be adopted: one is that the microwave output direction of the first microwave source 31 is parallel to the axis of the resonant cavity 1, and the microwave output direction of the second microwave source 32 is perpendicular to the axis of the resonant cavity 1; the other is that the output direction of the first microwave source 31 is perpendicular to the axis of the resonant cavity 1, and the output direction of the second microwave source 32 is parallel to the axis of the resonant cavity 1. Both arrangement schemes can achieve the technical effect of mutually perpendicular microwave output directions. The energy feed of the two microwave sources relies on the first waveguide subsystem 41 and the second waveguide subsystem 42 to achieve dedicated path transmission. The waveguide system includes at least two independent waveguide subsystems, specifically the first waveguide subsystem 41 and the second waveguide subsystem 42 in this embodiment. The number of waveguide subsystems corresponds to the number of microwave input ports, and they are connected to different microwave sources and their corresponding microwave input ports. The specific feed path is as follows: the first microwave source 31 feeds microwaves into the cylindrical resonant cavity 1 through the first microwave port 21 via the first waveguide subsystem 41; the second microwave source 32 feeds microwaves into the cylindrical resonant cavity 1 through the second microwave port 22 via the second waveguide subsystem 42.

[0039] The first waveguide subsystem 41 and the second waveguide subsystem 42 are the core components for microwave transmission. Both adopt a structure combining rectangular waveguides and ring waveguide slot antennas to form a waveguide slot array. By precisely controlling the position, tilt angle, and size of each slot, precise control of energy distribution is achieved, thereby enabling fine regulation of microwave energy distribution within the plasma electrothermal thruster. The rectangular and ring waveguides are internal components of their respective waveguide subsystems, which perform microwave transmission and energy regulation functions. The characteristic frequencies corresponding to the slot antennas of the first waveguide subsystem 41 and the second waveguide subsystem 42 are different. The characteristic frequency of the slot antenna used in the first waveguide subsystem 41 is consistent with the characteristic frequency of the first microwave source 31; the characteristic frequency of the slot antenna used in the second waveguide subsystem 42 is consistent with the characteristic frequency of the second microwave source 32.

[0040] The microwave source control logic for the corresponding working stages is as follows: During the ignition stage, only the first microwave source 31 operates; during the transition from the ignition stage to the sustaining stage, both the first microwave source 31 and the second microwave source 32 operate simultaneously; during the sustaining stage, the second microwave source 32 operates, and the first microwave source 31 can choose to continue operating or stop operating according to efficiency requirements. Depending on the critical plasma density required for the ignition and sustaining states in resonant cavity 1, the frequencies of the first microwave source 31 and the second microwave source 32 are adjusted accordingly.

[0041] In this embodiment, a nozzle 5 is configured as a thrust output terminal. The nozzle 5 is connected to the resonant cavity 1. The plasma generated in the resonant cavity 1 flows directly into the nozzle 5 and is ejected from the outlet of the nozzle 5. The nozzle 5 ejects the high-temperature plasma at high speed into the vacuum environment. The reverse thrust is generated through the directional ejection of the plasma, providing power support for the spacecraft's orbital maneuvers, attitude adjustments and other actions.

[0042] plasma cutoff frequency f p The relationship with plasma density is as follows:

[0043] in, n e It is electron density; m e It is electronic quality; It is the vacuum permittivity; e is the electron charge, or elementary charge, 1.6 × 10⁻⁶. -19 C; If the frequency of the incident microwave f m Below the plasma cutoff frequency f p Electromagnetic waves cannot propagate in plasma; they will be strongly reflected or drastically attenuated (cut off). For example, when an aircraft re-enters the atmosphere at high speed, the surrounding gas is rapidly compressed and heated, creating a layer of plasma on the aircraft's surface. Because the frequency of the incident microwaves at this time... f m Below the plasma cutoff frequency f p Plasma blocks the transmission of communication signals such as microwaves, preventing microwaves from transmitting between the aircraft and the ground, resulting in the blackout effect.

[0044] When microwaves interact with plasma, the microwave frequency must be higher than the plasma cutoff frequency. When the frequency of the incident microwave... f m Higher than plasma cutoff frequency f p At this time, electromagnetic waves can penetrate plasma and propagate, and the higher the frequency, the stronger the penetration ability.

[0045] This embodiment achieves multiple technical effects through a combination of multi-port excitation, frequency band separation design, discontinuous wave feeding, waveguide slot array modulation, and microwave source position optimization: First, it solves the problem of inefficient absorption of single-frequency microwaves, improving the overall operating efficiency of the thruster; second, it ensures stable plasma operation, avoids interruptions, and extends the service life of the equipment; third, it is compatible with the characteristics of various working fluids such as argon, helium, nitrogen, and air, ensuring uniform and efficient microwave energy supply when heating different working fluids, further expanding the working fluid adaptation scenarios and application scope of MET.

[0046] In the description of this application, it should be understood that the terms "upper", "lower", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application.

[0047] Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. Unless otherwise specified, the embodiments and features described in this application can be arbitrarily combined with each other.

Claims

1. A multi-frequency microwave-excited plasma electrothermal thruster, characterized in that, include: The resonant cavity (1), multiple microwave sources, and waveguide system; The resonant cavity (1) is provided with multiple independent microwave input ports. The waveguide system includes multiple independent waveguide subsystems. The multiple microwave sources, the multiple microwave input ports and the multiple waveguide subsystems correspond one-to-one. Different microwave sources output microwave signals of different frequency bands and are connected to the corresponding microwave input ports through the corresponding waveguide subsystems to feed microwave signals of different frequency bands into the resonant cavity (1). The resonant cavity (1) is used to receive the energy of the microwave signals and generate plasma.

2. The multi-frequency microwave-excited plasma electrothermal thruster as described in claim 1, characterized in that, The microwave source includes a first microwave source (31) and a second microwave source (32), wherein the frequency of the first microwave source (31) is lower than that of the second microwave source (32). The resonant cavity (1) has multiple inherent frequencies. The frequency of the first microwave source (31) is within a preset range centered on any one of the inherent resonant frequencies of the resonant cavity (1), and the frequency of the second microwave source (32) is outside the preset range centered on any one of the inherent resonant frequencies of the resonant cavity (1).

3. The multi-frequency microwave-excited plasma electrothermal thruster as described in claim 2, characterized in that, The width of the preset interval is set to be within ±10% of the inherent resonant frequency.

4. The multi-frequency microwave-excited plasma electrothermal thruster as described in claim 2, characterized in that, The microwave output directions of the first microwave source (31) and the second microwave source (32) are perpendicular to each other; The first microwave source (31) and the second microwave source (32) use discontinuous wave power supply as the power supply method, and the duty cycle of the power supply used by the first microwave source (31) and the second microwave source (32) is different.

5. The multi-frequency microwave-excited plasma electrothermal thruster as described in claim 4, characterized in that, The output direction of the first microwave source (31) is parallel to the axis of the resonant cavity (1), and the output direction of the second microwave source (32) is perpendicular to the axis of the resonant cavity (1), or The output direction of the first microwave source (31) is perpendicular to the axis of the resonant cavity (1), and the output direction of the second microwave source (32) is parallel to the axis of the resonant cavity (1).

6. The multi-frequency microwave-excited plasma electrothermal thruster as described in claim 2, characterized in that, The waveguide system includes a first waveguide subsystem (41) and a second waveguide subsystem (42), and the microwave input port includes a first microwave port (21) and a second microwave port (22). The first microwave source (31) is connected to the first microwave port (21) through the first waveguide subsystem (41), and the second microwave source (32) is connected to the second microwave port (22) through the second waveguide subsystem (42).

7. The multi-frequency microwave-excited plasma electrothermal thruster as described in claim 6, characterized in that, Both the first waveguide subsystem (41) and the second waveguide subsystem (42) adopt a structure combining a rectangular waveguide and a ring waveguide slot antenna to form a waveguide slot array; The characteristic frequency of the slot antenna in the waveguide subsystem is the same as the characteristic frequency of the corresponding microwave source of the waveguide subsystem.

8. The multi-frequency microwave-excited plasma electrothermal thruster as described in claim 1, characterized in that, Each microwave input port is equipped with a corresponding coupler and matching network, and the coupler and matching network have the same frequency as the corresponding microwave input port.

9. The multi-frequency microwave-excited plasma electrothermal thruster as described in claim 2, characterized in that, The control logic of the first microwave source (31) and the second microwave source (32) is as follows: during the ignition and start-up phase of the plasma electrothermal thruster, only the first microwave source (31) works; during the transition phase from ignition to steady-state maintenance, the first microwave source (31) and the second microwave source (32) work simultaneously; during the steady-state maintenance phase, the second microwave source (32) works.

10. The multi-frequency microwave-excited plasma electrothermal thruster as described in claim 1, characterized in that, It also includes a nozzle (5) connected to the resonant cavity (1), and the plasma generated by the resonant cavity (1) flows directly into the nozzle (5) and is ejected from the outlet of the nozzle (5).

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