A microwave resonant cavity

Abstract

The present invention discloses a microwave resonant cavity. The microwave resonant cavity of the present invention comprises a cavity and a film layer located on the surface of the inner wall of the cavity, and the film layer is made of a dielectric material capable of conducting heat. The internal film layer structure of the microwave resonant cavity of the present invention has a certain thermal conductivity, which can more effectively conduct the heat generated at the hot spot of the conductive wall to a low temperature away from the heating point, thereby reducing the maximum temperature rise at the hot spot, and further reducing the positive feedback effect caused by heat loss; the internal film layer is a dielectric, non-conductive, and has almost no interaction with the microwave electromagnetic field, so no additional heat loss is generated; thereby improving the overall operational stability of the microwave resonant cavity; the inner wall is coated with a dielectric film with stable chemical properties, which can also play a protective role. For example, magnesium boride is chemically unstable and easily decomposes when it comes into contact with water, and cannot be treated by high-pressure ultrapure water cleaning and other treatment methods. After adding a dielectric film, the inner wall of the resonant cavity can be protected.

Description

Technical Field

[0001] The invention relates to a microwave resonant cavity and belongs to the technical field of microwaves. Background Art

[0002] The resonant cavity is surrounded by a conductive wall of any shape, in which an electromagnetic standing wave is formed, thereby providing a resonant acceleration voltage for charged particles. It is an important component of the particle accelerator. The resonant cavity is divided into a normal conducting cavity and a superconducting cavity according to the state of the conductor during operation. Normal conducting cavities are generally made of pure copper and aluminum, while superconducting cavities are divided into pure metal cavities, including lead and niobium metal cavities, and coated cavities, including niobium-tin film cavities, niobium nitride film cavities, and magnesium boride film cavities. Due to the nature of microwaves, whether it is normal conducting or superconducting, the current distribution of the cavity is generally concentrated on the very shallow surface inside the cavity. High-frequency current passing through surface defects will cause hot spots. The increase in local resistance at the hot spots will increase more heat loss, bringing about a positive feedback effect of heat loss.

[0003] During the actual operation of the accelerator, the resonant cavity will be affected by factors such as the Lorentz force, coolant temperature fluctuations, heat loss and beam load, which will cause slight deformation of the cavity and lead to a shift in its resonant frequency. In particular, the resonant cavity often operates in the very high-frequency microwave band and is more affected by deformation. These factors can easily cause unstable operation.

[0004] For superconducting cavities, heat loss not only causes deformation, but also leads to thermal instability: superconductors only lose their thermal stability below the critical temperature (T c ) and below the critical magnetic field (H c ) environment to maintain the superconducting state. The temperature rise of the local hot spot of the superconductor will cause the temperature to be higher than T c Local quenching will also cause the corresponding H c Reduced, so that the operating magnetic field is higher than H c And quench. Local quench of a superconducting cavity will greatly increase the surface resistance, increase heat loss, cause a sharp drop in the quality factor, and even cause the entire cavity to quench and collapse. The higher the temperature of the central hot spot caused by the heating point, the worse the performance of the cavity (such as the maximum acceleration gradient). For example, Fig. 14 in the literature [H. Padamsee., HEAT TRANSFER AND MOOELS FOR BREAKDOWN. Proceedings of SRF Workshop 1980, Karlsruhe, Germany, srf80-7, p185] shows the distribution of temperature near the defect (i.e., local hot spot) under different magnetic fields. For example, at a distance of 0.5 mm from the defect, the temperature is about 1.7 K under a 503 Gauss magnetic field, and 2.5 K under a 723 Gauss magnetic field. The temperature change caused by heating and the RF loss are always corresponding.

[0005] In order to reduce the unstable operation caused by such heat loss, in addition to using coolant outside the cavity, various surface treatment processes to remove surface impurities and defects are always necessary, and various attempts to improve the thermal conductivity of cavity materials have also made great progress. However, with more than half a century of technological progress, the improvements brought about by various surface treatment technologies and technologies to improve the thermal conductivity of the cavity itself have reached their limits; at the same time, because the mechanical strength of the cavity should not be too low, the cavity material cannot be selected from high-purity but soft high-thermal conductivity materials, causing the performance of the resonant cavity to almost reach a bottleneck, and there is no systematic method to improve the performance of the resonant cavity in the near future. Summary of the invention

[0006] The object of the present invention is to provide a microwave resonant cavity, the inner wall of which has a film layer made of a dielectric material with a certain thermal conductivity, which can effectively conduct the heat generated by the hot spot to the low temperature of the resonant cavity, thereby reducing the local maximum temperature of the hot spot, and further reducing the probability of a positive feedback effect of increasing heat loss caused by temperature increase, thereby enabling the microwave resonant cavity to operate more stably.

[0007] The present invention provides a microwave resonance cavity, comprising a cavity and a film layer located on the surface of the inner wall of the cavity, wherein the film layer is made of a dielectric material capable of conducting heat.

[0008] The microwave resonant cavity may be a normal conducting cavity or a superconducting cavity, and the superconducting cavity may be a pure metal cavity or a coating cavity, such as a normal conducting cavity made of pure copper, a superconducting cavity made of pure niobium, or a superconducting cavity with niobium-tin coating on niobium.

[0009] The microwave resonant cavity may be an ellipsoidal cavity or other cavities.

[0010] In the microwave resonant cavity described above, preferably, the thickness of the film layer is less than or equal to 100 μm, such as 1-100 μm, 1 μm or 100 μm. The film layer may be a single layer or multiple layers.

[0011] In the above microwave resonant cavity, the dielectric material may be a single material or a composite material.

[0012] Preferably, the thermal conductivity of the dielectric material at the operating temperature of the resonant cavity is greater than 10 W / (mK), such as 4661 W / (mK) at 58 K to 5110 W / (mK) at 50 K, 100 W / (mK) at 4 K to 1675 W / (mK) at 23 K, and 100 W / (mK) at 4 K to 499 W / (mK) at 9 K.

[0013] As an example, the dielectric material capable of thermal conductivity is SiC or AlN.

[0014] In the microwave resonant cavity mentioned above, the film layer may be a coating layer, which may be specifically coated after being processed and treated by the resonant cavity.

[0015] The film layer structure inside the microwave resonant cavity of the present invention has a certain thermal conductivity, so it can more effectively conduct the heat generated at the hot spot of the conductive wall to a low temperature away from the heating point, thereby reducing the maximum temperature rise at the hot spot, and further reducing the positive feedback effect caused by heat loss; because the internal film layer is a dielectric, non-conductive, and has almost no interaction with the microwave electromagnetic field, no additional heat loss will be generated; thereby improving the overall operating stability of the microwave resonant cavity; the inner wall is plated with a dielectric film with stable chemical properties, which can also play a protective role. For example, magnesium boride is chemically unstable and easily decomposed when it comes into contact with water, and cannot be treated by high-pressure ultrapure water cleaning. After the dielectric film is added, it can protect the inner wall of the resonant cavity. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 The ellipsoid cavity and the corresponding cross-sectional view, the inner wall of the cavity is coated with a thermal conductive film;

[0017] The labels are as follows: 1 - ellipsoid cavity; 2 - membrane layer.

[0018] Figure 2 3D temperature distribution comparison of the structure without heat-conducting film (left) and the structure with heat-conducting film (right) in Example 1.

[0019] Figure 3 It is a comparison of the isotherms of the simulation results of Example 2 without a thermal conductive film structure (left) and with a thermal conductive film structure (right).

[0020] Figure 4 The simulation results of Example 2 show a comparison of the three-dimensional temperature distribution of the structure without a thermal conductive film (left) and the structure with a thermal conductive film (right).

[0021] Figure 5 The simulation results of Example 3 show the comparison of the three-dimensional temperature distribution of the structure without a thermal conductive film (left) and the structure with a thermal conductive film (right).

[0022] Figure 6 The simulation results of comparative example 1 compare the isotherms of the thermal conductive film in Example 1 when the film thickness is increased to 100 μm (left) and when the film thickness is increased to 1000 μm (right).

[0023] Figure 7 The simulation results for comparative example 2 are a comparison of isotherms of the structure without a thermally conductive film (left) and the thermally conductive film material using solid water material (right) in Example 1. DETAILED DESCRIPTION

[0024] The following is a clear and complete description of the technical solutions in the embodiments of the present invention in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. The methods are conventional methods unless otherwise specified. The raw materials can be obtained from public commercial channels unless otherwise specified.

[0025] The microwave resonant cavity of the present invention comprises a cavity 1 and a film layer 2 located on the surface of the inner wall of the cavity, and the film layer 2 is made of a dielectric material capable of conducting heat. The film layer 2 can more effectively conduct the heat generated at the hot spot of the conductive wall to a low temperature area far away from the heating point, thereby reducing the maximum temperature rise at the hot spot, and further reducing the positive feedback effect caused by heat loss. The film layer 2 is at least located in the high magnetic field area of ​​the cavity 1, that is, the area where hot spots are more likely to be generated. For example, Figure 1 As shown, the cavity (1) may be an ellipsoidal cavity or other cavities. In order to verify the effect of the inner wall film layer of the microwave resonant cavity of the present invention on reducing the heat loss caused by surface heating, the following is a detailed description using a normal conducting cavity made of pure copper, a superconducting cavity made of pure niobium, and a superconducting cavity with niobium-tin plated on niobium as examples.

[0026] Embodiment 1, microwave resonant cavity

[0027] Use COMSOL to simulate the effect of thermal conductive film: select copper (RRR value is 30) as the cavity material, use the thermal conductivity coefficient (Copper [solid, residual resistivity ratio of 30]) provided by the COMSOL material library, set the cavity wall thickness to 1mm, and the cooling temperature of the outer boundary of the wall to 50K. Select a disk-shaped local area with a radius of 0.1cm, and set a disk-shaped constant power heat source in the center of the inner surface with a power of 10 16 W / m 3 , with a radius of 0.01 mm and a thickness of 50 nm. The simulation results show that the maximum temperature at the center of this model is 57.4 K when it is in steady state (after 10 seconds).

[0028] A 1μm thick thermal conductive layer is added to the inner wall. The material is SiC, and the thermal conductivity is selected from the R66 material in Fig. 2.7 in the literature [Donald T. Morelli, GAS, High Lattice Thermal Conductivity Solids, in High Thermal Conductivity Materials, JSG Subhash L. Shind'e, Editor. 2006, Springer: USA.] (the thermal conductivity at the operating temperature of the resonant cavity is about 4661W / (mK) at 58K to 5110W / (mK) at 50K). The simulation results show that the maximum center temperature drops to 54.4K in steady state. The simulation results are shown in Figure 2 .

[0029] Embodiment 2, microwave resonant cavity

[0030] Use COMSOL to simulate the effect of thermal conductive film: select niobium as the cavity material, use the thermal conductivity coefficient (Nb[Solidpolycrystalline]) provided by COMSOL material library, set the cavity wall thickness to 1mm, and the outer boundary cooling temperature to 4K. Select a disk-shaped local area with a radius of 0.1cm, and set a disk-shaped constant power heat source in the center of the inner surface with a power of 10 16 W / m 3 , with a radius of 0.01 mm and a thickness of 50 nm. The simulation results show that the maximum temperature at the center of this model is 22.9 K when it is in steady state (after 10 seconds).

[0031] A 1μm thick thermal conductive layer is added to the inner wall. The material is AlN. The thermal conductivity above 15K is the default value of COMSOL. The thermal conductivity below 15K is selected from the w-201 material in Fig. 2.9 in the literature [Donald T. Morelli, GAS, High Lattice Thermal Conductivity Solids, in High Thermal Conductivity Materials, JSG Subhash L. Shind'e, Editor. 2006, Springer: USA.] (the thermal conductivity at the operating temperature of the resonant cavity is about 100W / (mK) at 4K to 1675W / (mK) at 23K). The simulation results show that the maximum center temperature drops to 18.1K in steady state. The simulation results are shown in Figure 3 , Figure 4 .

[0032] Embodiment 3, microwave resonant cavity

[0033] COMSOL was used to simulate the effect of the thermal conductive film: the cavity material was selected as niobium-tin-plated niobium, and the niobium used the thermal conductivity of the COMSOL material library (Nb[Solid, polycrystalline]). The thickness of the cavity wall niobium was set to 1 mm, and the thickness of the niobium-tin layer was set to 2 μm. The thermal conductivity of niobium-tin was taken from the literature [Andries den Ouden, HHJtK, Thermal conductivity of mica / glass insulation for impregnated Nb3Sn windings in accelerator magnets. Cryogenics, 1994. 34(1): p. 385-388]. The cooling temperature of the outer boundary of the wall was 4 K. A disk-shaped local area with a radius of 0.1 cm was selected, and a disk-shaped constant power heat source with a power of 10 was set in the center of the inner surface. 13 W / m 3 , with a radius of 0.01 mm and a thickness of 50 nm. The simulation results show that the maximum temperature at the center of this model is 8.97 K when it is in steady state (after 10 seconds).

[0034] A 1μm thick thermal conductive layer is added to the inner wall. The material is AlN, and the thermal conductivity is selected from the w-201 material in Fig. 2.9 in the literature [Donald T. Morelli, GAS, High Lattice Thermal Conductivity Solids, in High Thermal Conductivity Materials, JSG Subhash L. Shind'e, Editor. 2006, Springer: USA.] (the thermal conductivity at the operating temperature of the resonant cavity is about 100W / (mK) at 4K to 499W / (mK) at 9K). The simulation results show that the maximum center temperature drops to 4.5K in steady state. The simulation results are shown in Figure 5 .

[0035] Comparative Example 1

[0036] According to the steps in Example 1, the influence of the thermal conductive film is simulated. The inner wall thickness is increased to 100 μm. The simulation results show that the maximum center temperature is 50.96K in the steady state. If the inner wall thickness is increased to 1000 μm, the simulation results show that the maximum center temperature is 50.93K in the steady state. When the film thickness is increased by 10 times, the improvement of the center hot spot temperature is only 0.06K. The simulation results are shown in Figure 6 .

[0037] Comparative Example 2

[0038] The influence of the thermal conductive film was simulated according to the steps in Example 1, except that the coating material was replaced with solid water (H2O(water)[solid,tension]). The thermal conductivity coefficient provided by the COMSOL material library was used. The thermal conductivity of the resonant cavity at the operating temperature was about 9.7W / (mK) at 58K to 11.6W / (mK) at 50K. The simulation results showed that the maximum center temperature was 57.3K in steady state, which was only 0.1K lower than 57.4K without the film layer. The simulation results are shown in Figure 7 .

[0039] The specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A microwave resonant cavity, characterized in that: The invention comprises a cavity and a film layer located on the surface of the inner wall of the cavity, wherein the film layer is made of a dielectric material capable of conducting heat; The dielectric material capable of conducting heat is SiC or AlN; The thickness of the film layer is less than or equal to 100 μm; The thermal conductivity of the thermally conductive dielectric material at an operating temperature of the resonant cavity is greater than 10 W / (mK).

2. The microwave resonant cavity according to claim 1, characterized in that: The microwave resonant cavity is a normal conducting cavity or a superconducting cavity.

3. The microwave resonant cavity according to claim 2, characterized in that: The superconducting cavity is a pure metal cavity or a film-coated cavity.

4. The microwave resonant cavity according to any one of claims 1 to 3, characterized in that: The film layer is a single layer or multiple layers.

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