Microwave measurement system and radio frequency superconducting material surface resistance measurement method

Abstract

The invention discloses a microwave measurement system and a radio frequency superconducting material surface resistance measurement method, the microwave measurement system comprises a radio frequency cavity module immersed in liquid helium, the radio frequency cavity module comprises a pure niobium superconducting cavity provided with a mushroom-shaped inner cavity, and the pure niobium superconducting cavity is used for bearing a sample; the maximum radius of an inner cavity of the superconducting cavity is 62.8 mm, the minimum radius of the inner cavity of the superconducting cavity is 9.76 mm, the radius of the end, making contact with a sample, of the superconducting cavity is 33 mm, and in the inner cavity of the superconducting cavity in the resonance mode, the height of a magnetic field distribution area is 105.3 mm. According to the invention, by optimizing the radius of the superconducting cavity, the height of the magnetic field distribution area and the related size, the stability of the resonance mode is maintained while the size of the sample is reduced, and the diameter of the sample is reduced to 66mm under the same frequency on the basis of maintaining the high-field high-resolution measurement capability of the sample, so that the requirement of a test system on the size of the sample is greatly reduced; and large-scale preparation and testing of test samples are facilitated.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of superconducting accelerators, in particular to a microwave measurement system and a method for measuring the surface resistance of radio frequency superconducting material. BACKGROUND

[0002] As one of the core technologies of particle accelerators, quantum computing devices and large scientific devices, the research and development of new radio frequency superconducting (SRF) materials is the key to promoting the miniaturization and industrial application of superconducting devices. Precise evaluation of the performance of the material is necessary in the research and development process. In recent years, the research and development of new radio frequency superconducting materials (such as high critical temperature superconducting compounds and multi-layer superconducting-insulating film structure materials) has put forward higher requirements for testing technology, and it is urgent to realize high-precision and high-stability surface resistance (R s ) measurement under small sample size.

[0003] At present, there are various SRF performance test cavity systems developed at home and abroad, such as sapphire loaded impedance characteristic system, four-wire transmission line half-wave resonant cavity system, and mushroom type SRF test cavity system. Among them, the mushroom type test cavity is a sample loading cavity based on the end plate replacement method, which has the advantages of convenient sample replacement, large range of surface magnetic field measurement, high test precision, etc. However, it usually needs to expand the sample size to ensure sufficient magnetic field strength and measurement resolution, and the sample size often exceeds 120mm in diameter, which is not conducive to the mass production of new radio frequency superconducting samples. SUMMARY

[0004] The present application provides a microwave measurement system and a method for measuring the surface resistance of radio frequency superconducting material, which can solve the technical contradiction between small size sample and high field accurate measurement in the prior art.

[0005] A microwave measurement system, comprising: a radio frequency cavity module immersed in liquid helium, the radio frequency cavity module comprising a pure niobium superconducting cavity provided with a mushroom type inner cavity for carrying a sample, and a 0.5mm step is arranged on the contact surface between the sample and the superconducting cavity; the maximum radius of the inner cavity of the superconducting cavity is 62.8mm, the minimum radius is 9.76mm, and the radius of the contact end with the sample is 33mm, and in the resonant mode, the height of the magnetic field distribution area in the inner cavity of the superconducting cavity is 105.3mm; The superconducting cavity is connected with a coupling device, and the coupling device is used for transmitting radio frequency energy, cavity matching adjustment and signal extraction; The radio frequency cavity module is connected with a microwave feeding subsystem for establishing a radio frequency field and controlling the field strength parameter, and a signal extraction subsystem for collecting radio frequency signals; The superconducting cavity is connected with a cavity monitoring probe, and the cavity monitoring probe is used for monitoring the magnetic field intensity and temperature of the superconducting cavity surface.

[0006] Preferably, the resonance modes include TE011 and TE012, and in the TE011 mode, the resonance frequency is 3.9 GHz and the normal temperature design frequency is 3.898 GHz.

[0007] Preferably, the ratio of the sample surface peak magnetic field to the superconducting cavity surface peak magnetic field is not less than 0.9.

[0008] Preferably, the wall thickness of the superconducting cavity is 3 mm, and the superconducting cavity is made of pure niobium material by stamping and electron beam welding process.

[0009] Preferably, the superconducting cavity is provided with four functional interfaces, which are respectively a sample end flange, a vacuum pumping flange, a feed-in antenna flange and an extraction antenna flange, and are all made of niobium-titanium alloy. Preferably, the sample end flange is a blind flange with a diameter of 152.72 mm.

[0010] Preferably, the coupling device includes a radio frequency feed-in antenna and a radio frequency extraction antenna, and the external quality factors of the radio frequency feed-in antenna and the radio frequency extraction antenna are adjusted by adjusting the depth of insertion into the superconducting cavity.

[0011] Preferably, the microwave feed-in subsystem includes a power source, the power source is provided with a solid-state amplifier, the solid-state amplifier is connected with a circulator, the circulator is connected with a directional coupler, and the directional coupler is connected with the radio frequency feed-in antenna. Preferably, the directional coupler is provided with a forward coupling end and a reflection coupling end, and is respectively connected with a forward power meter and a reflection power meter.

[0012] Preferably, the signal extraction subsystem includes a power divider, the power divider is connected with the radio frequency extraction antenna, and an isolation port is arranged on the coaxial cable connected with the power divider and the radio frequency extraction antenna, and the two output ends of the power divider are respectively connected with an oscilloscope and an extraction power meter.

[0013] A surface resistance measurement method using the microwave measurement system, comprising: A standard sample is loaded in the superconducting cavity, the microwave cable and the measurement cable are connected, hoisted into the liquid helium shaft, a cooling curve is preset, the superconducting cavity is cooled to a specified temperature in the liquid helium temperature zone based on the cooling curve, and the superconducting cavity is ensured to be in a superconducting state. A stable power is fed into the superconducting cavity through the microwave feed-in subsystem to establish a radio frequency field, and the feed-in power is adjusted until the superconducting cavity loses superconductivity. For each power point, the forward power P f, reflected power P r , extraction power P t ; Then, the forward power is instantly cut off, triggering the oscilloscope to record the voltage decay curve of the RF extraction antenna. Based on the voltage decay curve, the loaded quality factor Q of the superconducting cavity after calibration with the standard sample is calculated. L , the loaded quality factor Q of the superconducting cavity after calibration based on the standard sample L , forward power, reflected power and extracted power, and calculate the intrinsic quality factor of the superconducting cavity after calibration with the standard sample ; The energy storage in the cavity is obtained based on the intrinsic quality factor of the superconducting cavity after calibration with the standard sample. The surface magnetic field strength of the standard sample is calculated based on the relationship between the energy storage and surface field strength of the superconducting cavity. -Magnetic field strength curve to determine the quench field strength; Replace the standard sample with the sample to be tested, and calculate the intrinsic quality factor of the superconducting cavity after the sample to be tested is loaded at the same power point according to the above method. ; The surface resistance Rs of the sample to be tested is calculated based on the formula:

[0014] Where: G is the geometric factor, which is 768 in TE011 mode and 1012 in TE012 mode; α is 0.122 in TE011 mode and 0.151 in TE012 mode.

[0015] Preferably, the voltage attenuation curve of the RF extraction antenna is ; Where: V0 is the voltage value read by the oscilloscope at time t=0; t is the time; is the decay time constant.

[0016] Beneficial effects of the present invention: (1) In the present invention, by optimizing the radius of the superconducting cavity, the height of the magnetic field distribution area and related dimensions, the stability of the resonant mode is maintained while reducing the sample size.

[0017] (2) In the present invention, by optimizing the structure of the superconducting cavity, the sample diameter is reduced to less than 66 mm at the same frequency while maintaining its high-field and high-resolution measurement capabilities. This greatly reduces the test system's requirements for sample size and facilitates the mass preparation and testing of test samples. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 A half-section structure schematic diagram of a superconducting cavity in a microwave measurement system provided by the present application; Figure 2 A magnetic field distribution schematic diagram of a superconductive cavity in a TE011 mode; Figure 3 A magnetic field distribution schematic diagram of a superconductive cavity in a TE012 mode; Figure 4 A structure schematic diagram of a superconducting cavity provided by the present application in a microwave measurement system after a flange interface is arranged; Figure 5 A Figure 4 A section structure schematic diagram at A in the present application; Figure 6 A structure schematic diagram of a radio frequency feeding antenna in a microwave measurement system provided by the present application; Figure 7 A structure schematic diagram of a radio frequency extracting antenna in a microwave measurement system provided by the present application; Figure 8 A structure schematic diagram of a microwave measurement system provided by the present application; Figure 9 A flowchart of a surface resistance measurement method provided by the present application; Figure 10 A diagram of a relative error formula of a sample surface resistance in the present application.

[0019] Explanation of reference signs: 1, superconducting cavity; 11, sample end flange; 2, radio frequency feeding antenna; 3, radio frequency extracting antenna; 4, microwave feeding subsystem; 41, directional coupler; 42, circulator; 43, power source; 5, signal extracting subsystem; 51, isolation port; 52, power divider; 53, oscilloscope; 54, power meter; 6, liquid helium shaft. DETAILED DESCRIPTION

[0020] The specific embodiments of the present application are described in detail below, but it should be understood that the protection scope of the present application is not limited by the specific embodiments.

[0021] As Figures 1-5 shown, the microwave measurement system provided by the present application embodiment comprises a radio frequency cavity module immersed in liquid helium, the radio frequency cavity module comprising a superconducting cavity 1 provided with a mushroom-shaped inner cavity and used for carrying a sample. The wall thickness of the superconducting cavity 1 is 3 mm, and the superconducting cavity 1 is prepared by stamping and electron beam welding processes using pure niobium material. Such processing belongs to a mature process, and thus is not described herein.

[0022] The machining precision requirement of the superconducting cavity 1 is <0.1 mm, the RRR requirement of the niobium material is ≥300, and the inner surface sequentially passes through conventional mechanical polishing, chemical polishing, and electro-polishing (chemical polishing liquid parameters: HF:HNO3:H3PO4=1:1:2; electro-polishing liquid parameters: HF:H2SO4=1:9) to a roughness Ra≤50 nm.

[0023] The superconducting cavity 1 is provided with four functional interfaces, which are respectively a sample end flange 11, a vacuum pumping flange, a feed-in antenna flange, and an extraction antenna flange, and are all made of a niobium-titanium alloy and connected to the cavity through electron beam welding. Among them, the connection between the sample and the cavity adopts a stepped structure and is fixed on the end face of the superconducting cavity 1 through the sample end flange 11 made of 316L stainless steel. The sample end flange 11 is a blind flange with a diameter of 152.72 mm, and the sealing is achieved in the manner of aluminum ring sealing, and the sealing surface is the flange surface of the cavity. After sealing is completed, a 0.5 mm step is arranged on the contact surface between the sample and the cavity, which satisfies the boundary condition, reduces the magnetic field strength of the sealing surface, and reduces the risk of microwave leakage.

[0024] The maximum radius of the inner cavity of the superconducting cavity 1 is 62.8 mm, the minimum radius is 9.76 mm, and the radius of the contact end with the sample is 33 mm (so that the sample diameter can be ≤66 mm), and in the inner cavity of the superconducting cavity 1 in the resonance mode, the height of the magnetic field distribution area is 105.3 mm. The resonance mode includes TE011 and TE012, and the TE011 mode is the core mode of optimization, and the TE012 mode is the available backup mode. In this type of cavity, the sample is placed on one end face of the mushroom-shaped cavity, and the magnetic field at the connection between the sample and the cavity is very small, and there is almost no radio frequency loss. At the same time, the electric field lines are closed loops around the cavity axis, and the electric field at the cavity wall and the sample is very small, and there is no electric field component perpendicular to the cavity surface, which can avoid problems such as secondary electron multiplication and local temperature rise caused by dark current.

[0025] The structure of the superconducting cavity 1 is optimized in detail through the CST microwave studio software to reduce the sample size as much as possible while ensuring the high-field performance of the superconducting cavity 1. The key points of the structure optimization design include the following: (1) In order to be the same as or an integer multiple of the frequency of the commonly used accelerator, the superconducting cavity 1 in the present application has a resonance frequency of 3.9 GHz in the TE011 mode, which is an integer multiple of the commonly used superconducting accelerator resonance frequency. Due to the cold contraction effect, its frequency at room temperature is 3.898 GHz.

[0026] (2) The ratio of the peak magnetic field on the surface of the sample to the peak magnetic field on the surface of the superconducting cavity 1 is not less than 0.9, and the measurement range of the magnetic field is increased. The larger the ratio, the better, which ensures a higher magnetic field measurement range, and the TE011 mode parameters are the focus of optimization in the optimization.

[0027] (3) Compared with the 120 mm diameter sample adapted by the traditional SRF performance test cavity system, this system can test samples with smaller size, but the superconducting cavity 1 needs to reserve a fixed slot distance for the sample.

[0028] (4) A 0.5 mm step is left at the contact point between the sample surface and the surface of the superconducting cavity 1 to reduce the magnetic field at this edge, thereby preventing the RF current from heating at the contact point.

[0029] The main structural parameters and main RF parameters of the cavity after optimization are shown in Table 1 below: Table 1 Main RF parameters and main structural parameters of the mushroom cavity

[0030] Note:

[0031] is the integral of the magnetic field energy in the cavity.

[0032] is the integral of the magnetic field energy on the sample surface.

[0033] α is the ratio of the geometric factor of the sample surface to the geometric factor of the entire cavity. Based on the above formula, it can be seen that α is independent of the cavity material.

[0034] In the above table, It is the peak value of the magnetic field in the sample area, that is, the maximum value of the magnetic induction intensity (magnetic field) in the sample surface area in the RF cavity, reflecting the concentration of the magnetic field energy at the sample.

[0035] It is the peak value of the magnetic field in the cavity, that is, the maximum value of the magnetic induction intensity (magnetic field) inside the entire RF cavity, reflecting the global distribution of the magnetic field energy in the cavity.

[0036] It represents the relative ratio of the peak magnetic field in the sample area to the peak magnetic field in the cavity, and is used to quantify the distribution relationship between the magnetic field energy on the "sample surface" and the "entire cavity".

[0037] like Figures 6-7 As shown, superconducting cavity 1 is connected to a coupling device, which is used to transmit RF energy, adjust cavity matching, and extract signals. The coupling device includes an RF feed antenna 2 and an RF extraction antenna 3. The external quality factor of RF feed antenna 2 and RF extraction antenna 3 is adjusted by adjusting the depth of insertion into superconducting cavity 1.

[0038] In this embodiment, the external quality factor of the RF feed antenna 2 reaches 1e 9 ~1e 10between 1e-4 and 1e-5. The external quality factor of the RF extraction antenna 3 reaches 1e-6 between 1e-4 and 1e-5. 11 1e-4 13 between 1e-4 and 1e-5. The external quality factor of the RF extraction antenna 3 reaches 1e-6 between 1e-4 and 1e-5. The specific structural parameters of the antennas can be fine-tuned as needed, or other forms of antennas can be used, which are alternative solutions. However, the matching relationship between the external quality factor and the intrinsic quality factor is a core requirement and is an unchanged element in the alternative solutions.

[0039] The coaxial line for cavity feeding is HUBER + SUHNER®, welded with the male N-type joint, connected to the N-type female joint of the cavity flange, and the cavity flange is fixedly connected to the antenna through threads.

[0040] The power required for sample RF testing is only a few W, and a solid-state amplifier in the s-band can be used for power feeding. Such equipment is a mature commercial product.

[0041] As shown in Figure 8 The low-temperature test of the superconducting cavity 1 is carried out by immersing in liquid helium, so in this embodiment, the superconducting cavity 1 is cooled by the liquid helium shaft 6, and the superconducting cavity 1 is hoisted into the liquid helium shaft 6. Before going down, the superconducting cavity 1 needs to be strictly pretreated, and the process is a general process for superconducting niobium cavities, including but not limited to ultrasonic, high-pressure water flushing, and slow vacuum pumping. The sample and the superconducting cavity 1 need to be cleaned by ultrasonic wave for about 40 min. Then the superconducting cavity 1 needs to be washed by high-pressure water for 3-4 times in a hundred-level clean room to remove dust particles attached to the surface, and then naturally air-dried. After air-drying, packaging is needed in a hundred-level clean room, and the flange 11 at the sample end is packaged with an aluminum ring, and the remaining flanges are packaged with oxygen-free copper rings. After packaging, a molecular pump set dedicated to a super-clean room is used for slow vacuum pumping, and a needle valve is used to control the vacuum pumping rate to ensure that turbulence does not cause dust to enter the superconducting cavity 1. After the vacuum is pumped to 1e-7 Pa, the valve is closed and the outlet is sealed. -4 Pa, the valve is closed and the outlet is sealed. After the superconducting cavity 1 is cleaned and installed, it can be hoisted into the liquid helium shaft 6 for vertical measurement.

[0042] Figure 8 It can also represent the microwave line of the superconducting cavity 1 during low-temperature performance testing. Among them, the RF cavity module is connected to the microwave feeding subsystem 4 for establishing a RF field and controlling the field strength parameter, and the signal extraction subsystem 5 for collecting RF signals.

[0043] Specifically, the microwave feed-in subsystem 4 cooperates with the radio frequency feed-in antenna 2 to form a radio frequency power transmission link. The microwave feed-in subsystem 4 comprises a power source 43 configured with a solid state amplifier connected to a circulator 42 connected to a directional coupler 41 connected to the input end of the radio frequency feed-in antenna 2. The directional coupler 41 is provided with a forward coupling end and a reflected coupling end, and is respectively connected with a forward power meter and a reflected power meter.

[0044] The signal extraction subsystem 5 comprises a power divider 52 connected to the output end of the radio frequency extraction antenna 3, and an isolation port 51 is arranged on the coaxial cable connected between the two. The two output ends of the power divider 52 are respectively connected with an oscilloscope 53 and an extraction power meter.

[0045] The superconducting cavity 1 is also connected with a cavity monitoring probe for monitoring the magnetic field intensity and temperature on the surface of the superconducting cavity 1.

[0046] During low-temperature testing, the magnetic field intensity and temperature on the surface of the superconducting cavity 1 need to be detected. The cavity monitoring probe of the type in Table 2 can be used. The purpose is to detect the surface temperature gradient and magnetic field intensity when the superconducting cavity 1 enters the superconducting state. According to the needs of different test samples, the temperature gradient can be controlled by adjusting the relevant control parameters of the liquid helium shaft 6 to achieve different test conditions. The magnetic field intensity can also be adjusted by an external coil.

[0047] Table 2: Cavity monitoring probe type

[0048] In this embodiment, the measurement method of the sample surface resistance adopts the direct radio frequency method. The principle of the direct radio frequency method is to directly calculate the surface resistance Rs by measuring the intrinsic quality factor of the superconducting cavity 1 before and after loading the standard sample and the sample to be measured. And This method of measurement can eliminate the error caused by the power loss of the microwave cable.

[0049] In this application, by optimizing the structure of the superconducting cavity 1, the sample diameter is reduced to less than 66 mm at the same frequency, greatly reducing the requirements of the test system on the sample size, facilitating the mass production and testing of test samples.

[0050] The inventors found that the conventional mushroom-shaped cavity needs to expand the sample size to ensure sufficient magnetic field strength and measurement resolution. For example, in the TE011 mode, if the sample size is too small, the magnetic field distribution inside the cavity will be distorted, which will affect the measurement accuracy of the quality factor and surface resistance. If the sample size is too small, the ratio of the peak value of the magnetic field on the surface of the sample to the peak value of the global magnetic field of the cavity will decrease significantly, resulting in insufficient proportion of the magnetic field energy in the sample area, which cannot meet the high-resolution measurement requirement. In order to ensure that the ratio is close to 1, the conventional design has to use a large-size sample (diameter > 120 mm). Moreover, the magnetic field concentration at the connection between the small-size sample and the cavity is easy to cause the heating of the radio frequency current, and the conventional design lacks effective structure optimization (such as the 0.5 mm step structure of the present application), so it can only disperse the heat loss by increasing the sample size to avoid the local temperature rise affecting the measurement accuracy.

[0051] In the present application, through electromagnetic field simulation optimization, structure parameter reconstruction and process upgrading, this limitation is broken, and the sample size is greatly reduced while ensuring the measurement accuracy.

[0052] Specifically, the parameters of the superconducting cavity 1 are designed in detail using CST software, so that the magnetic field peak value ratio of the TE011 mode is effectively improved to 0.935 (see Table 1), which can ensure the proportion of the magnetic field energy in the sample area even at a small size. By optimizing the radius of the superconducting cavity 1 and the height of the magnetic field distribution area, the resonance mode stability is maintained while the sample size is reduced. The superconducting cavity 1 uses pure niobium material (RRR≥300) and high-precision processing technology (roughness Ra≤50 nm), which can reduce the radio frequency loss and allow high field strength (≥100 mT) measurement at a small size.

[0053] As shown in Figure 9 The present application provides a surface resistance measurement method using a microwave measurement system, which comprises the following steps: S1, loading a standard sample in the superconducting cavity 1, connecting the microwave cable and the measurement cable, hoisting into the liquid helium shaft 6, presetting a cooling curve, and cooling the superconducting cavity 1 to a specified temperature in the liquid helium temperature zone based on the cooling curve to ensure that the superconducting cavity 1 is in a superconducting state. In this embodiment, the liquid helium temperature zone is 1.8K-4.2K, and the specified temperature is 4.2K. In some other embodiments, the liquid helium temperature zone and the specified temperature can be adjusted based on actual conditions.

[0054] During low-temperature testing, the magnetic field strength and temperature on the surface of the superconducting cavity 1 are detected by the cavity monitoring probe, and the temperature gradient can be controlled by adjusting the relevant control parameters of the liquid helium system to achieve different test conditions. The magnetic field strength can also be adjusted by an external coil.

[0055] S2. Feed stable power into the superconducting cavity 1 through the microwave feeding subsystem 4 to establish a radio frequency field, and adjust the feeding power until the superconducting cavity 1 quenches.

[0056] For each power point, the forward power P is measured by the power meter. f , reflected power P r , extraction power P t . Forward power P f , reflected power P r and extraction power P t Detected by forward power meter, reflected power meter and extracted power meter respectively.

[0057] S3, instantaneously cut off the forward power, triggering the oscilloscope 53 to record the voltage decay curve of the RF extraction antenna 3. The voltage decay curve of the RF extraction antenna 3 is .

[0058] Where: V0 is the voltage value read by the oscilloscope 53 at time t=0; t is the time; is the decay time constant.

[0059] S4. Calculate the loaded quality factor Q of the superconducting cavity 1 after calibration with the standard sample based on the voltage decay curve. L , the loaded quality factor Q of the superconducting cavity 1 after calibration based on the standard sample L , forward power, reflected power and extracted power, and calculate the intrinsic quality factor of the superconducting cavity 1 after calibration with the standard sample .

[0060] draw -Magnetic field strength (H) curve, determines the quench field strength, which is ≥100mT. This value reflects the characteristics of the material and the superconducting cavity 1, and reflects the magnetic field strength threshold that the superconducting material can withstand and maintain the superconducting state under specific test conditions.

[0061] Specifically, the loaded quality factor Q of the superconducting cavity 1 after calibration based on the standard sample is L It can be calculated by the following formula:

[0062] Where: W is the energy stored in the superconducting cavity 1; W T The energy consumption for one cycle; t is time; is the rate of change of energy with time; T is the resonance period; is the resonance frequency; is the decay time constant.

[0063] Coupling coefficient of RF feed-in antenna 2 and RF extraction antenna 3 and can be obtained by power relation according to loaded quality factor Q of superconducting cavity 1 L and intrinsic quality factor of superconducting cavity 1 The relationship can be obtained as follows:

[0064] Thus, the intrinsic quality factor of superconducting cavity 1 after calibration of the standard sample at different powers can be calculated .

[0065] At this time, by adjusting the size of the feed-in power, the size of the field strength in the superconducting cavity 1 can be adjusted, and according to the measurement of the intrinsic quality factor of the superconducting cavity 1 , the energy stored in the cavity can be calculated, and then the surface magnetic field strength of the standard sample can be calculated according to the relationship between the energy stored in the superconducting cavity 1 and the surface field strength in the CST simulation result.

[0066] S5, replace the standard sample with the sample to be measured, and calculate the intrinsic quality factor of the superconducting cavity 1 after loading the sample to be measured at the same power point . When calculating the intrinsic quality factor of the superconducting cavity 1 after loading the sample to be measured , the calculation formula of in S4 is used to calculate.

[0067] Based on the formula, the surface resistance Rs of the sample to be measured is calculated, and the formula is as follows:

[0068] In the formula: G is the geometric factor, which is 768 in TE011 mode and 1012 in TE012 mode; α is 0.122 in TE011 mode and 0.151 in TE012 mode.

[0069] Finally, after data processing, the surface resistance of the sample to be measured at different field strengths, i.e. Rs-H curve, is obtained.

[0070] Assuming that the relative errors of the intrinsic quality factors measured twice (i.e. and ) are the same, both are , in the formula, is the measurement error of the intrinsic quality factor.

[0071] According to the error transfer formula, the relative error of the sample surface resistance can be obtained :

[0072] Where, is the measurement error of the sample surface resistance.

[0073] From the above formula, we can see that the variable related to the surface resistance measurement error is the ratio of the sample surface resistance to the cavity surface resistance , relative error of intrinsic quality factor .

[0074] like Figure 10 As shown, in general The relative error of surface resistance measurement is 10%. Increases and decreases, typically, when α=0.122, =10, =18.6%; =100, =10.7%.

[0075] From the above calculation results, it can be seen that the relative error of surface resistance measurement is generally low, the measurement resolution is small, and the measurement accuracy is higher.

[0076] The above disclosures are only a few specific embodiments of the present invention. However, the embodiments of the present invention are not limited thereto. Any changes that can be conceived by those skilled in the art should fall within the scope of protection of the present invention.

Claims

1. A microwave measurement system, characterized in that: include: A radio frequency cavity module immersed in liquid helium, the radio frequency cavity module comprising a pure niobium superconducting cavity (1) provided with a mushroom-shaped inner cavity, used for carrying a sample, wherein a contact surface between the sample and the superconducting cavity (1) is provided with a step of 0.5 mm; The maximum radius of the inner cavity of the superconducting cavity (1) is 62.8 mm, the minimum radius is 9.76 mm, the radius of the end in contact with the sample is 33 mm, and the height of the magnetic field distribution area in the inner cavity of the superconducting cavity (1) in the resonant mode is 105.3 mm; The superconducting cavity (1) is connected to a coupling device, and the coupling device is used for transmitting radio frequency energy, cavity matching adjustment and signal extraction; The radio frequency cavity module is connected to a microwave feeding subsystem (4) for establishing a radio frequency field and controlling field intensity parameters and a signal extraction subsystem (5) for collecting radio frequency signals; The superconducting cavity (1) is connected to a cavity monitoring probe, and the cavity monitoring probe is used to monitor the magnetic field intensity and temperature on the surface of the superconducting cavity (1).

2. A microwave measurement system according to claim 1, characterized in that: The resonance modes include TE011 and TE012. In the TE011 mode, the resonance frequency is 3.9 GHz, and the design frequency at room temperature is 3.898 GHz.

3. A microwave measurement system according to claim 1, characterized in that: The ratio of the peak magnetic field on the surface of the sample to the peak magnetic field on the surface of the superconducting cavity (1) is not less than 0.

9.

4. A microwave measurement system according to claim 1, characterized in that: The wall thickness of the superconducting cavity (1) is 3 mm, and is made of pure niobium material through stamping and electron beam welding processes.

5. A microwave measurement system according to claim 1, characterized in that: The superconducting cavity (1) is provided with four functional interfaces, namely a sample end flange (11), a vacuum flange, a feed antenna flange and an extraction antenna flange, and all of them are made of niobium-titanium alloy; Wherein, the sample end flange (11) is a blind flange with a diameter of 152.72 mm.

6. A microwave measurement system according to claim 1, characterized in that: The coupling device comprises a radio frequency feeding antenna (2) and a radio frequency extraction antenna (3), and the external quality factor of the radio frequency feeding antenna (2) and the external quality factor of the radio frequency extraction antenna (3) are adjusted by adjusting the depth of insertion into the superconducting cavity (1).

7. A microwave measurement system according to claim 6, characterized in that: The microwave feeding subsystem (4) includes a power source (43), the power source (43) is configured with a solid-state amplifier, the solid-state amplifier is connected to a circulator (42), the circulator (42) is connected to a directional coupler (41), and the directional coupler (41) is connected to the radio frequency feeding antenna (2); The directional coupler (41) is provided with a forward coupling end and a reflection coupling end, and is respectively connected to a forward power meter and a reflection power meter.

8. A microwave measurement system according to claim 6, characterized in that: The signal extraction subsystem (5) includes a power distributor (52), the power distributor (52) is connected to the radio frequency extraction antenna (3), and an isolation port (51) is provided on the coaxial cable connecting the two, and two output ends of the power distributor (52) are respectively connected to an oscilloscope (53) and an extraction power meter.

9. A surface resistance measurement method using the microwave measurement system according to any one of claims 1 to 8, characterized in that: include: A standard sample is loaded into the superconducting cavity (1), a microwave cable and a measuring cable are connected, and the cavity (1) is hoisted into the liquid helium shaft (6). A cooling curve is preset, and the superconducting cavity (1) is cooled to a specified temperature in the liquid helium temperature range based on the cooling curve to ensure that the superconducting cavity (1) is in a superconducting state; Feeding stable power into the superconducting cavity (1) through the microwave feeding subsystem (4) to establish a radio frequency field, and adjusting the feeding power until the superconducting cavity (1) quenches; For each power point, the forward power P is measured by the power meter. f , reflected power P r , extraction power P t ; Then the forward power is instantly cut off, triggering the oscilloscope (53) to record the voltage decay curve of the radio frequency extraction antenna (3), and the loaded quality factor Q of the superconducting cavity (1) after calibration with the standard sample is calculated based on the voltage decay curve. L , the loaded quality factor Q of the superconducting cavity (1) after calibration based on the standard sample L , forward power, reflected power and extracted power, calculate the intrinsic quality factor of the superconducting cavity (1) after calibration with the standard sample ; The energy storage in the cavity is obtained based on the intrinsic quality factor of the superconducting cavity (1) after calibration with the standard sample. The surface magnetic field intensity of the standard sample is calculated based on the relationship between the energy storage and the surface field intensity of the superconducting cavity (1). -Magnetic field strength curve to determine the quench field strength; Replace the standard sample with the sample to be tested, and measure the intrinsic quality factor of the superconducting cavity (1) loaded with the sample to be tested at the same power point according to the above method. ; The surface resistance Rs of the sample to be tested is calculated based on the formula: Where: G is the geometric factor, which is 768 in TE011 mode and 1012 in TE012 mode; α is 0.122 in TE011 mode and 0.151 in TE012 mode.

10. A surface resistance measurement method according to claim 9, characterized in that: The voltage attenuation curve of the radio frequency extraction antenna (3) is: ; Where: V0 is the voltage value read by the oscilloscope (53) at time t=0; t is the time; is the decay time constant.

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