A radio frequency superconducting material performance testing system and surface resistance measurement method

Abstract

The present invention discloses a radio frequency (RF) superconducting material performance testing system and surface resistance measurement method. The RF superconducting material performance testing system comprises an RF cavity module, a cryogenic module, and a measurement module. The RF cavity module includes a mushroom-shaped superconducting cavity with a niobium-tin thin film coated on its inner wall. The cryogenic module is equipped with a multi-stage cooling structure and a magnetic shielding shell. The measurement module is used to collect sample temperature, magnetic field, vacuum level, RF power, and heater DC power. By optimizing the mushroom-shaped cavity design, utilizing Nb3Sn coating technology, and a cryogenic module with efficient conduction cooling, this RF superconducting material performance testing system eliminates the reliance of traditional RF material testing systems on liquid helium cryogenic systems, creating a compact testing platform that enables high-field and high-resolution measurement of the surface resistance of SRF materials.

Description

Technical Field

[0001] The present invention relates to the technical field of superconducting accelerators, and in particular to a radio frequency superconducting material performance testing system and a surface resistance measuring method. Background Art

[0002] Radiofrequency (SRF) superconducting technology plays a crucial role in modern particle accelerators. The development of novel SRF materials is crucial for improving superconducting cavity performance and promoting the miniaturization and industrialization of superconducting accelerators. In recent years, novel RF superconducting materials such as niobium tin (Nb3Sn), magnesium diboride (MgB2), and titanium niobium nitride (NbTiN) have garnered widespread attention, leading to a growing demand for testing their RF performance. To evaluate the performance of SRF materials, they must be directly exposed to RF fields, their power loss measured, and their surface resistance calculated. Samples can be flat, rod-shaped, or even inserted into the cavity.

[0003] Currently, various SRF performance testing systems have been developed, including sapphire-loaded impedance characterization systems, four-wire transmission line half-wavelength resonant cavity systems, and mushroom-shaped SRF test cavity systems. These systems each offer advantages in terms of measurement accuracy and surface magnetic field strength, but most require a liquid helium cryogenic system to provide a constant low-temperature environment. This results in long sample replacement and testing cycles, high costs, and stringent requirements for cavity materials. Traditional SRF performance testing systems and methods make it difficult to efficiently test and develop new SRF materials, hindering the miniaturization and industrialization of RF superconducting technology. Summary of the Invention

[0004] The present invention provides a radio frequency superconducting material performance testing system and a surface resistance measurement method, which can solve the problem in the prior art that it is difficult to efficiently test and develop new SRF materials, thereby limiting the miniaturization and industrialization development of radio frequency superconducting technology.

[0005] A radio frequency superconducting material performance testing system includes: a radio frequency cavity module, wherein the radio frequency cavity module includes a mushroom-shaped superconducting cavity for carrying a sample, wherein the superconducting cavity can operate in a TE011 or TE012 resonance mode and is plated with a niobium-tin thin film on the inner wall;

[0006] The superconducting cavity is provided with a coupling device, which is used for transmitting radio frequency energy, cavity matching adjustment and signal extraction;

[0007] A cryogenic module, the cryogenic module being provided with a multi-stage cooling structure and a magnetic shielding shell, for providing a temperature-stable non-magnetic environment for the superconducting cavity and the sample;

[0008] The low temperature module is connected to a heater and a measuring module, and the measuring module is used to collect sample temperature, magnetic field, vacuum degree, radio frequency power and DC power of the heater;

[0009] After establishing a radio frequency field in the superconducting cavity, the radio frequency loss of the sample is compensated by adjusting the DC power of the heater to keep the sample temperature constant, so that the measurement of the radio frequency loss power is converted into DC power measurement and temperature measurement.

[0010] Preferably, the thickness of the niobium-tin film is 1 μm-2 μm.

[0011] Preferably, the Nb-Sn thin film is plated on the inner wall of the superconducting cavity by a tin vapor diffusion method.

[0012] Preferably, the coupling device includes a radio frequency feeding antenna and a radio frequency extraction antenna, and the external quality factor of the radio frequency feeding antenna and the external quality factor of the radio frequency extraction antenna are adjusted by adjusting the depth of insertion into the superconducting cavity.

[0013] Preferably, the multi-stage cooling structure includes a cold shield and a plurality of cold heads, the cold heads are connected to an external refrigerator, and the superconducting cavity is arranged in the cold shield.

[0014] Preferably, the cold head is connected to a thermal anchor, the superconducting cavity and the cold head are connected via the thermal anchor, and the multi-stage cooling structure cooperates with the thermal anchor to perform cascade heat transfer to the superconducting cavity.

[0015] Preferably, the cold screen is made of an oxygen-free copper screen with a thickness of 3 mm, and the outer surface of the oxygen-free copper screen is mirror-gold-plated.

[0016] Preferably, the low-temperature module also includes a thermostat, which includes a vacuum-insulated outer shell and the magnetic shielding shell. The magnetic shielding shell is a thin shell structure and is fixedly installed on the inner wall of the cold screen. The cold screen is hoisted inside the outer shell by a pull rod.

[0017] Preferably, the heat anchor is provided with a heater at the connection with the cold head, for adjusting the cooling power of the cold head.

[0018] A method for measuring the surface resistance of a radio frequency superconducting material, using the radio frequency superconducting material performance testing system, comprising:

[0019] The standard sample is fixed to the end face of the superconducting cavity. The surface resistance of the standard sample is known. , cool the superconducting cavity to the specified temperature, preset a temperature reference point, establish the RF field and adjust the heater DC power to stabilize the reference point temperature at the specified temperature, record the heater DC power and incident power of the standard sample , reflected power and extraction power ;

[0020] Replace the sample to be tested, adjust the heater power under the same RF field conditions to stabilize the reference point temperature at the specified temperature, record the heater DC power of the sample to be tested, and calculate the difference between it and the heater DC power of the standard sample ;

[0021] The surface resistance of the sample to be tested is calculated based on the formula:

[0022] ;

[0023] Where:

[0024] α is 0.122 in TE011 mode and 0.151 in TE012 mode.

[0025] Beneficial effects of the present invention:

[0026] (1) In the present invention, the radio frequency superconducting material performance test system eliminates the dependence of the traditional radio frequency material test system on the liquid helium cryogenic system by optimizing the mushroom-shaped cavity design of the superconducting cavity, adopting Nb3Sn coating technology and a low-temperature module with efficient cooling conduction, forming a compact test platform, and realizing high-field and high-resolution measurement of the surface resistance of SRF materials.

[0027] (2) In the present invention, the surface resistance measurement method of RF superconducting materials uses power compensation to transfer the measurement error of RF loss power to the measurement error of temperature and DC power, significantly improving the measurement accuracy. This makes the present invention have the advantages of high resolution, low operating cost and compact design, and can achieve high-field and high-resolution low-cost and rapid measurement of the surface resistance of SRF materials, thereby significantly reducing the sample testing cycle. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 A schematic diagram of a half-section structure of a superconducting cavity in a radio frequency superconducting material performance testing system provided by the present invention;

[0029] Figure 2 Schematic diagram of the magnetic field distribution of the superconducting cavity in TE011 mode;

[0030] Figure 3 Schematic diagram of the magnetic field distribution of the superconducting cavity in TE012 mode;

[0031] Figure 4 A schematic structural diagram of a radio frequency feeding antenna in a radio frequency superconducting material performance testing system provided by the present invention;

[0032] Figure 5A schematic structural diagram of a radio frequency extraction antenna in a radio frequency superconducting material performance testing system provided by the present invention;

[0033] Figure 6 A schematic side cross-sectional view of a thermostat in a radio frequency superconducting material performance testing system provided by the present invention;

[0034] Figure 7 for Figure 6 A front view of the connection structure between the superconducting cavity and the thermal anchor structure;

[0035] Figure 8 for Figure 6 A side view of the connection structure between the superconducting cavity and the thermal anchor structure;

[0036] Figure 9 A partial schematic diagram of the cross-sectional structure of the sample connected and installed in the superconducting cavity;

[0037] Figure 10 The present invention provides a flow chart of a method for measuring the surface resistance of radio frequency superconducting materials.

[0038] Description of reference numerals:

[0039] 1. Superconducting cavity; 2. RF feed antenna; 3. RF extraction antenna; 4. Cryogenic module; 41. Housing; 42. Cold screen; 43. Magnetic shielding shell; 44. Cold head; 45. Thermal anchor. DETAILED DESCRIPTION

[0040] The specific embodiments of the present invention are described in detail below, but it should be understood that the protection scope of the present invention is not limited by the specific embodiments.

[0041] like Figure 1-Figure 3 As shown, an embodiment of the present invention provides a radio frequency superconducting material performance testing system, including: a radio frequency cavity module, the radio frequency cavity module includes a mushroom-shaped superconducting cavity 1 for carrying a sample, and the sample is circular. The mushroom-shaped cavity is a symmetrical structure with a circular cross-section selected based on the end plate replacement method. The superconducting cavity 1 can switch the resonant mode TE011 and TE012, and the magnetic field distribution is shown in FIG. Figure 2-Figure 3 In this type of cavity, the sample is placed at one end of the cavity. The magnetic field at the connection between the sample and the cavity is very small, with almost no RF loss. At the same time, the electric field lines are closed loops around the cavity axis, and the electric field is very small at the cavity wall and the sample. At the same time, 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.

[0042] The inner wall of the superconducting cavity 1 is plated with a niobium-tin film with a thickness of 1 μm-2 μm, which is used to reduce its radio frequency loss so that the heat generated in the cavity can be taken away by the cold head 44 and the refrigerator.

[0043] The main operating mode of the superconducting cavity 1 is the TE011 mode, the operating frequency is limited to 3.9 GHz, the operating temperature range is 2.5 K-5.5 K, and the main operating temperature is 4.2 K.

[0044] In this example, to facilitate batch production of samples, the structure of the RF cavity (i.e., superconducting cavity 1) was optimized to minimize the sample diameter while simultaneously improving the high-field performance of the RF cavity. After optimization, the minimum sample diameter can be 66 mm. The main RF parameters of superconducting cavity 1 are as follows:

[0045]

[0046] Note:

[0047]

[0048] is the integral of the magnetic field energy in the cavity.

[0049] is the integral of the magnetic field energy on the sample surface.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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 of the magnetic field on the "sample surface" and "the entire cavity".

[0054] In order to achieve conductive cooling of the superconducting cavity 1, the cavity material itself is selected to be pure niobium (the RRR value of the material is greater than or equal to 300, and the substrate surface roughness is required to be controlled within Ra≤50nm). A 1μm-2μm low-loss Nb3Sn film (i.e., niobium-tin thin film) is deposited on the inner surface of the niobium cavity, i.e., the RF contact surface, by the tin vapor diffusion method. The heat generation of the cavity is controlled within the cooling power of 2 to 3 cold heads 44 (≤6.6W).

[0055] The entire superconducting cavity 1 is equipped with four flange ports. Due to the requirements of the niobium-tin coating process, the flange material is also pure niobium material, which is the same as the cavity material requirements, and vacuum electron beam welding is adopted.

[0056] like Figure 4-Figure 5 As shown, superconducting cavity 1 is equipped with a coupling device for transmitting RF energy, adjusting cavity matching, and extracting 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.

[0057] For optimal matching, the external quality factor of RF feed antenna 2 should be equal to the intrinsic quality factor of superconducting cavity 1, while the external quality factor of RF extraction antenna 3 should be two orders of magnitude higher than the intrinsic quality factor of superconducting cavity 1. The quality factor of the niobium-tin thin film cavity is higher than that of the niobium cavity, reaching the level of a 2K niobium cavity (@1.3GHz) at a temperature of 4.2K.

[0058] Specifically, at a temperature of 4.2K, the external quality factor of the RF feed antenna 2 can be adjusted to 1e 9 to 1e 10 The external quality factor of the RF extraction antenna 3 can be adjusted to 1e 11 to 1e 13 Although the specific structural parameters of the antenna can be fine-tuned as needed, and even other forms of antennas can be used, the above-mentioned matching relationship between the external quality factor and the intrinsic quality factor is the core requirement and is an unchanging factor in the alternative solution.

[0059] The mushroom cavity feed system uses HUBER+SUHNER® coaxial cables equipped with N-type connectors for connection to the cavity antenna. During sample RF testing, the required power was only a few watts, which was relatively low, allowing the use of an S-band solid-state amplifier for power delivery. Currently, this type of solid-state amplifier is technologically mature and widely used commercially.

[0060] like Figure 6-Figure 8 As shown, the radio frequency superconducting material performance testing system also includes a low temperature module 4, which is provided with a multi-stage cooling structure and a magnetic shielding shell 43, which cooperate to provide a temperature-stable non-magnetic environment for the superconducting cavity 1 and the sample.

[0061] Specifically, the multi-stage cooling structure includes a cold shield 42 and multiple cold heads 44, each connected to an external refrigerator. In this embodiment, three cold heads 44 and three refrigerators are provided, each using the KDE420SA model. Each 4.2K cold head 44 has a cooling power of 2.2W. To ensure a stable low-temperature test environment for the test sample and resonant cavity, a cascaded heat transfer design is proposed, using the refrigerator's first 40K cold head 44 and second 4.2K cold heads 44 for cooling. The 40K cold heads 44 are connected to the cold shield 42 to reduce external heat radiation power to the cavity and sample, while also intercepting heat conduction from room temperature. The cold heads 44 are connected to thermal anchors 45, which are made of high-purity aluminum heat-conducting plates. The superconducting cavity 1 is connected to the cold heads 44 via the thermal anchors 45. A heater is installed at the connection between the thermal anchors 45 and the cold heads 44 to adjust the cooling power of the cold heads 44.

[0062] Among them, Figure 7-Figure 9 As shown, the superconducting cavity 1 is connected to two 4.2K cold heads 44 via thermal anchors 45. Thermal anchors 45 are designed near the coupling port (109.32 mm from the sample surface) and at the point of maximum RF heating (60.43 mm from the sample surface). These are rectangular plates and annular plates surrounding the cavity surface, each with a through hole. Both are made of pure niobium and are electron-beam welded to the cavity. The coupling port is connected to the 4K cold heads 44 to intercept heat transfer from the RF feed antenna 2. These two locations are connected to the two 4K cold heads 44 via high-purity aluminum (5N) structural plates. The contact surfaces are coated with Apiezon® N-type thermal grease and secured with screws. The sample is inserted into the end face through the slot of the superconducting cavity 1, and the bottom surface is in close contact with the heat anchor 45. The heat anchor 45 is installed and clamped with the cavity flange through the G10 flange base to ensure contact between the sample, the heat anchor 45 and the cavity, forming a heat transfer chain of heat anchor-sample-cavity, so that the cooling capacity on the sample can be adjusted, which is convenient for measuring the surface resistance of the sample by the power compensation method. After installation, there is a 0.5mm step between the sample surface and the cavity surface. The G10 flange is a stepped structure, such as Figure 9 Thermal grease is applied between the heat anchor 45 and the sample to improve contact.

[0063] like Figure 6As shown, the superconducting cavity 1 is housed within a cold shield 42, which is made of a 3mm-thick RRR30 oxygen-free copper screen with a mirror-gold finish on its outer surface. Radiation heat leakage is the primary source of heat load on the cold shield 42. Gold plating on the outer surface of the oxygen-free copper screen reduces the emissivity of the heat reflective screen to below 0.02, reducing radiation heat leakage while also preventing oxidation of the cold shield 42. Mirror gold plating requires a sufficiently bright surface to effectively reduce emissivity. Before mirror gold plating, the oxygen-free copper screen is polished and cleaned. After copper plating any uneven areas, it is polished again until a bright, flat copper surface is achieved. A 1μm-thick gold layer is then electroplated. The cold shield 42 is secured to the 40K cold head 44 with screws. Apiezon N-type thermal grease is applied to the contact surfaces to reduce thermal resistance and enhance contact.

[0064] The cryogenic module 4 also includes a thermostat, which consists of a vacuum-insulated outer shell 41 and a magnetic shield 43. The magnetic shield 43 is a thin shell structure and is fixed to the inner wall of the cold shield 42 by screws. The cold shield 42 is suspended within the outer shell 41 by tie rods. The thermostat shell is made of carbon steel (SA516GR60), commonly used in the manufacture of welded pressure vessels. As a ferromagnetic material, SA516GR60 generally has a magnetic permeability of 300-2000. Using it to construct the vacuum shell also provides magnetic shielding. The magnetic shield 43 is made of 1J79 (an iron-nickel alloy with excellent magnetic and mechanical properties). The initial magnetic permeability is 20,000, which increases with cooling, reaching a maximum of 80,000 at low temperatures. This magnetic shielding structure reduces the remanent magnetization of the superconducting cavity 1 to below 10 mG.

[0065] The thermostat housing 41 is 615mm tall and 600mm in diameter. It features an upper opening and is sealed with flange grooves and rubber rings. The end cap flange has 10 CF35 vacuum ports with an inner diameter of 35mm. These ports are used for heaters, vacuum pumps, measuring sensors, and microwave cables. The main functional interfaces are shown in the table below:

[0066]

[0067] The overall height of the cold shield 42 is 406 mm and the diameter is 456 mm. The cold shield 42 is hoisted on the top cover of the thermostat by four pure titanium tie rods. Two 38 mm diameter interfaces are left on the end cover of the cold shield 42 for use as radio frequency interfaces and measurement cable interfaces.

[0068] While the superconducting cavity 1 is partially connected and fixed via the cold shield 42, the main weight is suspended from the end caps of the cold shield 42 via four 6mm diameter G10 rods. G10 is made by laminating glass fiber cloth and epoxy resin under high temperature and high pressure. It has good mechanical strength and low thermal conductivity at low temperatures, providing excellent insulation. It is a commonly used connection or support material at low temperatures.

[0069] In this embodiment, the sample is fixed to the end of the superconducting cavity 1 by an insulated sample drag made of G10 material. The sample is 66 mm in size, with one side being the RF contact surface and the other side connected to the heat anchor 45. The heat anchor 45 and the sample are also fixed by a G10 plate. The thermal contact between the sample and the heat anchor 45 is increased by padding indium sheets or thermal grease. In this way, a chain heat conduction path is formed: cold head 44-heat anchor 45-superconducting cavity 1-sample-heat anchor 45-cold head 44.

[0070] To implement power compensation, three sets of heaters are connected to the thermal anchors 45 connected to the cold head 44 to adjust the cooling power of the cold head 44. The heaters are sheet-shaped resistive heaters connected by four leads. To reduce heat leakage, the middle sections of the leads ensure good contact with the 40K cold shield 42.

[0071] The low temperature module 4 is also connected to a measuring module, which is used to collect sample temperature, magnetic field, vacuum degree, radio frequency power and DC power of the heater.

[0072] After the radio frequency field is established in the superconducting cavity 1, the radio frequency loss of the sample is compensated by adjusting the DC power of the heater to keep the reference point temperature constant, so that the measurement of the radio frequency loss power is converted into DC power measurement and temperature measurement.

[0073] The principle of the power compensation method is: after the radio frequency field is established, the cooling power applied to the standard sample and the sample to be tested is adjusted to keep the sample temperature constant. Then the difference in power loss of the radio frequency field in the two samples is The absolute value of is the difference in DC power between the heaters of the two samples, and the surface resistance of the sample to be tested can be calculated based on the resistance of the known standard sample. The advantage of the power compensation method is that it can transfer the measurement error of RF loss power to the temperature measurement error and DC power measurement error, that is, the temperature measurement error can be reduced by using a higher-precision sensor, and the DC power measurement error is a mechanical error. The DC power measurement error of the heater belongs to the mechanical error of the equipment. The mechanical error is controllable. Compared with the measurement error of RF loss power in the existing technology (that is, the system error of actual operation that is difficult to control), the error is smaller, making the measurement data more accurate. By reducing the errors of these two parts, the surface resistance of the sample to be measured can be reduced. The measurement resolution reaches 0-5nΩ.

[0074] Specifically, the sample surface temperature is measured using a high-precision Cernox thermometer with an accuracy better than 2 mK. The measurement system utilizes a Keithley 2450 sourcemeter and an NI PXIe-4082 digital multimeter. The DC compensation current resolution is 0.1 μA, and the voltage sampling accuracy is 0.5 nV. The fluxgate, thermometer, and heater all utilize copper wires for signal and power transmission, connected to the sensor terminals of superconducting cavity 1 and the feedthrough of the thermostat flange. The fluxgate utilizes a four-wire system, while the thermometer utilizes a four-wire system. The feedthrough utilizes a 28-wire interface soldered to a CF35 flange.

[0075] The thermometer measures the following locations: the temperatures of the three cold heads 44, the coupling flange temperature of the superconducting cavity 1, the sample end temperature, the thermal anchor connection temperature of the superconducting cavity 1, and the end flange temperature of the superconducting cavity 1. The temperature near the end flange of the superconducting cavity 1 is used as the reference point temperature for power compensation measurement. The corresponding sensors for each parameter are shown in the following table:

[0076]

[0077] It should be noted that the material of superconducting cavity 1 can be replaced with other low-loss superconducting films (such as MgB2 or NbTiN). This requires adjustments to the heat transfer design. Furthermore, the refrigerator can be a pulse tube refrigerator with the same or greater cooling power at 4.2K, requiring corresponding modifications to the thermostat interface dimensions.

[0078] like Figure 10 As shown, in one embodiment, the present invention provides a method for measuring the surface resistance of a radio frequency superconducting material, which is measured using a radio frequency superconducting material performance testing system. The method for measuring the surface resistance of a radio frequency superconducting material includes:

[0079] S1. Fix a standard sample made of niobium (Nb) material (with a diameter of 50 mm) to the end face of the superconducting cavity 1. The surface resistance of the standard sample is known. , which is the typical value.

[0080] Before attaching the standard sample, the RF superconducting material performance test system must be assembled. First, the superconducting cavity 1 must be machined. A mushroom-shaped cavity is formed from a pure niobium substrate with a machining accuracy of <0.1mm. The niobium material RRR must be ≥300. The inner surface undergoes conventional niobium cavity polishing, followed by mechanical polishing, chemical polishing, and electropolishing (chemical polishing solution parameters: HF:HNO3:H3PO4 = 1:1:2; electropolishing solution parameters: HF:H2SO4 = 1:9) to a roughness Ra ≤ 50nm.

[0081] After the superconducting cavity 1 is fabricated, it undergoes desulfurization, ultrasonic cleaning, and high-pressure water rinsing to dry. The cavity is then placed in a vacuum furnace and a 1.5-2μm thick niobium tin thin film is deposited according to the coating process parameters. The Nb3Sn thin film is prepared in a dedicated furnace with high vacuum heating conditions. Due to the small size of the superconducting cavity 1, uniform diffusion of the tin vapor is easily achieved. Using 0.5-1g of high-purity tin, a proven temperature control scheme is employed: degassing at 180°C for 48 hours, nucleation at 500°C for 5 hours, film formation at 1200°C for 3 hours, and annealing at 1100°C for 1 hour. This process forms a niobium tin thin film on the inner cavity wall of the niobium-based superconducting cavity 1, completing the fabrication of the Nb3Sn thin film cavity. Specific process parameters can be adjusted based on the furnace and cavity conditions.

[0082] Then fix the superconducting cavity 1 to the 5N aluminum heat conducting plate (i.e., heat anchor 45) with screws, and coat the contact surface with Apiezon® N-type thermal grease or indium pads. The aluminum heat conducting plate of the superconducting cavity 1 is connected to the 4K cold head 44, and the contact surface is also coated with Apiezon® N-type thermal grease or indium pads. The same is true for the connection between the 40K cold head 44 and the cold shield 42. After the heat anchor 45 is connected, connect the heater wire, thermometer wire, and microwave wire, and thermally cut off the 40K cold shield 42. Finally, the entire thermostat is pumped down to 1e by a dry pump and a molecular pump. -5 Pa and below.

[0083] S2, when the vacuum drops to 1e -4 When the temperature is lowered, the refrigerator is turned on to start cooling. By adjusting the heater power, the temperature of the superconducting cavity 1 is lowered to the specified temperature according to the specified curve. At this time, the standard sample is also cooled to the specified temperature. In this embodiment, the specified temperature is 4.2K.

[0084] S3. Preset a reference point (the end flange of the superconducting cavity 1 is used as the reference point, and the temperature near it is used as the reference point temperature for the power compensation method). Establish an RF field, feed microwave power (frequency 3.9 GHz), and gradually increase the field intensity to 50 mT (or the specified magnetic field strength). Adjust the heater DC power to stabilize the reference point temperature at the specified temperature. In this embodiment, the specified temperature is 4.2 K. Record the heater DC power and incident power of the standard sample. , reflected power and extraction power .

[0085] S4. Replace the sample to be tested and repeat the above steps. That is, adjust the heater power under the same RF field conditions to stabilize the reference point temperature at 4.2K, record the heater DC power of the sample to be tested, and calculate the difference between it and the heater DC power of the standard sample. .

[0086] The surface resistance of the sample to be tested is calculated based on the formula:

[0087] ;

[0088] Where:

[0089] α is 0.122 in TE011 mode and 0.151 in TE012 mode.

[0090] The inventors discovered that traditional radio frequency superconducting material testing systems have technical bottlenecks such as strong dependence on liquid helium, limited test resolution, and high operating costs.

[0091] In this application, a radio frequency superconducting material performance testing system serves as a conduction-cooled SRF material performance testing platform. It uses a niobium-tin thin film on the inner wall of a superconducting cavity 1 and a cryogenic module equipped with a multi-stage cooling structure and a magnetic shield 43 to cool the cavity and sample. This allows for surface resistance measurements of the sample to be measured with an absolute error of 0-5 nΩ and a relative error of 0-2%, with a test field strength of ≥50 mT. Specific details are as follows:

[0092] The surface resistance measurement error of the standard sample is 5%. Assuming that the temperature measurement accuracy is 2mK and the RF power measurement error is 2.5%, according to the error transmission relationship, it can be obtained that the relative measurement error of the sample surface resistance decreases as its own value increases, and the absolute error first decreases and then increases.

[0093] According to the literature [RD Porter, etc. HIGH FREQUENCY Nb3Sn CAVITIES, 19th Int.Conf. on RF Superconductivity doi:10.18429 / JACoW-SRF2019-MOP011], those skilled in the art can reasonably predict based on the experimental results in the literature that the test field strength of this test system is ≥50 mT.

[0094] Compared to existing liquid helium-based measurement systems, this system is more compact and advanced. Furthermore, a low-temperature module using conduction cooling replaces liquid helium, eliminating the need for co-operation with liquid helium-related equipment, effectively reducing equipment construction and operating costs.

[0095] This RF superconducting material performance test system eliminates the reliance on liquid helium cryogenic systems by optimizing the mushroom-shaped cavity design of the superconducting cavity 1, adopting Nb3Sn coating technology and a low-temperature module with efficient conduction cooling, forming a compact test platform capable of high-field and high-resolution SRF material surface resistance measurements.

[0096] A method for measuring the surface resistance of radio frequency superconducting materials uses power compensation to shift measurement errors in radio frequency power loss to those in temperature and DC power, ensuring nΩ-level high-precision measurements. This method offers the advantages of high resolution, low operating costs, and a compact design. It enables rapid, low-cost measurement of the surface resistance of SRF materials at high field and high resolution, significantly reducing sample testing cycles.

[0097] It is understood that the RF superconducting material performance testing system can be used not only for the aforementioned RF superconducting material surface resistance measurement method, but also for measuring surface resistance using a direct RF method. That is, the measurement method in this application can be replaced with a direct RF method, where the surface resistance of the sample to be tested is calculated using a formula by measuring the cavity intrinsic quality factor of a standard sample with known surface resistance and the sample to be tested under the same conditions.

[0098] Specifically, step 1: Test the standard sample: Fix the standard sample to the end face of the superconducting cavity 1, assemble the RF superconducting material performance test system, connect the microwave cable and the measurement cable, and hoist it into the shaft. Cool the cavity to 4.2 K (or the specified temperature) according to the specified cooling curve. Measure the intrinsic quality factor of the superconducting cavity 1 at different feed powers. , increase the feeding power until superconducting cavity 1 quenches.

[0099] Step 2: Test the sample to be tested: Fix the sample to be tested on the end face of the superconducting cavity 1. The rest of the steps are the same as those for testing the standard sample. Measure the intrinsic quality factor of the superconducting cavity 1 under the same feed power group as in step 1. After that, the surface resistance of the sample under different field strengths is obtained by the following formula: curve.

[0100] .

[0101] 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 method for measuring the surface resistance of a radio frequency superconducting material, characterized in that: include: The standard sample is fixed on the end face of the superconducting cavity (1). The surface resistance of the standard sample is known. , cool the superconducting cavity (1) to a specified temperature, preset a reference point, establish a radio frequency field and adjust the heater DC power to stabilize the reference point temperature at the specified temperature, record the heater DC power and incident power of the standard sample , reflected power and extraction power ; Replace the sample to be tested, adjust the heater power under the same RF field conditions to stabilize the reference point temperature at the specified temperature, record the heater DC power of the sample to be tested, and calculate the difference between it and the heater DC power of the standard sample ; The surface resistance of the sample to be tested is calculated based on the formula: ； Where: α is the ratio of the geometric factor of the sample surface to the geometric factor of the entire cavity, and α is 0.122 in TE011 mode and 0.151 in TE012 mode.

2. A radio frequency superconducting material performance testing system, applicable to the radio frequency superconducting material surface resistance measurement method according to claim 1, characterized in that: include: A radio frequency cavity module, comprising a mushroom-shaped superconducting cavity (1) for carrying a sample, wherein the superconducting cavity (1) can operate in a TE011 or TE012 resonance mode and is plated with a niobium-tin film on its inner wall; The superconducting cavity (1) is provided with a coupling device, and the coupling device is used for transmitting radio frequency energy, cavity matching adjustment and signal extraction; A low-temperature module (4), the low-temperature module (4) being provided with a multi-stage cooling structure and a magnetic shielding shell (43), and being used for providing a temperature-stable non-magnetic environment for the superconducting cavity (1) and the sample; The low temperature module (4) is connected to a heater and a measuring module, and the measuring module is used to collect sample temperature, magnetic field, vacuum degree, radio frequency power and DC power of the heater; After establishing a radio frequency field in the superconducting cavity (1), the radio frequency loss of the sample is compensated by adjusting the DC power of the heater to keep the sample temperature constant, so that the measurement of the radio frequency loss power is converted into DC power measurement and temperature measurement.

3. A radio frequency superconducting material performance testing system according to claim 2, characterized in that: The thickness of the niobium-tin film is 1 μm-2 μm.

4. A radio frequency superconducting material performance testing system according to claim 3, characterized in that: The niobium-tin thin film is plated on the inner wall of the superconducting cavity (1) by a tin vapor diffusion method.

5. A radio frequency superconducting material performance testing system according to claim 2, 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).

6. A radio frequency superconducting material performance testing system according to claim 2, characterized in that: The multi-stage cooling structure comprises a cold shield (42) and a plurality of cold heads (44), wherein the cold heads (44) are connected to an external refrigerator, and the superconducting cavity (1) is arranged in the cold shield (42).

7. A radio frequency superconducting material performance testing system according to claim 6, characterized in that: The cold head (44) is connected to a thermal anchor (45), the superconducting cavity (1) and the cold head (44) are connected via the thermal anchor (45), and the multi-stage cooling structure cooperates with the thermal anchor (45) to perform cascade heat transfer on the superconducting cavity (1).

8. A radio frequency superconducting material performance testing system according to claim 7, characterized in that: The cold screen (42) is made of an oxygen-free copper screen with a thickness of 3 mm, and the outer surface of the oxygen-free copper screen is mirror-gold-plated.

9. The radio frequency superconducting material performance testing system according to claim 7, characterized in that: The low-temperature module (4) also includes a thermostat, which includes a vacuum-insulated shell (41) and the magnetic shielding shell (43). The magnetic shielding shell (43) is a thin shell structure and is fixedly installed on the inner wall of the cold screen (42). The cold screen (42) is hoisted inside the shell (41) by a pull rod.

10. The radio frequency superconducting material performance testing system according to claim 7, characterized in that: The heat anchor (45) is provided with a heater at the connection with the cold head (44) for adjusting the cooling power of the cold head (44).

Images