Magnetic Susceptibility Measurement Device and Method Based on Variable-Turn Coils and Atomic Magnetometers

Abstract

The present application discloses a magnetic susceptibility measurement device and method based on a variable-turns coil and an atomic magnetometer. The device includes an atomic magnetometer, a magnetic field generation module, and a magnetic field compensation module. The magnetic field generation module includes two pairs of coils C1 and C2 with different radii and coaxially arranged. The magnetic fields generated by the coils C1 and C2 at the gradient zero point are equal in magnitude and opposite in direction. The atomic magnetometer is arranged at the gradient zero point for measuring the low-field magnetic susceptibility. The magnetic field compensation module includes a pair of coils C3 with adjustable turns, which is used to achieve magnetic field compensation by changing the number of turns of the coil C3. The present application can accurately compensate for the residual magnetic field and magnetic field gradient of the coil at the atomic magnetometer by precisely adjusting the number of turns of the coil, enabling the atomic magnetometer to operate within the measurement range and gradient tolerance. At the same time, the high-precision variable-turns coil is used to precisely adjust the compensation magnetic field, improving the measurement accuracy of the magnetic susceptibility, so as to measure the low-field magnetic susceptibility of large-size samples.

Description

Technical Field

[0001] This application belongs to the technical field of material magnetic property measurement in space gravitational wave detection. More specifically, it relates to a magnetic susceptibility measurement device and method based on a variable-turns coil and an atomic magnetometer. Background Technique

[0002] Magnetic fields are ubiquitous, and materials are always in a magnetic field of a certain magnitude. These magnetic fields include the geomagnetic field in our living environment, the interstellar magnetic field in the cosmic space, etc. In the field of precision measurement, various instrument noises caused by magnetic fields are important factors affecting measurement accuracy. For example, in space gravitational wave detection, the magnetic force noise generated by the test mass under the action of the interstellar magnetic field is an important source of the acceleration noise of the test mass. The interstellar magnetic field is a weak magnetic field. Measuring the magnetic susceptibility of the test mass under a weak magnetic field is of great significance for evaluating the magnetic noise of the test mass. Due to the existence of the hysteresis phenomenon, the magnetization characteristics of materials are related to the intensity of the applied magnetic field and the magnetization history. Therefore, the magnetic susceptibility under a weak magnetic field is usually different from that under a strong magnetic field. In order to measure the magnetic susceptibility of the test mass under a weak magnetic field, make the applied magnetic field closer to the magnetic field of the test mass in the on-orbit working state, and provide a more reliable magnetic susceptibility measurement result for the space gravitational wave detection mission, a magnetic field induction measurement instrument is required to have a very high magnetic field resolution.

[0003] Atomic magnetometers and superconducting quantum interference devices (SQUIDs) are currently two types of magnetic field sensors with the highest measurement accuracy, and the sensitivity can reach Magnitude, capable of meeting the requirement of measuring the magnetic susceptibility of the test mass under weak magnetic fields. SQUID devices need to maintain a low-temperature environment and are bulky, making them inconvenient for movement and installation. Therefore, at room temperature, atomic magnetometers are more suitable for magnetic susceptibility measurement. In the prior art, the method of measuring the induced magnetic field generated by a sample using an atomic magnetometer is to place the sample in a strong magnetic field generated by a superconducting magnet or an electromagnet, etc., so that it is fully magnetized, thereby generating a sufficiently strong induced magnetic moment, and vibrating the sample at a certain frequency at high speed. By placing a pick-up coil near the sample, an induced current is generated in the pick-up coil when the sample vibrates. Then, the pick-up coil is connected in series with another magnetic field generating coil away from the electromagnet area, and the atomic magnetometer is used to measure the magnetic field of this magnetic field generating coil, thereby obtaining information about the induced current, and further obtaining information about the induced magnetic moment of the sample. In this technology, there are two defects that prevent it from meeting the requirement of measuring the magnetic susceptibility of the test mass. First, the magnetic field applied to the sample by this device is a high field above several hundred milliteslas, and it is impossible to measure the near-zero low-field magnetic susceptibility. Second, in this method, the sample needs to be vibrated at high speed, and the mass of the test mass reaches the order of kilograms. The rapid vibration will cause the test mass to receive too large an impact force, resulting in deformation or damage. Therefore, the existing measurement methods do not meet the requirement of low-field magnetic susceptibility measurement. The solution to the above problems is to directly measure the induced magnetic field generated by the sample, rather than measuring it after converting the induced magnetic field into current. Therefore, it is necessary to install the magnetization field generating coil, the sample, and the atomic magnetometer in the same set of devices to achieve direct measurement of the induced magnetic field of the sample.

[0004] Since the dynamic range of high-precision atomic magnetometers is usually very small, and due to the existence of magnetic field gradients, which will reduce the amplitude of the Larmor signal and other reasons, the measurement noise increases. Therefore, atomic magnetometers must operate in a near-zero magnetic field and a near-zero gradient field. While the coil applies a magnetic field to the sample, it also applies a magnetic field to the atomic magnetometer, which means that the magnetic field and gradient at the location of the atomic magnetometer must be compensated. The magnetic field compensation schemes proposed in the prior art do not meet the usage requirements of atomic magnetometers. For example, Patent No. CN 115718273 B proposes a device and its measurement method for measuring the magnetic susceptibility of an object based on the magnetic induction intensity. In this scheme, different coils are connected in parallel, so that the current of each coil can be independently adjusted to compensate for the residual magnetic field. However, since the impedances of the coils are not exactly the same, the currents no longer have coherence, and the power supply noise will be converted into magnetic field noise through the current, so that the noise components in the coil magnetic field can no longer cancel each other out by interference, resulting in the final magnetic field compensation accuracy being limited by the power supply noise. At present, the output noise of the highest-precision power supply does not meet the requirement of measuring the low-field magnetization signal of weakly magnetic objects such as copper. Therefore, there is an urgent need to develop a magnetic field compensation method that does not rely on power supply noise. Summary of the Invention

[0005] Aiming at the defects of the prior art, the purpose of this application is to provide a magnetic susceptibility measurement device and method based on a variable-turns coil and an atomic magnetometer; by using an atomic magnetometer with high precision and small size to measure the low-field magnetic susceptibility of large-size objects such as test masses, thereby improving the measurement accuracy of the magnetic susceptibility of test masses, reducing the required external magnetic field intensity, and realizing the measurement of the hysteresis loop of large-size weak magnetic objects, and solving the problem that the current conventional measurement instruments cannot measure the low-field magnetic susceptibility of large-size objects.

[0006] The magnetic susceptibility measurement device based on a variable-turns coil and an atomic magnetometer provided by this application includes an atomic magnetometer, a magnetic field generation module, and a magnetic field compensation module; the magnetic field generation module includes two pairs of coils C1 and C2 with different radii and coaxial settings; the magnetic fields generated by coils C1 and C2 at the gradient zero point are equal in magnitude and opposite in direction; the atomic magnetometer is arranged at the gradient zero point for measuring the low-field magnetic susceptibility of the sample to be measured; the magnetic field compensation module includes a pair of coils C3 with adjustable turns for realizing magnetic field compensation by changing the number of turns of coil C3.

[0007] In this application, coils C1, C2, and C3 are connected in series; coil C3 is coaxially arranged outside coil C1; the geometric centers of coils C1, C2, and C3 coincide.

[0008] Further preferably, the number of turns and radii of coils C1 and C2 satisfy the following relationship: , where N 1 represents the number of turns of coil C1, R 1 represents the radius of coil C1, N 2 represents the number of turns of coil C2, R 2 represents the radius of coil C2.

[0009] In this application, the change in the number of turns of coil C3 is achieved by rotating coil C3 around the axis through a bearing installed at the center of coil C3.

[0010] Among them, the rotation angle of coil C3 and the corresponding magnetic field change amount satisfy the following relationship: ; where

[0011] Further preferably, the magnetic field compensation module further includes a conductive reed, a conductive ring, and a wire winding coil T7; one end of the conductive reed is used to connect to a power source, and the other end is pressed against the conductive ring by an elastic force, so that there is always current passing between the two; the conductive ring is inlaid on the coil skeleton, the head end of the wire wound around the coil C3 is connected to the conductive ring, after winding a certain number of turns, the tail end of the wire is then wound around the wire winding coil T7; the wire winding coil T7 is arranged directly below the coil C3, and the two are connected together by a transmission belt to achieve synchronous rotation.

[0012] Further preferably, the magnetic susceptibility measurement device further includes a displacement module and a transmission module; the displacement module includes a displacement stage T1 and a displacement stage T2. The displacement stage T1 adjusts the relative position between the atomic magnetometer and the coil C1 to make the atomic magnetometer at the gradient zero point of the coil C1; the displacement stage T2 adjusts the relative position between the coil C2 and the atomic magnetometer to make the atomic magnetometer at the gradient zero point of the coil C2; thereby suppressing the magnetic field gradient within the gradient tolerance of the atomic magnetometer; the transmission module includes a conveyor belt, a rotating shaft, and a coupling. The conveyor belt is used to connect the coil C3 and the wire winding coil T7, so that the coil C3 can rotate precisely by a certain angle and generate a precise compensation magnetic field; the rotating shaft is used to connect the two coils in the coil C3 to enable the two coils to rotate synchronously; the coupling is used to control the connection and disconnection between the rotating shafts of the two coils.

[0013] This application also provides a magnetic susceptibility measurement method implemented based on the above magnetic susceptibility measurement device, including the following steps:

[0014] S1 Align the gradient zero points by adjusting the coincidence of the gradient zero points of the coil C1, the gradient zero point of the coil C2, and the position of the atomic magnetometer.

[0015] S2 Reduce the background magnetic field magnitude at the gradient zero point through magnetic field compensation.

[0016] S3 Measure the induced magnetic field of the sample by the atomic magnetometer, and obtain the low-field magnetic susceptibility of the sample to be measured according to the relationship between the induced magnetic field and the magnetic susceptibility of the sample of the sample.

[0017] Among them, step S1 is specifically:

[0018] S11 Pass a reverse current through the coil C1 to generate a centrally symmetric gradient magnetic field.

[0019] S12 Move the atomic magnetometer to reduce the magnetic field amplitude, and when the magnetic field amplitudes in all three directions reach the minimum value, the atomic magnetometer is already at the geometric center of the coil C1, thus completing the alignment of the atomic magnetometer and the gradient zero point of the coil C1.

[0020] S13 keeps the atomic magnetometer stationary and cuts off the power supply of the coil C1. After passing a reverse current through the coil C2, the coil C2 is moved to reduce the magnetic field amplitude. When the magnetic field amplitudes in all three directions reach the minimum, the atomic magnetometer is already at the gradient zero point of the coil C2, thereby realizing the coincidence of the gradient zero point of the coil C1, the gradient zero point of the coil C2, and the position of the atomic magnetometer.

[0021] Among them, step S2 is specifically as follows:

[0022] S21 Pass a first current in the same direction through the coil C1 and a second current in the same direction through the coil C2 to generate uniform magnetic fields in opposite directions; the magnitudes of the first current and the second current are the same and the directions are opposite;

[0023] When the coil C3 is not connected in series, a third current in the mA level is passed through the coil C1 and the coil C2 , and the third current is obtained by measuring the residual magnetic field value under the third current through the atomic magnetometer The residual magnetic field value generated ;

[0024] S22 When the coil C3 is connected in series and the coils C1 and C2 are disconnected, the third current is passed through the coil C3 , and the number of turns of the coil C3 is adjusted so that the magnetic field value generated by the coil C3 is close to the residual magnetic field value;

[0025] S23 When it is impossible to further reduce by adjusting the number of turns of the coil C3, the differential gear is connected to the coil rotating shaft by a coupling, and the coils C1, C2, and C3 are connected in series and a fourth current is passed through them to reduce the residual magnetic field;

[0026] S24 The residual magnetic field is continuously reduced by continuously increasing the value of the fourth current and continuously adjusting the number of turns of the coil C3 until the fourth current reaches the current required for magnetic susceptibility measurement ;

[0027] S25 When the current reaches , a signal generator is used to generate a current I , and the time-domain magnetic field data after passing the current I for a period of time is collected. After performing a frequency-domain conversion on the time-domain magnetic field data, it is determined whether the atomic magnetometer is in a normal working state by judging whether there is an obvious signal peak at the frequency f . If so, it proceeds to step S3;

[0028] Among them, , and f are respectively the amplitude and frequency of the sine signal.

[0029] Through the above technical solutions conceived by this application, compared with the prior art, since this application uses a variable-turn compensation coil C3 and an atomic magnetometer for magnetic susceptibility measurement, when the magnetic field gradient of the coil residual magnetic field at the measurement point where the atomic magnetometer is located is within the gradient tolerance of the atomic magnetometer, the coil residual magnetic field at the measurement point is compensated, so that the coil residual magnetic field at the measurement point is within the measurement range of the atomic magnetometer; at the same time, by changing the number of turns of the compensation coil, the compensation magnetic field is accurately adjusted, so that the measurement accuracy of the magnetic susceptibility is improved, and the magnetic susceptibility of large-size samples to be measured such as low-field magnetic susceptibility for inspection quality can be measured. Description of the Drawings

[0030] Figure 1 is a schematic structural diagram of a magnetic susceptibility measurement device based on a variable-turn coil and an atomic magnetometer provided by an embodiment of this application;

[0031] Figure 2 is a schematic structural diagram of a displacement stage, a differential gear, a wire-winding coil, a manual turntable and a coupling in the magnetic susceptibility measurement device provided by an embodiment of this application;

[0032] Figure 3 is a schematic structural diagram of a transmission belt, an atomic magnetometer support rod and a sample stage mounting bracket in the magnetic susceptibility measurement device provided by an embodiment of this application;

[0033] Figure 4 is a schematic principle diagram of the compensation coil C3 provided by an embodiment of this application;

[0034] Figure 5 is the coil rotation angle change amount and the magnetic field change amount measurement result schematic diagram;

[0035] Figure 6 is an implementation flowchart of a magnetic susceptibility measurement method based on a variable-turn coil and an atomic magnetometer provided by an embodiment of this application;

[0036] Figure 7 is a schematic diagram of the experimental results of the sample magnetization curve measurement provided by an embodiment of this application.

[0037] Among them, C1 represents a large Helmholtz coil, C2 represents a small Helmholtz coil, C3 represents a variable turns compensation coil, T1 represents an atomic magnetometer displacement stage, T2 represents a coil C2 displacement stage, T3 represents a conductive spring, T4 represents a sample stage, T5 represents an atomic magnetometer, T6 represents a differential gear, T7 represents a take-up coil, T8 represents a manual turntable, T9 represents a coupling, T10 represents a transmission belt, T11 represents an atomic magnetometer support rod, T12 represents a sample stage mounting bracket, 1 represents a power supply, 2 represents a conductive spring, 3 represents a bearing, 4 represents a compensation coil winding frame, 5 represents a conductive ring, 6 represents a suspended wire, and 7 represents a winding frame of the take-up coil T7. DETAILED DESCRIPTION

[0038] In order to make the purpose, technical solution and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0039] The present application utilizes a high-precision, small-volume atomic magnetometer to measure the low-field magnetic susceptibility of large-sized objects such as proof mass, thereby improving the measurement accuracy of the proof mass magnetic susceptibility, reducing the required external magnetic field strength, and realizing the measurement of the hysteresis loop of large-sized weakly magnetic objects, thereby solving the problem that current conventional measuring instruments cannot measure the low-field magnetic susceptibility of large-sized objects.

[0040] like Figures 1 to 3 As shown, the magnetic susceptibility measuring device based on variable-turn coils and atomic magnetometer provided in the present application includes an atomic magnetometer, a magnetic field generating module and a magnetic field compensation module; the magnetic field generating module includes two pairs of coils C1 and coils C2 with different radii and coaxially arranged; the magnetic fields generated by coils C1 and coils C2 at the gradient zero point are equal in size and opposite in direction; the atomic magnetometer is arranged at the gradient zero point, and is used to measure the low-field magnetic susceptibility of the sample to be measured; the magnetic field compensation module includes a pair of coils C3 with adjustable turns, which are used to achieve magnetic field compensation by changing the number of turns of coil C3.

[0041] The atomic magnetometer can use an optically pumped atomic magnetometer with a measurement accuracy of up to the fT level. The atomic magnetometer can measure the magnetic field in the three directions of x, y, and z.

[0042] Among them, both the coil C1 and the coil C2 can adopt Helmholtz coils, and the radius of the Helmholtz coil C1 is greater than that of the Helmholtz coil C2. The geometric centers of the coil C1 and the coil C2 coincide, so the magnetic field gradient zero points of the two coincide. After the magnetic fields of the two are superimposed, a point with a magnetic field gradient of zero is generated at the geometric center, which is called the gradient zero point. Placing the atomic magnetometer at this gradient zero point for measurement can ensure that the magnetic field gradient does not exceed the gradient tolerance of the atomic magnetometer, and at the same time can greatly reduce the magnetic field noise caused by the coupling of mechanical vibration and magnetic field gradient. At the same time, in order to make the magnetic field at the gradient zero point small enough so that the residual magnetic field of the coil is within the range of the atomic magnetometer, the gradient zero point also needs to be a magnetic field zero point at the same time, which requires the magnetic fields generated by the coil C1 and the coil C2 at the gradient zero point to be equal in magnitude and opposite in direction. Therefore, when designing the coil in this application, it is necessary to ensure that the number of turns N and the radius R of the coil approximately satisfy the following relationship: , where, N 1 represents the number of turns of the coil C1, R 1 represents the radius of the coil C1, N 2 represents the number of turns of the coil C2, R 2 represents the radius of the coil C2.

[0043] In this application, since the radius becomes a fixed value after the Helmholtz coil is processed, then is equal to the fixed value K. Therefore, to make the central magnetic fields generated by the coil C1 and the coil C2 equal in magnitude, it is necessary to precisely adjust to make it infinitely close to K, so that the residual magnetic field is infinitely close to zero. However, the current processing accuracy of the coil can only meet Close to 99.9% K, which means that after the mutual cancellation of coil C1 and coil C2, the central magnetic field is about one-thousandth of the magnetic field of coil C1. In order to generate a sufficiently strong magnetization signal for the sample, the central magnetic field value of coil C1 needs to reach the mT level. Therefore, the residual magnetic field reaches the μT level, while the background magnetic field of the atomic magnetometer needs to be controlled below a few nT. Therefore, the residual magnetic field needs to be reduced by more than three orders of magnitude to meet the normal working range of the atomic magnetometer. The compensation methods for the residual magnetic field can be divided into current compensation method and voltage compensation method. The voltage compensation method is mentioned in the prior art (CN 115718273 B), that is, by adjusting the voltage to change the current in the coil so as to adjust the residual magnetic field of the coil. This method is applicable to the situation where the measurement accuracy requirements for the induced magnetic field are not high. When the accuracy needs to be further improved, since the currents of the compensation coil and the magnetic field generating coil in the voltage compensation method are not completely coherent, the magnetic field noise caused by the power supply noise cannot be completely cancelled, resulting in the inability to further improve the accuracy of this method. Therefore, to further improve the measurement accuracy, all coils need to be connected in series. After the coils are connected in series, the magnitude of the compensation magnetic field can only be adjusted by changing the position and number of turns of the coil in these two ways. However, the method of changing the coil position has the following two deficiencies. First, due to the limited movement accuracy of a single compensation coil, in order to achieve a higher magnetic field compensation accuracy, a multi-stage compensation method needs to be adopted, using multiple compensation coils, so that the distances from the compensation coils to the gradient zero point increase in sequence. However, since the entire coil system is placed in a magnetic shielding room for measurement and the size of the shielding room is limited, it is impossible to meet the requirement of placing multiple compensation coils. Second, changing the position of the compensation coil will also cause the position of the gradient zero point of the compensation coil to change, and the gradient zero point of the compensation coil no longer coincides with the gradient zero point of the Helmholtz coil, thereby increasing the magnetic field gradient at the atomic magnetometer. Changing the number of turns of the coil can avoid the above deficiencies, so it is more suitable for adjusting the magnitude of the compensation magnetic field.

[0044] In the embodiment of the present application, as Figure 4 shown, coil C3 is a pair of variable-turn magnetic field compensation coils, which are arranged outside coil C1, coaxial with coil C1, and the geometric center of coil C3 coincides with the geometric center of coil C1, so that the gradient zero point of coil C3 coincides with the gradient zero point of coil C1.

[0045] The change in the number of turns of coil C3 is achieved by rotating coil C3 around the axis through a bearing installed at the center of coil C3. Specifically, the number of turns of the coil can be precisely adjusted through a coil rotation mechanism, so as to precisely adjust the magnitude of the compensation magnetic field and suppress the coil residual magnetic field within the range of the atomic magnetometer.

[0046] Among them, the bearing can be made of non-magnetic materials such as plastic or ceramic.

[0047] In addition, asFigure 5 As shown, the angle by which the coil C3 rotates satisfies the following relationship: ; where represents the magnetic field generated by a single-turn coil.

[0048] Since the rotation of the coil causes the input and output ends of the wire to rotate along with it, the position of the wire that is not wound around the coil skeleton (referred to as the floating wire) changes. And because the position of the floating wire is not fixed, the generated magnetic field cannot be precisely controlled, resulting in the magnetic field change not satisfying the linear relationship with the coil rotation angle anymore. Thus, it is impossible to precisely adjust the magnetic field generated by the coil C3 by precisely adjusting the rotation angle. Therefore, a conductive reed and an additional take-up coil T7 can be used to solve this problem. The conductive reed is fixed on the mounting platform of the coil C3 and remains stationary during the rotation of the coil C3. One end of the conductive reed is connected to the power supply, and the other end presses against the conductive ring by elastic force, so that there is always current passing between the two. The conductive ring is embedded in the coil skeleton. The leading end of the wire wound around the coil C3 is connected to the conductive ring. After winding a certain number of turns, the trailing end of the wire is then wound around another rotatable coil identical to the C3 coil, that is, the take-up coil T7. The take-up coil T7 is installed directly below the coil C3, and the two are connected together by a transmission belt to achieve synchronous rotation. When the number of turns of the coil C3 increases, the number of turns of the coil T7 decreases, and the change amounts of their numbers of turns satisfy the relationship of being equal in magnitude and opposite in direction. Therefore, the floating wire between the two can always maintain a stretched state without changing its position. Since the coil T7 is far enough from the central region of the Helmholtz coil, its contribution to the compensation magnetic field can be ignored. Therefore, the compensation magnetic field is adjusted by the change in the number of turns of the coil C3.

[0049] In this application, the magnetic field compensation module further includes a conductive reed, a conductive ring, and a take-up coil T7; one end of the conductive reed is used to connect to the power supply, and the other end presses against the conductive ring by elastic force, so that there is always current passing between the two; the conductive ring is embedded in the coil skeleton, the leading end of the wire wound around the coil C3 is connected to the conductive ring, and after winding a certain number of turns, the trailing end of the wire is then wound around the take-up coil T7; the take-up coil T7 is arranged directly below the coil C3, and the two are connected together by a transmission belt to achieve synchronous rotation.

[0050] In this application, the magnetic susceptibility measurement device further includes a displacement module and a transmission module; the displacement module is used to precisely adjust the relative position between the atomic magnetometer and the coil, so that the measurement unit of the atomic magnetometer is in the region with the minimum magnetic field gradient, thereby suppressing the magnetic field gradient within the gradient tolerance of the atomic magnetometer. Since the precision displacement stage contains weakly magnetic materials, in order to avoid the magnetic properties of the displacement stage interfering with the measurement results, the atomic magnetometer displacement stage is placed in a region far from the atomic magnetometer, and the displacement stage and the atomic magnetometer are rigidly connected by a non-magnetic support rod.

[0051] Among them, the displacement module includes a displacement stage T1 and a displacement stage T2. The displacement stage T1 adjusts the relative position between the atomic magnetometer and the coil C1, so that the atomic magnetometer is at the gradient zero point of the coil C1; the displacement stage T2 adjusts the relative position between the coil C2 and the atomic magnetometer, so that the atomic magnetometer is at the gradient zero point of the coil C2; thereby suppressing the magnetic field gradient within the gradient tolerance of the atomic magnetometer.

[0052] The transmission module includes a conveyor belt, a rotating shaft and a coupling. The conveyor belt is used to connect the coil C3 and the take-up coil T7, so that the coil C3 can rotate precisely by a certain angle and generate a precise compensation magnetic field; the rotating shaft is used to connect the two coils in the coil C3, so that the two coils can rotate synchronously; the coupling is used to control the connection and disconnection between the rotating shafts of the two coils.

[0053] The magnetic susceptibility measurement device in this application further includes a differential speed module, a sample mechanism and a magnetic shielding mechanism. Among them, the differential speed module includes a number of differential speed gears with different radii and a coupling. The differential speed gears are used to change the speed ratio between the manual turntable and the coil C3, so that the large-angle and low-precision rotation of the manual turntable is converted into the small-angle and high-precision rotation of the coil C3, thereby improving the coil rotation resolution, and further improving the magnetic field adjustment resolution, so that the residual magnetic field of the coil is adjusted to a lower level to meet the measurement range of the atomic magnetometer. The coupling is used to control the connection and disconnection between the rotating shaft of the differential speed gear and the rotating shaft of the coil, so as to realize the switching between large-angle low-precision rotation and small-angle high-precision rotation. The sample mechanism is used to place the sample to be measured. The sample placement mechanism includes a sample stage and a bracket. The bracket is not in contact with the coil system, but is directly connected to the ground, so as to prevent the mechanical vibration during the placement or movement of the sample from being transmitted to the atomic magnetometer and causing magnetic field fluctuation noise. The magnetic shielding mechanism is used to shield environmental magnetic fields such as the geomagnetic field, so that the magnitude of the background magnetic field meets the workload range of the atomic magnetometer, and the background noise meets the requirements of the magnetic field measurement resolution.

[0054] The magnetic susceptibility measurement device provided by this application precisely compensates for the residual magnetic field and magnetic field gradient of the coil at the atomic magnetometer by precisely adjusting the number of coil turns, enabling the atomic magnetometer to operate within the range and gradient tolerance, and at the same time, precisely adjusting the compensation magnetic field using the high-precision coil C3 to improve the magnetic susceptibility measurement accuracy. This application solves the problem that the presence of power supply noise in the existing measurement method prevents the further improvement of magnetic field measurement accuracy. At the same time, an atomic magnetometer is used for inductive magnetic field measurement to improve the magnetic field measurement accuracy, so that the low-field magnetic susceptibility of large-size samples such as inspection masses can be measured.

[0055] As Figure 6 shown, this application also provides a magnetic susceptibility measurement method implemented based on the above magnetic susceptibility measurement device, including the following steps:

[0056] S1 Align the gradient zero points by adjusting the gradient zero points of coil C1, the gradient zero point of coil C2, and the position of the atomic magnetometer to coincide.

[0057] S2 Reduce the background magnetic field magnitude at the gradient zero point through magnetic field compensation.

[0058] S3 Measure the induced magnetic field of the sample by the atomic magnetometer, and obtain the low-field magnetic susceptibility of the sample to be measured according to the relationship between the induced magnetic field and the sample magnetic susceptibility .

[0059] Among them, step S1 is specifically:

[0060] S11 Pass a reverse current through coil C1 to generate a centrally symmetric gradient magnetic field.

[0061] S12 Move the atomic magnetometer to reduce the magnetic field amplitude. When the magnetic field amplitudes in all three directions reach the minimum value, the atomic magnetometer is already at the geometric center of coil C1, thus completing the alignment of the gradient zero point of the atomic magnetometer and coil C1.

[0062] S13 Keep the atomic magnetometer stationary and cut off the power supply of coil C1. After passing a reverse current through coil C2, move coil C2 to reduce the magnetic field amplitude. When the magnetic field amplitudes in all three directions reach the minimum value, the atomic magnetometer is already at the gradient zero point of coil C2, thus realizing the coincidence of the gradient zero points of coil C1, the gradient zero point of coil C2, and the position of the atomic magnetometer.

[0063] In the embodiment of this application, the specific operation process for making the gradient zero points of coil C1, the gradient zero point of coil C2, and the sensitive unit of the atomic magnetometer coincide is as follows:

[0064] (1) Install the atomic magnetometer on the translation stage T1. During installation, it is necessary to ensure that the measurement axis of the atomic magnetometer remains parallel to the axis of the coil. A pair of coils C1 are fed with reverse current and start recording magnetic field data, generating a centrally symmetrical gradient magnetic field. It should be pointed out that since the magnetic field at this time is a gradient magnetic field, and the magnetic field generated by the coil is only used for positioning, and not for measuring the sample magnetization signal, the current applied to the coil at this time needs to be as small as possible while meeting the magnetic field measurement resolution of the atomic magnetometer (the difference between the magnetic field generated by the coil and the magnetic field measurement resolution of the atomic magnetometer can be 10%), so that the magnetic field gradient does not exceed the gradient tolerance of the atomic magnetometer.

[0065] (2) After the coil is powered on and the atomic magnetometer outputs a magnetic field waveform corresponding to the current waveform, the atomic magnetometer is moved toward the positive direction of the z-axis with a certain step length (millimeter step length) using the displacement stage T1. If the magnetic field amplitude increases, the step length is increased and the movement is made toward the negative direction of the z-axis. If the magnetic field amplitude decreases, the step length is reduced and the movement is made toward the negative direction of the z-axis. This process is then repeated (micrometer step length) until the magnetic field amplitude can no longer be reduced by moving the atomic magnetometer.

[0066] (3) Move the atomic magnetometer in the y and z directions respectively according to the above method until the magnetic field amplitude reaches the minimum value (the minimum moving step that the translation stage can achieve). When the magnetic field amplitudes in the three directions all reach the minimum value, it means that the measurement unit of the atomic magnetometer is now at the geometric center point of coil C1, that is, the gradient zero point when the coil C1 is passing current in the same direction. It should be pointed out that due to the coupling between the magnetic fields of the three axes, the above adjustment process may need to be repeated several times before the magnetic fields of the three axes reach the minimum value. At this point, the alignment of the atomic magnetometer measurement unit and the gradient zero point of coil C1 is completed.

[0067] (4) The atomic magnetometer remains stationary, coil C1 is powered off, and reverse current is passed through coil C2. According to the method of moving the atomic magnetometer in step 2, coil C2 is moved using displacement stage T2 until the magnetic field amplitudes in the three directions are all minimized, indicating that the measurement unit of the atomic magnetometer is now at the gradient zero point of coil C2. The operations of steps 2 and 3 place the atomic magnetometer at the gradient zero point of coil C1. Therefore, at this time, the atomic magnetometer measurement unit, the gradient zero point of coil C1, and the gradient zero point of coil C2 are aligned, indicating that the atomic magnetometer is in the measurement state with the minimum external magnetic field gradient, which means that the gradient tolerance index of the atomic magnetometer is met.

[0068] In this application, step S2 is specifically:

[0069] Apply a first current in the same direction to coil C1 and a second current in the same direction to coil C2 to generate uniform magnetic fields with opposite directions; wherein the magnitudes of the first current and the second current are the same and their directions are opposite.

[0070] When coil C3 is not connected in series, apply a third current in the milliamperes range to coil C1 and coil C2 and measure the residual magnetic field value under the third current through an atomic magnetometer to obtain the residual magnetic field value generated by the third current ;

[0071] S22 When coil C3 is connected in series and coils C1 and C2 are disconnected, apply the third current to coil C3 and adjust the number of turns of coil C3 so that the magnetic field value generated by coil C3 is close to the residual magnetic field value

[0072] S23 When it is impossible to further reduce by adjusting the number of turns of coil C3, connect the differential gear to the coil shaft using a coupling, and connect coils C1, C2, and C3 in series and apply a fourth current to reduce the residual magnetic field;

[0073] S24 Continuously reduce the residual magnetic field by continuously increasing the value of the fourth current and continuously adjusting the number of turns of coil C3 until the fourth current reaches the current required for magnetic susceptibility measurement ;

[0074] S25 When the current reaches , use a signal generator to generate a current I , and collect the time-domain magnetic field data after applying the current I for a period of time. After performing a frequency-domain conversion on the time-domain magnetic field data, determine whether the atomic magnetometer is in a normal working state by judging whether there is an obvious signal peak at the frequency f . If so, proceed to step S3;

[0075] Among them, , and f are the amplitude and frequency of the sine signal respectively.

[0076] In the embodiment of the present application, the method for reducing the background magnetic field magnitude at the gradient zero point in step S2 can be specifically operated according to the following method:

[0077] (1) After completing the alignment of the gradient zero point, by changing the coils C1 and C2 to the uniform field mode (the current directions in the two coils C1 are the same, the current directions in the two coils C2 are the same, and the current direction in coil C1 is opposite to that in coil C2), and at the same time, the directions of the uniform fields generated by coils C1 and C2 are opposite. Since the processing accuracy of the coils cannot ensure that the central magnetic fields generated by the two are strictly equal, the central magnetic fields cannot be completely cancelled out, resulting in at least of residual magnetic field existing, and for the atomic magnetometer to work properly, this magnetic field needs to be lower than . To compensate for this residual magnetic field, first, without connecting the compensation coil, apply a small current (in the order of milliamperes) to coils C1 and C2, and use the atomic magnetometer to measure the value of the residual magnetic field under this small current, obtaining the value of the residual magnetic field generated .

[0078] (2) Disconnect the current in coils C1 and C2, and pass current through coil C3 , and use the rotating device to adjust the number of turns of coil C3. First, perform a rough adjustment to make the magnetic field value close to . At this time, usually a relatively large number of turns of coil C3 need to be changed, and the presence of the differential gear will make the rotation speed of the coil very slow, which is not conducive to large-angle adjustment (any angle above 10° is called a large angle). Therefore, it is necessary to use a coupling to disconnect the connection between the differential gear and the coil shaft so that the coil can rotate freely.

[0079] (3) When the magnetic field value generated by coil C3 is close to , and it cannot be further reduced by rotation, start to connect the differential gear and the coil shaft using the coupling. Then connect coils C1, C2, and C3 in series and pass current through them. When wiring, it is necessary to ensure that the current flow direction can make the magnetic field opposite to the direction of the magnetic field generated by and . At this time, the residual magnetic field is preliminarily compensated by coil C3, reducing by about one order of magnitude. Then increase the current to , and observe the value of the residual magnetic field . Then start to finely adjust the rotation angle of coil C3 clockwise through the manual turntable and observe the change in the value of the residual magnetic field. If the residual magnetic field decreases, continue to adjust clockwise; otherwise, adjust counterclockwise to continuously reduce the residual magnetic field.

[0080] (4) When the value of the residual magnetic field is reduced to about , increase the current to , the residual magnetic field value will increase by 10 times, and then rotate the coil according to the method in Step 3 to continuously reduce the residual magnetic field. Then, continuously increase the current and reduce the residual magnetic field by rotating the coil until the current reaches the current required for susceptibility measurement. . When the current reaches , in order to verify whether the measurement conditions are met at this time, that is, whether the atomic magnetometer can work properly in the magnetic field generated by the current and whether the magnetic field background noise meets the measurement requirements, use a signal generator to generate a current , where and f are the amplitude and frequency of the sine signal respectively. Assume that the magnetic field measurement resolution of the atomic magnetometer is , and the value of the current amplitude should satisfy: The magnitude of the residual magnetic field generated by is close to , and then by judging whether the atomic magnetometer can successfully detect a weak magnetic field signal close to the measurement resolution when it is in the residual magnetic field generated by a large current , to determine whether the atomic magnetometer is in a normal working state.

[0081] (5) Measure the time-domain magnetic field data after passing the current for a period of time, perform frequency-domain conversion on the time-domain data, and observe whether there is an obvious signal peak at the frequency f. If this peak appears, it proves that the atomic magnetometer has achieved the measurement of the modulation signal . In order to increase the confidence in the measured modulation signal, change the frequency f , keep the current unchanged, measure the magnetic field data at more than 5 different frequency values. If similar signal peaks can be obtained at different frequencies, it proves that the atomic magnetometer has successfully achieved the precise measurement of the weak magnetic field under the condition of passing a large current through the coil. Therefore, the atomic magnetometer is in a working state where it can measure the magnetization magnetic field of the sample.

[0082] In the embodiment of the present application, the specific operation process of Step S3 is as follows:

[0083] (1)Measure the induced magnetic field of the sample. There are two measurement modes for the sample, namely the dynamic field measurement mode (liquid samples are not suitable for movement, and the magnetic field must change with time, such as a square wave magnetic field) and the static field measurement mode (solid samples are suitable for movement, and the magnetic field does not change with time). In the dynamic field measurement mode, the sample remains stationary, and for DC susceptibility measurement, the waveform of the applied magnetic field is generally a square wave. It should be noted that the slope of the rising edge of the square wave magnetic field is very large, corresponding to an extremely high magnetic field change rate. According to Maxwell's equations , a changing magnetic field will generate an induced electromotive force in the conductive sample and thus form eddy currents. The presence of eddy currents will cause the sample to generate an additional eddy current magnetic field. Experiments have found that directly applying a square wave magnetic field will cause the eddy current magnetic field to be too large, resulting in the magnetic field exceeding the range of the atomic magnetometer, thereby affecting the performance of the atomic magnetometer. Therefore, it is necessary to optimize the square wave to reduce the slope of the rising edge and then the falling edge, making it into a trapezoidal wave, which can solve the over-range problem. Assume that the difference between the high and low levels of the magnetic field measured without a sample is , and the difference between the high and low levels of the magnetic field measured when the sample is placed near the atomic magnetometer is , then the induced magnetic field generated by the sample can be obtained as . In the static field measurement mode, a constant current is applied to the coil, and the external magnetic field does not change with the application. Then, the magnetic field change can be obtained in two cases where the sample approaches and moves away from the atomic magnetometer. Thus, the induced magnetic field generated by the sample is .

[0084] (2)Perform a sweep field measurement on the sample. To complete a hysteresis loop measurement, the current changes in the following way: . The susceptibility of the sample at different field strengths can be obtained from the slope of the magnetization curve. For a uniformly magnetized isotropic sample with volume V, the relationship between the induced magnetic field generated by the sample and the susceptibility of the sample is:

[0085]

[0086] where represents the unit current, represents the magnetic field applied to the sample by the coil when the current is , The calculation formula is a general formula in the field and will not be elaborated here. R is the position vector between a point on the sample and the measurement unit of the atomic magnetometer, and C is a constant related to the size and shape of the sample and the relative position between the sample and the atomic magnetometer. The value of C can be obtained by measuring the relative position R and substituting it into the above formula for calculation, or it can be calibrated by measuring a standard sample with a known magnetic susceptibility. Then the magnetic susceptibility of the sample The relationship with the slope K of the measured hysteresis loop is as follows: .

[0087] To further verify the reliability of the magnetic susceptibility measurement device and method of the present application, Figure 1 The entire device shown was placed in a zero magnetic chamber, and the low-field magnetization curves of two cubic samples with dimensions of were measured. The samples were composed of titanium and tungsten with a purity of 99.99%. The minimum magnetic field applied to the center of the samples was , which is much smaller than the geomagnetic field level and the external magnetic field level that the current test instruments can achieve, realizing the low-field magnetic susceptibility measurement conditions. The measured magnetization curves are as shown in Figure 7 . Since the measured samples are high-purity metals with extremely low ferromagnetic impurity content or the samples have a large coercive force, no obvious hysteresis phenomenon was observed. The magnetization curves around were linearly fitted, and the magnetic susceptibilities of the samples were calculated according to the obtained slopes and are respectively: , , which is in good agreement with the reference values reported in the literature, proving the reliability of the device and measurement method of the present application.

[0088] It is easy for those skilled in the art to understand that the above are only the preferred embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A magnetic susceptibility measuring device based on a variable turns coil and an atomic magnetometer, characterized in that: It includes an atomic magnetometer, a magnetic field generating module and a magnetic field compensating module; The magnetic field generating module comprises two pairs of coils C1 and C2 with different radii and arranged coaxially; the magnetic fields generated by the coils C1 and C2 at the gradient zero point are equal in magnitude and opposite in direction; The atomic magnetometer is set at the gradient zero point and is used to measure the low-field magnetic susceptibility of the sample to be tested; The magnetic field compensation module includes a pair of coils C3 with adjustable turns, which are used to achieve magnetic field compensation by changing the turns of the coils C3; The coil C1, the coil C2 and the coil C3 are connected in series; The coil C3 is coaxially arranged outside the coil C1; The geometric centers of the coil C1 , the coil C2 and the coil C3 coincide with each other.

2. The magnetic susceptibility measuring device according to claim 1, characterized in that: The number of turns and radius of the coil C1 and the coil C2 satisfy the following relationship: ,in, N 1 represents the number of turns of coil C1, R 1 represents the radius of coil C1, N 2 represents the number of turns of coil C2, R 2 represents the radius of coil C2.

3. The magnetic susceptibility measuring device according to claim 1, characterized in that: The change of the number of turns of the coil C3 is achieved by rotating the coil C3 around its axis via a bearing installed at the center of the coil C3.

4. The magnetic susceptibility measuring device according to claim 3, characterized in that: The angle of rotation of coil C3 The corresponding magnetic field change Satisfies the following relationship: ;in Represents the magnetic field generated by a single turn coil.

5. The magnetic susceptibility measuring device according to claim 3, characterized in that: The magnetic field compensation module also includes a conductive reed, a conductive ring and a take-up coil T7; One end of the conductive spring is used to connect to a power source, and the other end is pressed against the conductive ring by elastic force, so that current always flows between the two. The conductive ring is embedded in the coil frame, and the head end of the wire wound on the coil C3 is connected to the conductive ring. After a certain number of turns, the end of the wire is wound on the take-up coil T7; The wire-receiving coil T7 is arranged directly below the coil C3, and the two are connected together by a transmission belt to achieve synchronous rotation.

6. The magnetic susceptibility measuring device according to any one of claims 1 to 5, characterized in that: The magnetic susceptibility measuring device also includes a displacement module and a transmission module; The displacement module includes a displacement stage T1 and a displacement stage T2. The displacement stage T1 adjusts the relative position of the atomic magnetometer and the coil C1 so that the atomic magnetometer is at the gradient zero point of the coil C1. The displacement stage T2 adjusts the relative position of the coil C2 and the atomic magnetometer so that the atomic magnetometer is at the gradient zero point of the coil C2. Thus, the magnetic field gradient is suppressed within the gradient tolerance of the atomic magnetometer. The transmission module includes a conveyor belt, a rotating shaft and a coupling. The conveyor belt is used to connect the coil C3 and the take-up coil T7, so that the coil C3 can accurately rotate a certain angle and generate an accurate compensation magnetic field; the rotating shaft is used to connect the two coils in the coil C3, so that the two coils can rotate synchronously; the coupling is used to control the connection and disconnection between the rotating shafts of the two coils.

7. A magnetic susceptibility measurement method implemented based on the magnetic susceptibility measurement device according to any one of claims 1 to 6, characterized in that: The steps include: S1 achieves the alignment of the gradient zero point by adjusting the gradient zero point of coil C1, the gradient zero point of coil C2 and the position of the atomic magnetometer; S2 reduces the background magnetic field at the gradient zero point by magnetic field compensation; S3 measures the induced magnetic field of the sample through an atomic magnetometer and The magnetic susceptibility of the sample The low-field magnetic susceptibility of the sample to be tested is obtained by the relationship between 8. The magnetic susceptibility measurement method according to claim 7, characterized in that: Step S1 is specifically as follows: S11 passes a reverse current through the coil C1 to generate a centrally symmetrical gradient magnetic field; S12: reducing the magnetic field amplitude by moving the atomic magnetometer, and when the magnetic field amplitudes in the three directions all reach minimum values, the atomic magnetometer is already at the geometric center of the coil C1, thereby completing the alignment of the atomic magnetometer with the gradient zero point of the coil C1; S13 keeps the atomic magnetometer stationary and the coil C1 is powered off, and after a reverse current is passed through the coil C2, the coil C2 is moved to reduce the magnetic field amplitude, and when the magnetic field amplitudes in three directions are all minimum, the atomic magnetometer is already at the gradient zero point of the coil C2, thereby achieving the coincidence of the gradient zero point of the coil C1, the gradient zero point of the coil C2 and the position of the atomic magnetometer.

9. The magnetic susceptibility measurement method according to claim 7, characterized in that: Step S2 is specifically as follows: S21: a first current in the same direction is passed through the coil C1 and a second current in the same direction is passed through the coil C2 to generate a uniform magnetic field in opposite directions; the first current and the second current have the same magnitude and opposite directions; When the coil C3 is not connected in series, a third current of the order of milliamperes is passed through the coil C1 and the coil C2. and obtain the third current by measuring the residual magnetic field value under the third current through the atomic magnetometer The residual magnetic field value generated ; S22 When the coil C3 is connected in series and the coils C1 and C2 are disconnected, the third current is passed to the coil C3 , and adjust the number of turns of coil C3 so that the magnetic field value generated by coil C3 With the residual magnetic field value near; S23 When the number of turns of coil C3 cannot be adjusted, To further reduce the residual magnetic field, the differential gear and the coil shaft are connected by a coupling, and the coils C1, C2 and C3 are connected in series and then a fourth current is passed through, thereby reducing the residual magnetic field. S24 continuously reduces the residual magnetic field by continuously increasing the value of the fourth current and continuously adjusting the number of turns of the coil C3 until the fourth current reaches the current required for magnetic susceptibility measurement. ; S25 When the current reaches When the signal generator generates current I , and collect the current for a period of time I After the time domain magnetic field data is converted into the frequency domain, the frequency f Whether an obvious signal peak appears at the position to determine whether the atomic magnetometer is in a normal working state, if so, proceed to step S3; in, , and f are the amplitude and frequency of the sinusoidal signal respectively.

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