Self-excited oscillation dynamic nuclear polarization magnetic sensor

Abstract

A self-excited oscillation type dynamic nuclear polarization magnetic sensor, the sensor includes: a high-frequency oscillator, which generates a frequency-adjustable radio frequency signal and acts on a probe; a probe, which outputs a sine wave signal with a variable amplitude; a differential amplifier, which amplifies the sine wave signal output by the probe; and a low-frequency feedback coil, which generates a rotating magnetic field under the action of the signal output by the differential amplifier. A frequency meter, which receives the output voltage signal of the differential amplifier and measures the frequency of the output voltage signal. The self-excited oscillation type dynamic nuclear polarization magnetic sensor of the present invention can continuously output a sine wave signal whose frequency is proportional to the measured magnetic field, and has the advantages of continuous magnetic field measurement, high accuracy, sensitivity and simple structure.

Description

Technical Field

[0001] The invention relates to the technical field of magnetic field measurement, and in particular to a self-excited oscillation type dynamic nuclear polarization magnetic sensor. Background Art

[0002] Weak magnetic field measurement technology is widely used in geophysical resource survey, aviation and navigation magnetic survey, industrial manufacturing and biomedicine and other fields. Magnetic sensors are the most important tools for detecting magnetic fields. From the first magnetometer to various magnetic sensors today, people have studied and produced them. At present, magnetic sensors can be divided into mechanical, Hall, superconducting, fluxgate, optical pumping and proton magnetic sensors according to their working principles. Each of the above sensors has its own advantages and disadvantages. For example, superconducting quantum interference magnetometers have extremely high sensitivity, optical pumping magnetic sensors have high sensitivity and accuracy, and proton magnetic sensors have been studied by experts and scholars in my country for a long time due to their high absolute accuracy. They have been widely used in commercial instruments.

[0003] Proton precession magnetic sensors can be divided into two types at present. One is the DC pulse polarization type, that is, the ordinary proton magnetic sensor. The macroscopic magnetic moment of the proton is made to perform Larmor precession motion around the measuring magnetic field by prepolarization. After the acquisition coil receives the precession signal, the size of the measuring magnetic field can be calculated. The DC pulse polarization type magnetic sensor has a low sensitivity due to its relatively small gyromagnetic magnetism, but has a high resolution. The other is the dynamic nuclear polarization type, that is, the Overhauser magnetic sensor. The Overhauser magnetic sensor, which is improved on the basis of the proton sensor, changes the original working material of the proton precession magnetic sensor and uses a free radical solution. By applying a radio frequency magnetic field, the electrons undergo electron paramagnetic resonance. Under the interaction between electrons and protons, the degree of proton excitation is greatly improved, thereby improving the signal-to-noise ratio of the sensor. The Overhauser magnetic sensor uses a high-frequency oscillator for high-frequency excitation, and its power consumption is greatly reduced compared to the DC pulse polarization type magnetic sensor.

[0004] The current Overhauser magnetic sensors are all RF-pulse excitation type. The principle of this excitation is that the proton system in the free radical solution is fully polarized under the action of the RF signal. Secondly, a DC pulse signal is applied to the sensor's inductor coil to generate a deflection magnetic field. The sensor will output a Larmor signal that decays exponentially with time. Repeated application of DC pulse signals can achieve intermittent measurement. This measurement method cannot meet the needs of continuous magnetic field measurement. Summary of the invention

[0005] In view of the intermittent measurement problem of the current radio frequency-pulse excitation Overhauser magnetic sensor, the present invention provides a self-excited oscillation dynamic nuclear polarization magnetic sensor, which can continuously output a sinusoidal wave signal with a frequency proportional to the measured magnetic field, and has the advantages of continuous magnetic field measurement, high accuracy, sensitivity and simple structure.

[0006] The technical solution adopted by the present invention is:

[0007] A self-excited oscillation dynamic nuclear polarization magnetic sensor, the sensor comprising:

[0008] A high frequency oscillator generates a frequency-adjustable radio frequency signal and acts on the probe;

[0009] The probe outputs a sine wave signal with varying amplitude;

[0010] The differential amplifier amplifies the sine wave signal output by the probe;

[0011] The low-frequency feedback coil generates a rotating magnetic field under the action of the signal output by the differential amplifier.

[0012] The sensor comprises a frequency meter, which receives the output voltage signal of the differential amplifier and measures the frequency of the output voltage signal.

[0013] The probe is connected to a differential amplifier, which is connected to a low-frequency feedback coil. The probe, the differential amplifier and the low-frequency feedback coil form a self-excited oscillation circuit. When a high-frequency oscillator generates a radio frequency signal that acts on the probe, the self-excited oscillation starts, and the probe outputs a sinusoidal wave signal with a gradually increasing signal amplitude.

[0014] The probe comprises a first resonant cavity and a second resonant cavity;

[0015] A first signal receiving coil is wound around the outside of the first resonant cavity. The winding direction of the first signal receiving coil is left-handed. The magnetization intensity exhibited by the first resonant cavity is positive. The positive magnetization intensity cuts the first signal receiving coil to generate a positive induced electromotive force.

[0016] A second signal receiving coil is wound outside the second resonant cavity. The winding direction of the second signal receiving coil is right-handed. The magnetization intensity exhibited by the second resonant cavity is positive. The positive magnetization intensity cuts the second signal receiving coil to generate a positive induced electromotive force.

[0017] The first signal receiving coil and the second signal receiving coil adopt a differential structure.

[0018] A low-frequency feedback coil is wound around the connection between the first resonant cavity and the second resonant cavity, and the winding direction of the low-frequency feedback coil is right-handed.

[0019] The first resonant cavity is a positively polarized resonant cavity, including a first coaxial glass bottle and an LC resonant circuit. A first free radical solution is provided in the first coaxial glass bottle to produce a positive dynamic nuclear polarization effect. A high-frequency coil and an adjustable capacitor constitute an LC resonant circuit. Under the action of a radio frequency signal generated by a high-frequency oscillator, a polarized magnetic field is generated to act on the first resonant cavity.

[0020] The first free radical solution is prepared from free radicals and a first organic solvent.

[0021] The second resonant cavity is a negatively polarized resonant cavity, including a second coaxial glass bottle and an LC resonant circuit. A second free radical solution is provided in the second coaxial glass bottle to produce a negative dynamic nuclear polarization effect. The high-frequency coil and the adjustable capacitor constitute an LC resonant circuit. Under the action of the radio frequency signal generated by the high-frequency oscillator 1, a polarized magnetic field is generated to act on the second resonant cavity.

[0022] The second free radical solution is prepared from free radicals and a second organic solvent.

[0023] The free radical is 4-carbonyl-tetramethylpiperidinyloxy free radical,

[0024] The first organic solvent is methanol; the second organic solvent is ethylene glycol dimethyl ether.

[0025] The resonant frequencies of the first resonant cavity and the second resonant cavity are the same as the frequency of the sensor output signal, that is, the frequency measured by the frequency meter;

[0026] The resonant frequencies of the first resonant cavity and the second resonant cavity are the same as the frequency at which electrons of the free radical solution undergo electron paramagnetic resonance;

[0027] The frequency of the signal emitted by the high-frequency oscillator to the first and second resonant cavities is the same as the frequency of the electron paramagnetic resonance of the electrons of the free radical solution;

[0028] The rotation frequency of the magnetic field generated by the low-frequency feedback coil is the same as the resonance frequency of the first resonant cavity and the second resonant cavity.

[0029] The present invention provides a self-excited oscillation dynamic nuclear polarization magnetic sensor, and the technical effects are as follows:

[0030] 1) The magnetic sensor of the present invention can perform continuous measurement for a long time. Under the action of the high-frequency oscillator, the proton system remains in an excited state, and under the action of the rotating magnetic field generated by the low-frequency feedback coil, the proton magnetization intensity continues to precess around the measuring magnetic field, and the sensor continuously outputs a sinusoidal wave signal with a frequency proportional to the measured magnetic field value.

[0031] 2) The magnetic sensor of the present invention has higher accuracy and sensitivity. The sensor adopts a dual-resonance cavity structure. Positive polarization and negative polarization occur at the same frequency. The dynamic nuclear polarization factors of the two resonant cavities reach the maximum value. A dual signal receiving coil is adopted. After differential amplification by a differential amplifier, the output signal amplitude is twice that of the differential amplifier.

[0032] 3) The magnetic sensor of the present invention has a simple structure. Compared with the Overhauser magnetometer, the magnetic sensor does not require a DC pulse generator and can generate a transverse deflection magnetic field through a self-excited oscillation circuit formed by itself. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a schematic diagram of the overall structure of the magnetic sensor of the present invention.

[0034] Figure 2 It is a schematic diagram of the probe structure of the magnetic sensor of the present invention.

[0035] Figure 3(1) is a magnetization vector diagram of the proton system.

[0036] FIG3(2) is a schematic diagram showing a proton system generating a magnetization intensity M0 in the same direction as the measured magnetic field B0.

[0037] Figure 3 (3) is a schematic diagram of the precession of the magnetization intensity M0 after a disturbance is applied to the proton system.

[0038] FIG3(4) is a schematic diagram of the positive dynamic nuclear polarization effect occurring in the first resonant cavity;

[0039] FIG3(5) is a schematic diagram of the negative dynamic nuclear polarization effect occurring in the second resonant cavity;

[0040] Figure 3 (6) is a schematic diagram of the magnetic field distribution in the first resonant cavity under nuclear magnetic resonance;

[0041] Figure 3(7) is a schematic diagram of the magnetic field distribution in the second resonant cavity under nuclear magnetic resonance.

[0042] Figure 3 (8) is a schematic diagram of the magnetic field distribution in the first resonant cavity of the oscillation circuit in equilibrium state;

[0043] Figure 3 (9) is a schematic diagram of the magnetic field distribution in the second resonant cavity in the equilibrium state of the oscillation circuit. DETAILED DESCRIPTION

[0044] like Figure 1 As shown, a self-excited oscillation dynamic nuclear polarization magnetic sensor, the sensor comprises:

[0045] A high frequency oscillator 1 generates a frequency-adjustable radio frequency signal and acts on a probe 2;

[0046] Probe 2, outputs a sine wave signal with varying amplitude;

[0047] The differential amplifier 3 amplifies the sine wave signal output by the probe 2;

[0048] The low-frequency feedback coil 13 generates a rotating magnetic field under the action of the signal output by the differential amplifier 3.

[0049] The frequency meter 4 receives the output voltage signal of the differential amplifier 3 and measures the frequency of the output voltage signal.

[0050] The probe 2 is connected to the differential amplifier 3, and the differential amplifier 3 is connected to the low-frequency feedback coil 13. The probe 2, the differential amplifier 3 and the low-frequency feedback coil 13 constitute a self-excited oscillation circuit; when the high-frequency oscillator 1 generates a radio frequency signal acting on the probe 2, the self-excited oscillation starts to oscillate, and the probe 2 outputs a sinusoidal signal with a gradually increasing signal amplitude. After amplification by the differential amplifier 3, the sensor outputs a sinusoidal signal, and the output signal frequency is proportional to the external magnetic field. The external magnetic field refers to the magnetic field measured by the magnetic sensor.

[0051] like Figure 2 As shown, the probe 2 includes a first resonant cavity 7 and a second resonant cavity 9;

[0052] A first signal receiving coil 11 is wound around the outside of the first resonant cavity 7. The winding direction of the first signal receiving coil 11 is left-handed. The magnetization intensity exhibited by the first resonant cavity 7 is positive. The positive magnetization intensity cuts the first signal receiving coil 11 to generate a positive induced electromotive force.

[0053] A second signal receiving coil 12 is wound around the outside of the second resonant cavity 9 . The winding direction of the second signal receiving coil 12 is right-handed. The magnetization intensity exhibited by the second resonant cavity 9 is positive. The positive magnetization intensity cuts the second signal receiving coil 12 to generate a positive induced electromotive force.

[0054] The first signal receiving coil 11 and the second signal receiving coil 12 adopt a differential structure, which can effectively reduce external electromagnetic interference and increase the output signal amplitude.

[0055] The receiving coil has two connection ends, namely the upper end and the lower end. The differential structure means that the two coils are connected in reverse series. The lower end of the first signal receiving coil 11 is connected to the lower end of the second receiving coil 12. The upper end of the first receiving coil 11 and the upper end of the second receiving coil 12 are the two output ends of the sensor.

[0056] The differential amplifier 3 amplifies the voltage signals output by the first and second signal receiving coils.

[0057] A low-frequency feedback coil 13 is wound around the connection between the first resonant cavity 7 and the second resonant cavity 9. The winding direction of the low-frequency feedback coil 13 is right-handed. Under the action of the voltage signal output by the differential amplifier 3, the low-frequency feedback coil 13 generates a rotating magnetic field.

[0058] The first resonant cavity 7 is a positively polarized resonant cavity, including a first coaxial glass bottle and an LC resonant circuit. A first free radical solution is provided in the first coaxial glass bottle to produce a positive dynamic nuclear polarization effect. The high-frequency coil 6 and the adjustable capacitor 5 constitute an LC resonant circuit. Under the action of the radio frequency signal generated by the high-frequency oscillator 1, a polarized magnetic field is generated to act on the first resonant cavity 7.

[0059] The first free radical solution is prepared from free radicals and a first organic solvent 8 .

[0060] The second resonant cavity 9 is a negatively polarized resonant cavity, including a second coaxial glass bottle and an LC resonant circuit. A second free radical solution is provided in the second coaxial glass bottle to produce a negative dynamic nuclear polarization effect. The high-frequency coil 6 and the adjustable capacitor 5 constitute an LC resonant circuit. Under the action of the radio frequency signal generated by the high-frequency oscillator 1, a polarized magnetic field is generated to act on the second resonant cavity 9.

[0061] The first coaxial glass bottle and the second coaxial glass bottle are two independent glass containers, and the bottom surfaces of the two glass bottles are tightly fitted together.

[0062] The second free radical solution is prepared from free radicals and a second organic solvent 10 .

[0063] The free radical is 4-carbonyl-tetramethylpiperidinyloxy free radical;

[0064] The first organic solvent 8 is methanol; the second organic solvent 10 is ethylene glycol dimethyl ether.

[0065] The first resonant cavity 7, the second resonant cavity 9, the first signal receiving coil 11, the second signal receiving coil 12, the differential amplifier 3 and the low-frequency feedback coil 13 form a self-excited oscillation circuit.

[0066] The resonant frequencies of the first resonant cavity 7 and the second resonant cavity 9 are the same as the frequency of the sensor output signal, that is, the frequency measured by the frequency meter 4;

[0067] When the sensor is placed in the measuring magnetic field, the protons in the free radical solution will perform weak Larmor precession around the measuring magnetic field. The Larmor precession frequency is f0. The receiving coil receives a voltage signal with the same precession frequency. The voltage signal passes through the feedback coil to generate a magnetic field with a frequency of f0 acting on the resonant cavity. Then, a stronger signal with a frequency of f0 is output from the receiving coil. The Larmor precession frequency f0 is linearly related to the measuring magnetic field B0. The relationship is:

[0068] B0=23.48720378f0.

[0069] The resonance frequency of the first resonance cavity 7 and the second resonance cavity 9 is the same as the frequency of the electron paramagnetic resonance of the electrons of the free radical solution. The sensor resonance cavity is manufactured so that the resonance frequency is the same as the resonance frequency of the free radical solution.

[0070] The signal frequency emitted by the high-frequency oscillator 1 to the first and second resonant cavities is the same as the frequency of the electron paramagnetic resonance of the electrons of the free radical solution. The closer the signal frequency emitted by the high-frequency oscillator 1 is to the electron paramagnetic resonance frequency of the electrons of the free radical solution, the higher the polarization intensity and the higher the output signal amplitude.

[0071] The rotation frequency of the magnetic field generated by the low-frequency feedback coil 13 is the same as the resonant frequency of the first resonant cavity 7 and the second resonant cavity 9. The resonant cavity is equivalent to the frequency selection network in the self-excited oscillation circuit, the resonant frequency is equal to the output signal frequency, and the voltage signal acting on the feedback coil is the output signal.

[0072] The first and second resonant cavities are capacitively loaded coaxial resonant cavities.

[0073] The present invention discloses a self-excited oscillation dynamic nuclear polarization magnetic sensor. A radio frequency signal is generated by adjusting a high-frequency oscillator 1. The signal acts on a high-frequency coil 6 in a probe 2 to fully polarize a free radical solution. A positive dynamic nuclear polarization effect is generated in a first resonant cavity 7. A negative dynamic nuclear polarization effect is generated in a second resonant cavity 9. The high-frequency coil 6 and an adjustable capacitor 5 form an LC resonant circuit. The resonant frequency of the LC resonant circuit is made the same as the resonant frequency of the resonant cavity by adjusting the adjustable capacitor 5. A polarized magnetic field is generated under the action of the radio frequency signal generated by the high-frequency oscillator 1 to act on the resonant cavity.

[0074] The present invention is a self-excited oscillation dynamic nuclear polarization magnetic sensor, the low-frequency feedback coil 13 generates a rotating magnetic field. Since the voltage signal fed back to the low-frequency feedback coil 13 is a Larmor precession signal, the rotating magnetic field frequency is consistent with the proton precession frequency in the resonant cavity.

[0075] The present invention discloses a self-excited oscillation type dynamic nuclear polarization magnetic sensor. A free radical solution needs to be configured with free radicals and an organic solution. The solution will undergo dynamic nuclear polarization under the action of a radio frequency signal of a fixed frequency. The frequencies of positive polarization and negative polarization of the same solution are different. In order to realize the simultaneous positive polarization of a first resonant cavity 7 and negative polarization of a second resonant cavity 9 at the same frequency, two free radical solutions are configured using the same free radical and two different organic solvents, that is, the first free radical solution is configured using 4-carbonyl-tetramethylpiperidinyloxy free radical and methanol, and the second free radical solution is configured using 4-carbonyl-tetramethylpiperidinyloxy free radical and ethylene glycol dimethyl ether.

[0076] The present invention is a self-excited oscillation type dynamic nuclear polarization magnetic sensor, the working process of which can be divided into the following stages:

[0077] Phase 1: In the absence of an external magnetic field, the hydrogen protons in the free radical solution undergo nuclear spin motion, and the spin directions are all chaotic. At this time, the proton system's magnetization vector is macroscopically 0, as shown in Figure 3(1). A single proton has magnetism, so a proton system composed of a large number of protons also has magnetism. The magnetization intensity of the proton system refers to the vector sum of the proton magnetic moments per unit volume.

[0078] The second stage: the sensor is placed in the measuring magnetic field. After being affected by the external magnetic field, the spin directions of some protons in the proton system become the same as the measuring magnetic field direction, while the spin directions of the remaining protons remain chaotic. The entire proton system generates a magnetization intensity M0 with the same direction as the measuring magnetic field B0, as shown in Figure 3(2). If a disturbance is applied to M0, M0 will begin to precess, and the precession frequency is the Larmor frequency ω=γB0, as shown in Figure 3(3).

[0079] The third stage: When the sensor starts to power on, the high-frequency oscillator 1 delivers energy to the resonant cavity. Under the action of the radio frequency signal, as shown in Figure 3 (4) and Figure 3 (5), a positive dynamic nuclear polarization effect occurs in the first resonant cavity 7, and M0 is continuously enhanced along the B0 direction to M p1 , a negative dynamic nuclear polarization effect occurs in the second resonant cavity 9, and M0 is continuously enhanced in the reverse direction of B0 to M p2 At the same time, the low-frequency feedback coil 13 in the sensor self-excited oscillation circuit will generate a rotating magnetic field H0 that increases with time. H0 is perpendicular to the direction of B0 and the rotation frequency is the Larmor precession frequency ω. As shown in Figure 3 (6) and Figure 3 (7), under the joint action of the high-frequency oscillator 1 and the low-frequency feedback coil 13, M p1 、M p2 It will make a precession motion with a gradually increasing radius around B0 along the direction of B0. Due to the rotation frequency of H0 and M p1 、M p2 The precession frequencies are equal, both ω, and nuclear magnetic resonance will occur at this time.

[0080] Stage 4: When the energy delivered by the high-frequency resonator 1 to the resonant cavity is balanced with the energy consumed by the sensor, the oscillation circuit reaches a balanced state. At this time, M p1 、M p2 No longer increases, it will rotate around the measuring magnetic field B0 at a fixed frequency, as shown in Figure 3(8) and Figure 3(9), and the signal receiving coil receives M p1 、M p2 The frequency of the precession signal around B0 is proportional to the measured magnetic field. The magnitude of the magnetic field can be calculated by measuring the frequency of the precession signal.

Claims

1. A self-excited oscillation dynamic nuclear polarization magnetic sensor, characterized in that: The sensor includes: A high frequency oscillator (1) generates a frequency-adjustable radio frequency signal and applies it to a probe (2); The probe (2) outputs a sine wave signal with a varying amplitude; A differential amplifier (3) amplifies the sinusoidal wave signal output by the probe (2); The low-frequency feedback coil (13) generates a rotating magnetic field under the action of the signal output by the differential amplifier (3); The probe (2) is connected to a differential amplifier (3), and the differential amplifier (3) is connected to a low-frequency feedback coil (13). The probe (2), the differential amplifier (3) and the low-frequency feedback coil (13) form a self-excited oscillation circuit. When the high-frequency oscillator (1) generates a radio frequency signal that acts on the probe (2), the self-excited oscillation starts, and the probe (2) outputs a sinusoidal wave signal with a gradually increasing signal amplitude. The probe (2) comprises a first resonant cavity (7) and a second resonant cavity (9); A first signal receiving coil (11) is wound around the outside of the first resonant cavity (7), the winding direction of the first signal receiving coil (11) is left-handed, the magnetization intensity exhibited by the first resonant cavity (7) is positive, and the positive magnetization intensity cuts the first signal receiving coil (11) to generate a positive induced electromotive force; A second signal receiving coil (12) is wound around the outside of the second resonant cavity (9); the second signal receiving coil (12) is wound in a right-hand direction; the second resonant cavity (9) exhibits a positive magnetization intensity; the positive magnetization intensity cuts the second signal receiving coil (12) to generate a positive induced electromotive force.

2. The self-excited oscillation dynamic nuclear polarization magnetic sensor according to claim 1, characterized in that: The sensor comprises a frequency meter (4), which receives an output voltage signal of a differential amplifier (3) and measures the frequency of the output voltage signal.

3. The self-excited oscillation dynamic nuclear polarization magnetic sensor according to claim 1, characterized in that: A low-frequency feedback coil (13) is wound around the connection between the first resonant cavity (7) and the second resonant cavity (9), and the winding direction of the low-frequency feedback coil (13) is right-handed.

4. The self-excited oscillation dynamic nuclear polarization magnetic sensor according to claim 1, characterized in that: The first resonant cavity (7) is a positively polarized resonant cavity, comprising a first coaxial glass bottle and an LC resonant circuit. A first free radical solution is provided in the first coaxial glass bottle for generating a positive dynamic nuclear polarization effect. The high-frequency coil (6) and the adjustable capacitor (5) form an LC resonant circuit. Under the action of a radio frequency signal generated by a high-frequency oscillator (1), a polarized magnetic field is generated to act on the first resonant cavity (7).

5. The self-excited oscillation dynamic nuclear polarization magnetic sensor according to claim 4, characterized in that: The first free radical solution is prepared from free radicals and a first organic solvent (8), the free radicals are 4-carbonyl-tetramethylpiperidinyloxy free radicals, and the first organic solvent (8) is methanol.

6. The self-excited oscillation dynamic nuclear polarization magnetic sensor according to claim 1, characterized in that: The second resonant cavity (9) is a negatively polarized resonant cavity, comprising a second coaxial glass bottle and an LC resonant circuit. A second free radical solution is provided in the second coaxial glass bottle for generating a negative dynamic nuclear polarization effect. The high-frequency coil (6) and the adjustable capacitor (5) form an LC resonant circuit. Under the action of a radio frequency signal generated by a high-frequency oscillator (1), a polarized magnetic field is generated to act on the second resonant cavity (9).

7. The self-excited oscillation dynamic nuclear polarization magnetic sensor according to claim 6, characterized in that: The second free radical solution is prepared from a free radical and a second organic solvent (10), the free radical is a 4-carbonyl-tetramethylpiperidinyloxy free radical, and the second organic solvent (10) is ethylene glycol dimethyl ether.

8. The self-excited oscillation dynamic nuclear polarization magnetic sensor according to any one of claims 1 to 7, characterized in that: A radio frequency signal is generated by adjusting the high frequency oscillator (1). The signal acts on the high frequency coil (6) in the probe (2), so that the free radical solution is fully polarized, a positive dynamic nuclear polarization effect is generated in the first resonant cavity (7), and a negative dynamic nuclear polarization effect is generated in the second resonant cavity (9). The high frequency coil (6) and the adjustable capacitor (5) form an LC resonant circuit. The resonant frequency of the LC resonant circuit is made the same as the resonant frequency of the resonant cavity by adjusting the adjustable capacitor (5). Under the action of the radio frequency signal generated by the high frequency oscillator (1), a polarized magnetic field is generated and acts on the resonant cavity.

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