Enhanced diffusion pumping suppressed optical frequency shift virtual magnetic field serf atomic magnetometer

Abstract

An enhanced diffusion-pump suppression optical frequency shift virtual magnetic field SERF atomic magnetometer, by using only a single laser and a beam splitter, can polarize atoms in a gas chamber using the diffusion effect and detect them using a detection light. Furthermore, the pump light does not directly interact with the atoms detected by the detection light, thereby suppressing the influence of the pump light's optical frequency shift virtual magnetic field on the magnetic field measurement and improving the magnetometer's sensitivity. The magnetometer is characterized by comprising a first pump light and a second pump light, both incident on the gas chamber along the z-axis, spaced apart along the y-axis, with a detection light incident on the gas chamber along the x-axis within the interval. All three pump lights originate from the same laser.

Description

Technical Field

[0001] This invention relates to the field of SERF atomic magnetometer technology, specifically to an enhanced diffusion-pump suppression optical frequency shift virtual magnetic field SERF atomic magnetometer. By using only a single laser and a beam splitter, it can utilize the diffusion effect to pump and detect atoms in a gas chamber, and the pump light does not directly interact with the atoms detected by the detection light, thereby suppressing the influence of the pump light frequency shift virtual magnetic field on the magnetic field measurement and improving the magnetometer's sensitivity. Background Technology

[0002] Existing optical frequency shift suppression schemes include adjusting the gas composition within the chamber, regulating laser parameters, measuring the virtual magnetic field of optical frequency shift, and magnetically compensating for suppression. Theoretically, suppressing the virtual magnetic field of optical frequency shift through diffusion effects can achieve complete suppression. This invention uses diffusion pumping to suppress the virtual magnetic field of optical frequency shift. To enhance polarization efficiency, two circularly polarized pump beams with the same polarization state and optical power are pumped in parallel. The detection light passes perpendicularly between the two pump beams. Atoms between the two pump beams are polarized by diffusion. Since the atoms detected by the detection light are not directly affected by the pump light, the influence of the pump light's optical frequency shift can be suppressed.

[0003] After searching, the relevant existing technologies are listed below:

[0004] CN115560749A discloses a system for suppressing transverse optical frequency shift in a spin-free exchange-relaxation inertial measurement device. Taking a spin-free exchange-relaxation inertial measurement device as the research object, this system addresses the problem of transverse optical frequency shift caused by detection laser detuning. It utilizes two detection lasers, one blue and one red, to detect the transverse optical frequency shift caused by the blue detuned detection laser and the other red detuned laser, thereby canceling out the transverse optical frequency shift caused by the red detuned laser.

[0005] CN115219966A discloses a system and method for suppressing the virtual magnetic field of optical frequency shift in an optically pumped magnetometer. By averaging the magnetic field detection results of left-handed and right-handed circularly polarized light, the true value of the magnetic field modulus to be measured can be obtained, enabling the magnetometer to measure the true value of the magnetic field modulus. This effectively suppresses the virtual magnetic field of optical frequency shift caused by the AC Stark effect and improves the accuracy of magnetic detection by the magnetometer.

[0006] CN114485638A discloses a method for decoupling and suppressing transverse optical frequency shift in an atomic spin inertial measurement device. This method addresses the coupling problem between transverse optical frequency shift caused by the circular polarization component of a laser and the transverse optical frequency shift caused by the circular dichroism of the atomic ensemble. It decouples the transverse optical frequency shift by changing the circular polarization degree of the detection laser, resulting in a transverse optical frequency shift solely caused by the circular dichroism of the atomic ensemble. Furthermore, it adjusts the magnitude of the transverse optical frequency shift caused by the circular polarization component of the detection laser by regulating the circular polarization degree of the detection laser, thus canceling out the transverse optical frequency shift caused by the circular polarization component of the detection laser and the transverse optical frequency shift caused by the circular dichroism of the atomic ensemble.

[0007] CN113280801A is a method for suppressing optical frequency shift in a hybrid pump SERF spin inertial measurement system. By changing the working temperature of the alkali metal gas cell, the atomic density ratio of the alkali metal is altered, thereby suppressing the total optical frequency shift of the hybrid pump SERF atomic spin inertial measurement system and bringing the total optical frequency shift of the system to zero.

[0008] CN112684386A discloses a closed-loop suppression method for hybrid optical frequency shift based on atomic collisions. This method involves filling a KRb hybrid atomic gas chamber in a SERF inertial measurement unit with a quantitative gas. The quantitative gas is used to change the working center frequency, adjusting the position where the KRb hybrid optical frequency shift is zero to the D1 line frequency of K. Simultaneously, a saturated absorption frequency stabilization system is used to achieve frequency stabilization, thereby suppressing optical frequency shift.

[0009] CN106226713A discloses a method for suppressing optical frequency shift in a SERF atomic magnetometer. Under two different pump light intensities, the method utilizes three-dimensional in-situ magnetic compensation technology to measure the magnitude of the magnetic field sensed by atoms at different pump light frequencies. After theoretical fitting and calculation of the measurement data, the zero point of optical frequency shift can be accurately found, thereby achieving sufficient suppression of optical frequency shift.

[0010] CN110426652A describes an experimental apparatus and method for suppressing the virtual magnetic field of optical frequency shift in a SERF magnetometer. The method involves adjusting or repeatedly setting the pump light frequency to measure the center frequency of the optical frequency shift, and then tuning the pump light to the center frequency to eliminate the optical frequency shift and the magnetic field it generates.

[0011] Atomic magnetometers have the advantage of smaller size compared to SQUID magnetometers (SQUID stands for Superconducting Quantum Interference Device). Among them, the SERF atomic magnetometer (SERF, Spin-Exchange Relaxation-Free) exhibits very high theoretical sensitivity because it suppresses spin exchange relaxation while achieving a very high atomic number density. The SERF atomic magnetometer uses circularly polarized light to polarize electrons of alkali metal atoms in the gas chamber. When the circularly polarized light is not at its resonant frequency, it causes a vector light displacement, at which point the atoms experience a virtual magnetic field along the pump light direction. This light displacement introduces a magnetic field gradient into the gas chamber. This is because the circularly polarized pump light attenuates as it propagates within the chamber, resulting in varying virtual magnetic field magnitudes at different locations, which cannot be fully compensated for by shimming coils. Furthermore, fluctuations in the optical power, frequency, and polarization state of the circularly polarized pump light lead to fluctuations in optical frequency shift and virtual magnetic field, which in turn become magnetic field noise, affecting the magnetometer's sensitivity. Due to the significant pressure broadening in the gas cell of the SERF atomic magnetometer, it is difficult to tune the circularly polarized pump light to the resonance point. Furthermore, after the magnetometer has been running for some time, the laser emitted by the laser will experience frequency drift compared to when it was first turned on. Additionally, because of the pressure frequency shift in the gas cell, conventional saturated absorption stabilization methods cannot be used to lock the laser frequency at the resonance point after the pressure shift. These factors make it difficult to avoid the optical frequency shift by simply adjusting the laser. Summary of the Invention

[0012] This invention addresses the deficiencies or shortcomings of existing technologies by providing an enhanced diffusion-pump-suppressed optical frequency shift virtual magnetic field SERF atomic magnetometer. By using only a single laser and a beam splitter, it can polarize atoms in the gas chamber using the diffusion effect and detect them using a detection light. Furthermore, the pump light does not directly interact with the atoms detected by the detection light, thereby suppressing the influence of the pump light's optical frequency shift virtual magnetic field on the magnetic field measurement and improving the magnetometer's sensitivity.

[0013] The technical solution of the present invention is as follows:

[0014] An enhanced diffusion pump suppression optical frequency shift virtual magnetic field (SERF) atomic magnetometer is characterized by comprising a first pump light and a second pump light that are both incident on the gas chamber along the z-axis direction, the first pump light and the second pump light being spaced apart along the y-axis direction, and a detection light that is incident on the gas chamber along the x-axis direction within the spaced intervals, wherein the first pump light, the second pump light and the detection light all originate from the same laser.

[0015] A first polarizing beam splitter is provided on the pump light incident side of the gas chamber. After the light passes through the transmission side of the first polarizing beam splitter, it passes through the first quarter-wave plate and reaches the gas chamber. After the light passes through the reflection side of the first polarizing beam splitter, it passes through the reflecting mirror and the second quarter-wave plate in sequence and reaches the gas chamber. The light generated by the laser passes through the first polarization-maintaining fiber, the first collimator, and the first half-wave plate in sequence and reaches the input side of the first polarizing beam splitter.

[0016] The light generated by the laser passes sequentially through the second polarization-maintaining fiber, the second collimator, and the linear polarizer to the incident side of the detection light in the gas chamber; the exit side of the detection light in the gas chamber passes sequentially through the second half-wave plate, the second polarizing beam splitter, the differential photodetector, and the transimpedance amplifier, and is finally connected to the signal acquisition system.

[0017] The air chamber is located inside a magnetically shielded barrel equipped with a coil. An electric heating film is provided on the outer circumference of the air chamber. The electric heating film is connected to a temperature control system, and the coil is connected to a current source.

[0018] The first pump beam and the second pump beam are two circularly polarized pump beams with the same polarization state and the same optical power.

[0019] The diffusion pumping expression within the interval of the gas chamber is as follows:

[0020]

[0021] Where S represents atomic spin polarization, t is time, D0 is the diffusion constant, T0 is the temperature corresponding to the diffusion constant D0, P0 is the pressure corresponding to the diffusion constant D0, T is the current operating temperature, and P is the buffer gas pressure. It is the Laplace operator.

[0022] The features and advantages of this invention are as follows:

[0023] 1. This invention pumps atoms through a diffusion effect. The pump light does not directly interact with the atoms detected by the detection light. Theoretically, this can completely suppress the influence of the virtual magnetic field of the pump light frequency shift on the magnetic field measurement.

[0024] 2. This invention uses a beam splitting method, employing two circularly polarized pump beams with the same polarization state and optical power for parallel pumping, thereby enhancing pumping efficiency.

[0025] 3. This invention uses only one laser, which reduces the number of light sources compared to the traditional dual-beam SERF atomic magnetometer that uses two lasers. The laser source in this invention is detuned, and the detuned pump light can ensure more uniform atomic polarization in the gas chamber, which is beneficial to the performance improvement of the magnetometer. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the structure of an enhanced diffusion pump suppression optical frequency shift virtual magnetic field SERF atomic magnetometer implementing the present invention.

[0027] Figure 2 yes Figure 1 A schematic diagram of the pump light detection light position.

[0028] The reference numerals in the attached figures are explained as follows: 1-Laser; 2-First polarization-maintaining fiber; 3-First collimator; 4-Second polarization-maintaining fiber; 5-Second collimator; 6-First half-wave plate; 7-First polarizing beam splitter prism; 8-Reflector; 9-First quarter-wave plate; 10-Second quarter-wave plate; 11-Gas cell; 12-Electrically heated film; 13-Linear polarizer; 14-Second half-wave plate; 15-Second polarizing beam splitter prism; 16-Balanced differential detector; 17-Magnetic shielding barrel; 18-Coil; 19-Transimpedance amplifier; 20-Signal acquisition system; 21-Temperature control system; 22-Current source. Detailed Implementation

[0029] The following is in conjunction with the attached diagram ( Figures 1-2 The invention will be described in the following sections and examples.

[0030] Figure 1 This is a schematic diagram of the structure of an enhanced diffusion pump suppression optical frequency shift virtual magnetic field SERF atomic magnetometer implementing the present invention. Figure 2 yes Figure 1 A schematic diagram showing the position of the pump light detection light. (Reference) Figures 1 to 2 As shown, an enhanced diffusion-pump suppression optical frequency shift virtual magnetic field (SERF) atomic magnetometer includes a first pump beam (e.g., incident along the z-axis direction on the gas cell 11) that is incident on the gas cell 11. Figure 1 or Figure 2 The middle horizontally lower pump light) and the second pump light (e.g. Figure 1 or Figure 2 The first and second pump lights are spaced apart along the y-axis, and a detection light (e.g., incident on the gas chamber along the x-axis) is disposed within the spaced area. Figure 1 or Figure 2 The longitudinal detection light, the first pump light, the second pump light and the detection light all come from the same laser 1.

[0031] A first polarizing beam splitter 7 is provided on the pump light incident side of the gas chamber 11. After passing through the transmission side of the first polarizing beam splitter 7, the light passes through the first quarter-wave plate 9 to reach the gas chamber 11. After passing through the reflection side of the first polarizing beam splitter 7, the light passes through the reflecting mirror 8 and the second quarter-wave plate 10 in sequence to reach the gas chamber 11. The light generated by the laser 1 passes through the first polarization-maintaining fiber 2, the first collimator 3, and the first half-wave plate 6 in sequence to reach the input side of the first polarizing beam splitter 7. The light generated by the laser 1 passes through the second polarization-maintaining fiber 4, the second collimator 5, and the linear polarizer 13 in sequence to reach the detection light incident side of the gas chamber 11. The detection light exiting side of the gas chamber 11 passes through the second half-wave plate 14, the second polarizing beam splitter 15, the balanced differential detector 16, and the transimpedance amplifier 19 in sequence, and is finally connected to the signal acquisition system 20.

[0032] The gas chamber 11 is located inside a magnetically shielded barrel 17 equipped with a coil 18. An electric heating film 12 is provided on the outer peripheral surface of the gas chamber 11. The electric heating film 12 is connected to a temperature control system 21, and the coil 18 is connected to a current source 22. The first pump light and the second pump light are two circularly polarized pump lights with the same polarization state and the same optical power.

[0033] The diffusion pumping expression within the interval of the gas chamber 11 is as follows:

[0034]

[0035] Where S represents atomic spin polarization, t is time, D0 is the diffusion constant, T0 is the temperature corresponding to the diffusion constant D0, P0 is the pressure corresponding to the diffusion constant D0, T is the current operating temperature, and P is the buffer gas pressure. It is the Laplace operator.

[0036] To address the adverse effects of optical frequency-shifted virtual magnetic fields on atomic magnetometers, this invention employs diffusion-pump polarization of alkali metal atoms, avoiding direct interaction between the pump light and the atoms detected by the detection light, thus suppressing the optical frequency-shifted virtual magnetic field. To enhance the polarization effect on alkali metal atoms, a beam splitting method is used to generate two parallel beams of equal power and polarization state, incident on the atomic gas cell. The detection light passes perpendicularly between the two pump beams, without directly intersecting with them. The alkali metal atoms between the two pump beams are polarized by diffusion, ultimately suppressing the optical frequency-shifted virtual magnetic field. A laser is used as the light source to simplify the magnetometer's structure. The laser generates both the pump and detection lasers; the detuned pump laser further enhances the uniform longitudinal polarization within the atomic gas cell.

[0037] The detuned light generated by the laser is split inside the laser and led out through two polarization-maintaining fibers connected by two fiber couplers. One fiber serves as the pump light, and the other as the detection light. After exiting the first collimator, the pump light is split by a combination of a first half-wave plate and a first polarizing beam splitter. The light in the transmission direction becomes circularly polarized after passing through the first quarter-wave plate, and the light in the reflection direction is reflected by a mirror and then becomes circularly polarized with the same rotation direction as the circularly polarized light in the transmission direction after passing through a second quarter-wave plate. The first and second quarter-wave plates are adjusted to maintain the same polarization state of the two pump beams. The first half-wave plate is used to ensure that the power of the circularly polarized light in the transmission and reflection directions is the same. The mirror is used to adjust the direction of the light so that the two pump beams are parallel, and the two circularly polarized beams with the same polarization state enter the gas cell in parallel. After exiting the second collimator, the detection light passes through a linear polarizer and enters the gas cell. Its incident direction is perpendicular to the pump light direction, passing through the middle of the two pump beams without intersecting them. After passing through the gas cell, it passes through a second half-wave plate and a second polarizing beam splitter, converting the optical rotation angle signal into the difference between the optical power signals transmitted and reflected by the second polarizing beam splitter. This difference is then detected using a balanced differential detector. The second half-wave plate is used to zero the signal of the balanced differential detector before the magnetometer operates. The balanced differential detector converts the difference in optical power into a photocurrent signal. This photocurrent signal is converted into a voltage signal by a transimpedance amplifier and finally acquired by the signal acquisition system to obtain the information of the magnetic field to be measured.

[0038] The detection light passes perpendicularly between the two pump beams. Atoms between the two pump beams are polarized due to diffusion. Since the atoms detected by the detection light are not directly affected by the pump beams, the influence of the pump beam frequency shift can be suppressed. Using only a single laser as the light source simplifies the magnetometer structure and reduces cost. Furthermore, because the light source is detuned, the detuned pump beam ensures more uniform atomic polarization within the gas chamber, which is beneficial for improving the magnetometer's performance.

[0039] Using two circularly polarized beams with the same polarization state and power in parallel pumping is to improve polarization efficiency.

[0040] In the SERF atomic magnetometer, when the circularly polarized pump light is not at its resonant frequency, it causes a vector light displacement. At this point, the atom experiences a virtual magnetic field along the pump light direction. This virtual magnetic field B... LS The expression is,

[0041]

[0042] Where, r e Let f be the classical electron radius, c be the speed of light, and f be the velocity of light. D1 For the oscillator strength, Ф pumpγ represents the pump light power, tr represents the transmittance of the gas chamber glass, and γ represents the spectral density. e Where is the electron gyromagnetic ratio, h is Planck's constant, and A is... pump The area of ​​the pump light spot is v, and the laser frequency is v. D1 The D1 line frequency of alkali metal atoms, Γ D1 The pressure broadening of alkali metal atoms is given, and s represents the photon spin component. The existence of optical displacement introduces a certain magnetic field gradient into the gas chamber. This is because the circularly polarized pump light attenuates as it propagates within the chamber, resulting in different magnitudes of the virtual magnetic field at different locations, which cannot be fully compensated for by the shimming coil. Furthermore, fluctuations in the optical power, frequency, and polarization state of the circularly polarized pump light lead to fluctuations in optical frequency shift and virtual magnetic field, which in turn become magnetic field noise affecting the magnetometer's sensitivity.

[0043] This invention utilizes diffusion pumping to suppress the influence of pump light frequency shift on an atomic magnetometer. The atoms detected by the detection light are not directly affected by the pump light, and the detection and pump lights do not intersect, thus suppressing the vector light frequency shift caused by the pump light. Furthermore, to enhance the diffusion pumping effect, the pump light is split into two parallel beams using a beam splitting method. The detection light passes perpendicularly between the two pump beams. The atoms between the two beams are polarized by diffusion. Since the atoms detected by the detection light are not directly affected by the pump light, the influence of the pump light frequency shift on the atoms can be suppressed, thereby suppressing the problems caused by the virtual magnetic field of the light frequency shift to the magnetometer. The principle of diffusion pumping is as follows:

[0044]

[0045] Where S represents atomic spin polarization, D0 represents the diffusion constant at temperature T0 and pressure P0, and the diffusion constant of alkali metal atoms differs in different buffer gases. T represents the current operating temperature, and P represents the buffer gas pressure. It is the Laplace operator. In the atomic cell, alkali metal atoms are polarized by the pump light, and the diffusion effect causes the spatial distribution of atomic polarization in the cell to redistribute. This manifests as atomic polarization spreading from the parts with high polarizability, that is, the parts directly affected by the pump light, to other parts, so that atoms outside the coverage of the pump beam can also be polarized.

[0046] like Figure 1As shown, the enhanced diffusion-pump SERF atomic magnetometer structure consists of a laser 1 generating detuned light, which is internally split and led out via two fiber optic couplers connected to a first polarization-maintaining fiber 2 and a second polarization-maintaining fiber 4, respectively. One fiber serves as the pump light, and the other as the detection light. After exiting through a first collimator 3, the pump light is split by a combination of a first half-wave plate 6 and a first polarizing beam splitter prism 7. The first half-wave plate 6 can adjust the splitting ratio to ensure that the power of the circularly polarized light in the transmission and reflection directions is the same. The transmitted light becomes circularly polarized after passing through a first quarter-wave plate 9, and the reflected light is reflected by a mirror 8 and then becomes circularly polarized with the same rotation direction as the transmitted light after passing through a second quarter-wave plate 10. The mirror 8 is used to adjust the direction of the light to ensure that the two pump beams are parallel. The first quarter-wave plate 9 and the second quarter-wave plate 10 are used to adjust the polarization state of the two pump beams to ensure that their polarization states are consistent. Two circularly polarized beams with the same polarization state are incident parallel to each other in the gas cell 11. The atoms between the two pump beams are polarized by the diffusion effect. The parallel incident of the two pump beams can enhance the diffusion effect compared to the configuration of a single pump beam.

[0047] After exiting the second collimator 5, the detection light passes through the linear polarizer 13 and then enters the gas chamber 11. The incident direction is perpendicular to the pump light direction, passing between the two pump beams. Since the atoms detected by the detection light are not directly affected by the pump light, the influence of the pump light frequency shift can be suppressed. After passing through the gas chamber 11, the light rotation angle signal is converted into the difference between the light power signals transmitted and reflected by the second polarizing beam splitter 15 via the second half-wave plate 14 and the second polarizing beam splitter 15. This difference is then detected using a balanced differential detector 16. The second half-wave plate 14 is used to zero the signal of the balanced differential detector 16 before the magnetometer operates. The balanced differential detector 16 converts the difference in light power into a photocurrent signal. This photocurrent signal is converted into a voltage signal by the transimpedance amplifier 19 and acquired by the signal acquisition system 20, ultimately obtaining the information of the magnetic field to be measured.

[0048] The temperature control system 21 is used to control the heating power of the electric heating film 12, thereby controlling the temperature of the air chamber 11. The magnetic shielding barrel 17 is used to shield the external geomagnetic field. The current source 22 drives the coil 18 to compensate for the remaining magnetic field and ensure that the magnetometer works in SERF state.

[0049] The detection light passes perpendicularly between the two pump beams, as shown in the schematic diagram below. Figure 2As shown, the atoms between the two pump beams are polarized due to diffusion. Since the atoms detected by the detection beam are not directly affected by the pump beam, the influence of the pump beam's frequency shift can be suppressed. Using only a single laser as the light source simplifies the magnetometer structure and reduces cost. Furthermore, because the light source is detuned, the detuned pump beam ensures more uniform atomic polarization within the gas chamber, which is beneficial for improving the magnetometer's performance. Using two circularly polarized beams with the same polarization state and power for parallel pumping is to improve polarization efficiency.

[0050] Contents not described in detail in this specification are prior art known to those skilled in the art. It is hereby indicated that the above description is intended to help those skilled in the art understand this invention, but does not limit the scope of protection of this invention. Any equivalent substitutions, modifications, improvements, and / or simplifications of the above descriptions that do not depart from the essential content of this invention fall within the scope of protection of this invention.

Claims

1. An enhanced diffuse pumping suppressed optical frequency shift virtual magnetic field (SERF) atomic magnetometer, comprising: It includes a first pump light and a second pump light that are both incident on the gas chamber along the z-axis direction. The first pump light and the second pump light are distributed at intervals along the y-axis direction. A detection light that is incident on the gas chamber along the x-axis direction is arranged within the interval. The first pump light, the second pump light and the detection light all come from the same laser. The diffusion pumping expression within the interval of the gas chamber is as follows: ， Where S is atomic spin polarization, t is time, D0 is the diffusion constant, T0 is the temperature corresponding to the diffusion constant D0, P0 is the pressure corresponding to the diffusion constant D0, T is the current operating temperature, P is the buffer gas pressure, and ∇ 2 It is the Laplace operator; The first pump beam and the second pump beam are two circularly polarized pump beams with the same polarization state and the same optical power.

2. The enhanced diffuse pumping suppressed optical frequency shift virtual magnetic field (SERF) atomic magnetometer of claim 1, wherein, A first polarizing beam splitter is provided on the pump light incident side of the gas chamber. After the light passes through the transmission side of the first polarizing beam splitter, it passes through the first quarter-wave plate and reaches the gas chamber. After the light passes through the reflection side of the first polarizing beam splitter, it passes through the reflecting mirror and the second quarter-wave plate in sequence and reaches the gas chamber. The light generated by the laser passes through the first polarization-maintaining fiber, the first collimator, and the first half-wave plate in sequence and reaches the input side of the first polarizing beam splitter.

3. The enhanced diffuse pumping suppressed optical frequency shift virtual magnetic field (SERF) atomic magnetometer of claim 1, wherein, The light generated by the laser passes sequentially through the second polarization-maintaining fiber, the second collimator, and the linear polarizer to the incident side of the detection light in the gas chamber; the exit side of the detection light in the gas chamber passes sequentially through the second half-wave plate, the second polarizing beam splitter, the differential photodetector, and the transimpedance amplifier, and is finally connected to the signal acquisition system.

4. The enhanced diffusion-pumped suppression optical frequency shift virtual magnetic field SERF atomic magnetometer according to claim 1, characterized in that, The air chamber is located inside a magnetically shielded barrel equipped with a coil. An electric heating film is provided on the outer circumference of the air chamber. The electric heating film is connected to a temperature control system, and the coil is connected to a current source.

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