Optical fiber current sensor for measuring plasma current and temperature compensation method thereof

Abstract

This application discloses a fiber optic current sensor for plasma current measurement and its temperature compensation method. The fiber optic current sensor includes a light source, a coupler, a polarizer, a phase modulator, a sensing fiber, a reflector, a detector, and a demodulation module. It also includes a quarter-wave plate positioned between the phase modulator and the sensing fiber, a wave plate temperature sensor positioned near the quarter-wave plate, distributed temperature-measuring fibers laid along the same path as the sensing fiber, and a temperature compensation module. The temperature compensation module compensates for the nonlinear error introduced by the quarter-wave plate based on the wave plate temperature, and calculates a weighted equivalent temperature based on the temperature and magnetic field distributions obtained from the distributed temperature-measuring fibers. Based on this equivalent temperature, the Wilder constant is corrected to obtain the measured current value. This application employs a dual-temperature-zone independent temperature compensation strategy, solving the nonlinear error and temperature compensation problems in high-current measurement and improving measurement accuracy.

Description

Technical Field

[0001] This application relates to the field of current measurement and temperature compensation, specifically to an optical fiber current sensor for plasma current measurement and its temperature compensation method. Background Technology

[0002] In the research and operation of fusion devices such as tokamaks, plasma current, as a key parameter characterizing the plasma state and ensuring the safe operation of the device, is crucial for accurate measurement. Precise plasma current measurement is not only fundamental to achieving plasma feedback control but also a necessary prerequisite for conducting advanced physics research, such as magnetohydrodynamic instability analysis and current distribution reconstruction. As fusion research progresses from short-pulse experimental devices to long-pulse and even steady-state operating devices, the requirements for current diagnostic systems have become increasingly stringent.

[0003] Currently, in the field of plasma current measurement, the most widely used current diagnostic methods are inductive sensors, represented by Rokowski coils. These sensors operate based on the principle of electromagnetic induction, and during measurement, they can only obtain the rate of change of current, requiring an integrator to obtain the specific current value. There are also fiber optic current sensors based on the Faraday magneto-optical effect, which utilize the physical phenomenon of the rotation of the polarization plane when polarized light propagates in a magnetic field to measure current.

[0004] However, existing measurement methods have certain drawbacks. Taking inductive sensors as an example, under long-pulse operation conditions, the zero-point drift of the integrator accumulates over time, leading to a continuous increase in measurement error. Simultaneously, strong transient magnetic fields and electromagnetic interference in fusion devices can severely contaminate coil signals. Furthermore, inductive sensors cannot directly measure DC current components in principle, limiting their application under steady-state operating conditions. Existing fiber optic current sensor technology faces the following problems when applied to high-current measurements in fusion devices:

[0005] (1) During high current (megaampere level) measurements, the non-ideal nature of the quarter-wave plate (phase delay angle and azimuth angle deviation) introduces a periodic nonlinear error related to the Faraday rotation angle, which increases significantly with increasing current. Existing technologies typically use empirical polynomial fitting for compensation, but polynomial fitting has poor convergence and insufficient compensation accuracy when the Faraday rotation angle is large;

[0006] (2) In the fusion device, the sensing fiber surrounds the vacuum chamber along the polar direction. The temperature gradient at different locations can reach tens of degrees. Traditional single-point temperature measurement or simple arithmetic average temperature cannot accurately reflect the equivalent thermal effect of the entire fiber, resulting in a systematic deviation in the temperature correction of the Wilder constant.

[0007] (3) The vacuum chamber of the fusion device needs to be baked at high temperature (150°C-200°C), which puts extremely high requirements on the temperature resistance and maintainability of the sensor. Existing fiber optic current sensors lack systematic engineering integration solutions. Summary of the Invention

[0008] To address the technical problems in the prior art, this application provides an optical fiber current sensor for plasma current measurement and a temperature compensation method thereof.

[0009] This application provides a fiber optic current sensor for plasma current measurement and its temperature compensation method, which adopts the following technical solution:

[0010] A fiber optic current sensor for plasma current measurement includes a light source, a coupler, a polarizer, a phase modulator, a sensing fiber, a reflector, a detector, and a demodulation module. The sensing fiber forms a sensing fiber loop around the conductor carrying the measured current. Light emitted from the light source enters the sensing fiber after passing through the coupler, polarizer, and phase modulator, and is reflected back to the detector via the reflector. The demodulation module obtains a feedback phase based on the output signal of the detector. The sensor also includes:

[0011] A quarter-wave plate is disposed in the optical path between the phase modulator and the sensing fiber.

[0012] A waveplate temperature sensor is positioned near the quarter-waveplate to acquire the waveplate temperature in real time. ;

[0013] Distributed temperature-sensing optical fiber is laid along the same path as the sensing optical fiber to acquire the temperature distribution along the sensing optical fiber. ;

[0014] Temperature compensation module, the temperature compensation module is based on the waveplate temperature The nonlinear error introduced by the quarter-wave plate is compensated, and the temperature distribution is considered. and the magnetic field distribution along the sensing fiber path Calculate the weighted equivalent temperature Based on the weighted equivalent temperature The Wilder constant of the sensing fiber is corrected to obtain the measured current value.

[0015] In some embodiments, the temperature compensation module includes a waveplate error compensation unit and a Wilder constant compensation unit;

[0016] The waveplate error compensation unit is based on the waveplate temperature. Calculate waveplate error parameters The feedback phase is then corrected for nonlinear error based on the following formula to obtain the corrected Faraday phase shift. :

[0017] ,

[0018] in, The feedback phase output by the demodulation module;

[0019] The Wilder constant compensation unit is based on the weighted equivalent temperature. Find or calculate the corrected Wild constant The measured current is calculated using the following formula:

[0020] ,

[0021] in, The number of turns of the sensing fiber.

[0022] In some embodiments, the waveplate error parameters Obtained through the following linear temperature model:

[0023] ,

[0024] in, Reference temperature The waveplate error parameter values ​​obtained by calibration This is the temperature coefficient obtained through full-temperature calibration.

[0025] In some embodiments, the weighted equivalent temperature Calculate using the following formula:

[0026] ,

[0027] in, The total length of the sensing optical fiber. The temperature distribution along the fiber path measured by the distributed temperature-sensing fiber. This represents the magnetic field strength distribution along the sensing fiber path.

[0028] In some embodiments, the magnetic field strength distribution The value is calculated using a geometric model based on the geometric position of the conductor being measured and the radius of the sensing fiber optic loop.

[0029] In some embodiments, the fiber optic current sensor for plasma current measurement further includes an engineering integration structure, the engineering integration structure comprising:

[0030] A metal protective tube is laid along the measurement path of the vacuum chamber, and its two ends are connected to the vacuum chamber by welded flanges to form a vacuum-sealed boundary;

[0031] A high-temperature resistant insulating protective tube is installed inside the metal protective tube, and the sensing optical fiber and the distributed temperature measurement optical fiber are installed together inside the high-temperature resistant insulating protective tube.

[0032] A cooling channel is formed in the annular gap between the metal protective tube and the high-temperature resistant insulating protective tube, for introducing a cooling medium.

[0033] In some embodiments, the high-temperature resistant insulating protective tube is a PEEK tube; the cooling medium is water or gas, and the cooling channel is compatible with both water cooling and air cooling modes; the cooling medium is introduced from one end flange and led out from the other end flange to form a circulating cooling loop.

[0034] In some embodiments, the quarter-wave plate is disposed outside the vacuum chamber and encapsulated inside an aluminum tube; the sensing optical fiber and the distributed temperature measurement optical fiber are disposed inside the vacuum chamber; the sensing optical fiber can be extracted and replaced from the high-temperature resistant insulating protective tube without damaging the vacuum seal boundary.

[0035] A temperature compensation method for a fiber optic current sensor used for plasma current measurement, applicable to the aforementioned fiber optic current sensor, includes the following steps:

[0036] Step 1: Obtain the waveplate temperature measured by the waveplate temperature sensor. The waveplate error parameters at the current temperature are calculated based on a pre-calibrated model relating waveplate error parameters to temperature. ;

[0037] Step 2: Obtain the feedback phase output by the demodulation module. Based on the waveplate nonlinear error model Calculate the corrected Faraday phase shift ;

[0038] Step 3: Obtain the temperature distribution along the sensing fiber path measured by the distributed temperature-sensing fiber. Combined with the magnetic field strength distribution along the sensing fiber path The equivalent temperature is calculated using the weighted average formula:

[0039] ;

[0040] Step 4: Based on the equivalent temperature The relationship between the pre-calibrated Wilder constant and temperature Calculate the Wilder constant at the current equivalent temperature. ;

[0041] Step 5: Calculate the measured current using the following formula:

[0042] .

[0043] In some embodiments, the relationship model between the waveplate error parameter and temperature in step one is as follows: ,in and The quarter-wave plate was obtained by full-temperature calibration at different temperatures.

[0044] In step three, the magnetic field strength distribution The result is obtained through analytical or numerical model calculations based on the position of the conductor being measured and the geometric relationship of the sensing fiber optic loop.

[0045] In summary, this application includes at least one of the following beneficial technical effects:

[0046] 1. By setting up a waveplate temperature sensor to acquire the temperature of a quarter-wave plate in real time, and using a deterministic physical model derived from the Jones matrix theory to compensate and correct the nonlinear error introduced by the non-ideality of the quarter-wave plate based on temperature, the periodic nonlinear error can still be accurately corrected in the megaampere-level high current measurement scenario. This overcomes the problem of insufficient compensation accuracy of traditional empirical polynomial fitting methods in the high current range and improves the accuracy of high current measurement.

[0047] 2. By laying distributed temperature-measuring optical fibers along the same path of the sensing optical fiber to obtain the temperature distribution along the sensing optical fiber, and combining it with the magnetic field intensity distribution along the sensing optical fiber path to calculate the magnetic field-weighted equivalent temperature to correct the Wilder constant, compared with simple arithmetic average temperature or single-point temperature measurement methods, it can more accurately reflect the equivalent thermal effect under the condition of uneven temperature distribution along the sensing optical fiber, improve the accuracy of Wilder constant temperature compensation, and the compensation of the two temperature zones is independent and uncoupled, resulting in a significant improvement in overall measurement accuracy.

[0048] 3. By setting up an integrated engineering structure including a metal protective tube, a high-temperature resistant insulating protective tube, and a cooling channel, the four functions of vacuum sealing, mechanical protection, active cooling, and temperature measurement are integrated into one. The metal protective tube is connected to the vacuum chamber through a welded flange to form a vacuum sealing boundary. The cooling channel is compatible with both water cooling and air cooling modes to protect the optical fiber during high-temperature baking. Moreover, the sensing optical fiber can be pulled out from the protective tube for replacement without damaging the vacuum sealing boundary, which improves the engineering reliability and maintainability of the sensor in the extreme environment of the fusion device. Attached Figure Description

[0049] Figure 1This is a schematic diagram of the system structure of an optical fiber current sensor for plasma current measurement provided in one embodiment of this application;

[0050] Figure 2 This is a flowchart of a temperature compensation method for a fiber optic current sensor for plasma current measurement provided in one embodiment of this application;

[0051] Figure 3 This is a schematic diagram of the engineering integration structure of an optical fiber current sensor for plasma current measurement provided in one embodiment of this application.

[0052] Explanation of reference numerals in the attached diagram: 1. Light source; 2. Coupler; 3. Polarizer; 4. Polarization-maintaining fiber; 5. Phase modulator; 6. Quarter-wave plate; 61. Aluminum tube; 62. Waveplate temperature sensor; 7. Sensing fiber; 8. Mirror; 9. Detector; 10. Demodulation module; 11. Distributed temperature measurement fiber; 12. Temperature compensation module; 13. Engineering integrated structure; 131. Metal protective tube; 132. Welded flange; 133. High-temperature resistant insulating protective tube; 134. Cooling channel; 1341. Cooling medium inlet; 1342. Cooling medium outlet; 14. Distributed temperature measurement system; 15. Acquisition device; 16. Conductor of the current being measured. Detailed Implementation

[0053] The technical solutions in the embodiments of this application will be further described in detail below with reference to the accompanying drawings. The described embodiments are only possible technical implementations of this application, but are not limited thereto. Other embodiments obtained by those skilled in the art in conjunction with the embodiments of this application without creative effort are also within the protection scope of this application.

[0054] This application mainly adopts a dual-temperature zone independent temperature compensation and engineering integrated structure 13, which achieves the effect of high-precision measurement of megaampere-level plasma current and enhances engineering reliability. The following is a further detailed description of this application.

[0055] Example 1

[0056] Please refer to Figure 1The fiber optic current sensor for plasma current measurement provided in this application includes a light source 1, a coupler 2, a polarizer 3, a phase modulator 5, a sensing fiber 7, a reflector 8, a detector 9, a demodulation module 10, a quarter-wave plate 6, a waveplate temperature sensor 62, a distributed temperature sensing fiber 11, and a temperature compensation module 12. The light emitted from the light source 1 passes through the coupler 2, polarizer 3, and phase modulator 5 before entering the sensing fiber 7. After being reflected at the reflector 8, it returns to the detector 9 along the same path. The demodulation module 10 demodulates the output signal from the detector 9 to obtain the feedback phase. The quarter-wave plate 6 is positioned in the optical path between the phase modulator 5 and the sensing fiber 7. The waveplate temperature sensor 62 is positioned near the quarter-wave plate 6 to acquire the waveplate temperature in real time. The distributed temperature-measuring fiber 11 is laid along the same path as the sensing fiber 7 to acquire the temperature distribution along the sensing fiber 7. The temperature compensation module 12 compensates for the nonlinear error introduced by the quarter-wave plate 6 based on the waveplate temperature, and calculates the weighted equivalent temperature based on the temperature distribution and the magnetic field distribution along the sensing fiber 7. Based on the weighted equivalent temperature, the Wilder constant of the sensing fiber 7 is corrected to obtain the measured current value. This effectively solves the nonlinear error compensation and temperature compensation problems of existing fiber optic current sensors in high current measurement, and improves the measurement accuracy.

[0057] Specifically, light source 1 can be a broadband superluminescent diode (SLD), with a center wavelength typically of... bandwidth approximately The low coherence characteristics of broadband light source 1 are beneficial for suppressing parasitic interference noise caused by various connections and interface reflections in the optical path, thereby improving the system's signal-to-noise ratio. Alternatively, a center wavelength of [missing value] can be selected. Light source 1. Coupler 2 is typically used. The fiber optic coupler distributes the light output from light source 1 to the sensing optical path at a predetermined splitting ratio (e.g., 50:50), while simultaneously guiding the signal light returning from the sensing optical path to detector 9. Coupler 2 is fabricated using a fused biconical tapered process, exhibiting low insertion loss and excellent polarization characteristics. Polarizer 3 is a linear polarizer that converts the transmitted light into linearly polarized light with a high extinction ratio. High extinction ratio (e.g., ...) The polarizer 3 can effectively suppress measurement errors caused by polarization crosstalk and ensure the purity of the polarization state entering the subsequent optical path.

[0058] The light source 1 is connected to the polarizer 3 and the phase modulator 5 through a polarization-maintaining fiber 4. The polarization-maintaining fiber 4 can maintain the polarization state of the light during transmission without degradation, ensuring that the polarized light reaches the phase modulator 5 in a definite polarization direction, thereby ensuring the working stability and measurement accuracy of the closed-loop demodulation system.

[0059] Phase modulator 5 is an integrated optical phase modulator based on lithium niobate ( The electro-optic effect operates by applying an electrically driven phase modulation to the passing polarized light. In closed-loop operation mode, the feedback phase generated by phase modulator 5... Used to track and lock onto the non-reciprocal phase shift caused by the Faraday effect, thus significantly extending the dynamic measurement range of the system and enabling it to measure megaampere-level currents. Quarter-wave plate 6 ( A quarter-wave plate 6 is placed in the optical path between the phase modulator 5 and the sensing fiber 7. Its function is to convert linearly polarized light into circularly polarized light. Only when circularly polarized light propagates in the sensing fiber 7 can a non-reciprocal Faraday rotation be generated, causing a phase difference between the light propagating in opposite directions. An ideal quarter-wave plate 6 has a phase delay angle precisely equal to 90° and an azimuth angle precisely aligned with the polarization direction of the optical path. However, actual quarter-wave plates 6 have manufacturing and installation deviations, causing the phase delay angle and azimuth angle to deviate from the ideal values. These deviations introduce nonlinear error terms related to the feedback phase. According to Jones matrix theory, the non-ideality of the quarter-wave plate 6 (phase delay angle deviation) is considered... and azimuth deviation By substituting the Jones matrix into the optical path and performing a rigorous derivation, it can be seen that, under closed-loop equilibrium conditions, the non-ideality of the quarter-wave plate 6 introduces a periodic nonlinear error term related to the Faraday rotation angle. This error term has... The parsing form consists of a single parameter. Full description, in which The phase delay angle deviation and azimuth angle deviation of the quarter-wave plate 6 are jointly determined. Unlike traditional empirical polynomial fitting methods, this deterministic physical model is naturally applicable to megaampere-level high-current measurement scenarios with large Faraday rotation angles, without the need to increase the fitting order, fundamentally solving the problem of insufficient accuracy in nonlinear error compensation in the high-current range.

[0060] Waveplate temperature sensor 62 is mounted near the quarter-wave plate 6, for example, attached to the outer surface of the aluminum tube 61 encapsulated by the quarter-wave plate 6, for real-time acquisition of the waveplate temperature. Waveplate error parameters The waveplate exhibits temperature dependence because the birefringence properties of the quarter-wave plate material change with temperature. A linear temperature model describing the relationship between the waveplate error parameters and temperature can be established by performing full-temperature calibration within the expected operating temperature range (e.g., 20°C to 60°C). In one specific embodiment, calibration is performed by applying a known standard current at five temperature points: 20°C, 30°C, 40°C, 50°C, and 60°C, and the reference temperature is then fitted. Waveplate error parameter values ​​at 25°C and temperature coefficient / °C. This linear model is simple in form, with clear physical meaning of its parameters, and is easy to implement. The sensing fiber 7 is a low-birefringence single-mode fiber, forming a sensing fiber loop around the conductor 16 (i.e., the plasma in the tokamak) to be measured, with a number of turns... The appropriate value (e.g., 1, 2, or 4) is selected based on the range and sensitivity requirements. The sensing fiber 7 is routed along the poloidal section of the vacuum chamber, forming a closed (or nearly closed) loop to surround the plasma current. The total rotation angle of the light in the sensing fiber 7 due to the Faraday effect is:

[0061] ,

[0062] Among them, factor This is because in a reflective structure, light travels back and forth twice through the sensing fiber 7. The Wilder constant at each point along the fiber path (and local temperature) (related) The magnetic field strength distribution along the fiber optic path, This is the total length of the sensing fiber 7. For In the ideal case of a looped fiber and assuming the Wilder constant is uniform along the fiber, the above equation simplifies to: ,in Let be the measured current. This formula reveals the differential contribution of the Faraday rotation angle. The physical nature of the relationship between local magnetic field strength and local temperature (via the Wilder constant) provides a theoretical basis for the subsequent calculation of magnetic field-weighted equivalent temperature.

[0063] A reflector 8 is located at the end of the sensing fiber 7, reflecting the light back along its original path. The advantage of this reflective structure is that the Faraday effect is non-reciprocal, meaning the Faraday rotation angle doubles after two round trips through the sensing fiber 7; while linear birefringence is reciprocal, automatically canceling out after two round trips, thus reducing the requirement for birefringence in the sensing fiber 7. The distributed temperature-sensing fiber 11 is laid along the same path as the sensing fiber 7 and operates based on Raman or Brillouin scattering principles. The distributed temperature-sensing system 14 (e.g., an optical time-domain reflectometer (OTDR) system based on Raman scattering) injects pulsed light into the distributed temperature-sensing fiber 11 and obtains the temperature distribution of the entire fiber by analyzing the backscattered signals at various points along the fiber. Spatial resolution can reach Temperature resolution can reach 1°C.

[0064] Temperature distribution obtained by distributed temperature measurement fiber optic cable 11 Magnetic field intensity distribution along the path of sensing fiber 7 Combined, calculate the magnetic field-weighted equivalent temperature. :

[0065] ;

[0066] The physical basis of this formula is as follows: the total Faraday rotation angle can be written as... If the temperature dependence of the Wilder constant is not too strong (the change in the Wilder constant is small relative to its absolute value within the normal operating temperature range), then... At a certain equivalent temperature Approximating at, such that This naturally leads to the definition of weighted equivalent temperature. Its physical meaning is: the stronger the magnetic field, the greater its contribution to the total Faraday rotation angle; therefore, the temperature at that location has a greater impact on the equivalent correction of the Wilder constant and should be assigned a higher weight. Compared to simple arithmetic mean temperature or single-point temperature measurement, magnetic field-weighted equivalent temperature can more accurately reflect the equivalent thermal effect under uneven temperature distribution conditions throughout the optical fiber, improving temperature compensation accuracy.

[0067] Among them, the magnetic field strength distribution The magnetic field strength is calculated using a geometric model based on the geometric position of the current-measuring conductor 16 (plasma) and the radius of the sensing fiber 7 loop. In a tokamak device, the plasma can be approximated as a ring current located at the magnetic axis. According to the Biot-Savart law or Ampere's circuital law, combined with the coordinates of the fiber optic loop path (such as the distance from each point on the fiber to the plasma center), the magnetic field strength at each point along the fiber optic path can be calculated analytically or numerically. This calculation method does not require additional magnetic field measurement equipment; it can be achieved using the device's own geometric parameters (such as the plasma's large and small radii, and the coordinates of the fiber optic loop path).

[0068] In practical digital signal processing implementations, the above integral formula is calculated using a discretized numerical method:

[0069] ,

[0070] in, This represents the number of sampling points for distributed temperature measurement. For the first The location of each sampling point The sampling interval is denoted as .

[0071] Detector 9 is an InGaAs photodetector that converts the returned optical signal into an electrical signal. Demodulation module 10 is a digital signal processing unit that executes a closed-loop feedback control algorithm and outputs the feedback phase. Simultaneously, a temperature compensation algorithm is executed, and the final output is the measured current value. Taking all compensations into account, the final formula for calculating the measured current is:

[0072] ;

[0073] The waveplate nonlinearity correction in the numerator addresses the temperature effect in the first temperature region (quarter-wave plate region 6), while the Wilder constant correction in the denominator addresses the temperature effect in the second temperature region (sensing fiber region 7). The two are independent of each other and are not coupled.

[0074] The entire system operation process of the above device is as follows:

[0075] The broadband light emitted by light source 1 is transmitted through polarization-maintaining fiber 4 to Coupler 2 distributes light to the sensing optical path according to a predetermined splitting ratio. After passing through polarizer 3, the light is converted into linearly polarized light and then enters integrated optical phase modulator 5. Phase modulator 5 applies phase modulation to the polarized light under the drive of a closed-loop control signal. The modulated light enters quarter-wave plate 6, is converted into circularly polarized light (or approximately circularly polarized light), and then enters sensing fiber 7. Sensing fiber 7 is laid around the plasma current conductor 16. When the circularly polarized light propagates in sensing fiber 7 along the direction of the magnetic field generated by the current being measured, the polarization plane rotates due to the Faraday magneto-optical effect. The rotation angle is proportional to the integral of the magnetic field strength and the fiber path length, and thus proportional to the current being measured. The light is reflected by mirror 8 at the end of sensing fiber 7 and returns along the original path. When it passes through sensing fiber 7 again, the Faraday rotation angle doubles (non-reciprocal effect), while the inherent linear birefringence effect of the fiber is automatically canceled out during the round trip due to reciprocity. The returning light passes sequentially through quarter-wave plate 6, phase modulator 5, and polarizer 3, returning to its starting point. Coupler 2, guided detector 9. Detector 9 converts the optical signal into an electrical signal, demodulation module 10 performs digital signal processing on the electrical signal, and outputs a feedback phase through a closed-loop feedback control algorithm. .

[0076] At the same time, the waveplate temperature sensor 62 collects the temperature of the quarter-wave plate 6 in real time. The distributed temperature measurement system 14 acquires the temperature distribution along the sensing fiber 7 in real time through the distributed temperature measurement fiber 11 laid along the same path as the sensing fiber 7. Temperature compensation module 12 adjusts according to waveplate temperature. Calculate the waveplate error parameters at the current temperature. For feedback phase By performing nonlinear error correction, the corrected Faraday phase shift is obtained. Then, based on the temperature distribution... and magnetic field distribution Calculate the magnetic field weighted equivalent temperature This leads to the correction of Wilder's constant. Finally, according to the formula Calculate and output the measured current value .

[0077] During system operation, the data update frequency varies among different measurement stages: closed-loop feedback phase. The read and update frequency is relatively high, typically at the kilohertz level (e.g. To meet the real-time tracking requirements when plasma current changes rapidly; waveplate temperature The reading frequency can be relatively low (e.g., once per second or every few seconds) because the temperature change in the quarter-wave plate 6 region (located outside the vacuum chamber) is typically slow; the data update cycle of the distributed temperature measurement depends on the performance of the distributed temperature measurement system 14, and is typically on the order of seconds to minutes. Between distributed temperature measurement data update cycles, the temperature compensation module 12 uses the previously acquired temperature distribution data for calculations.

[0078] The implementation principle of this embodiment is as follows: The fiber optic current sensor achieves plasma current measurement through the coordinated operation of its components. Light source 1 provides a stable optical signal, which, after processing by coupler 2, polarizer 3, and phase modulator 5, enters sensing fiber 7. Under the influence of a magnetic field, Faraday rotation occurs, and reflector 8 returns the light to detector 9 along its original path. Demodulation module 10 obtains the feedback phase based on the output signal of detector 9. Quarter-wave plate 6 converts linearly polarized light into circularly polarized light, waveplate temperature sensor 62 monitors the waveplate temperature in real time, and distributed temperature-measuring fiber 11 acquires the temperature distribution along sensing fiber 7. Temperature compensation module 12 compensates for nonlinear errors and Wilder constants based on this information, thereby obtaining an accurate measured current value. This design effectively solves the nonlinear error compensation and temperature compensation problems existing in the prior art, improving measurement accuracy. Simultaneously, the reasonable layout and coordination of the components ensure the stability and reliability of the system.

[0079] Please refer to Figure 1 and Figure 2 Specifically, the temperature compensation module 12 includes a waveplate error compensation unit and a Wilder constant compensation unit. The waveplate error compensation unit calculates the waveplate error parameters based on the waveplate temperature and performs nonlinear error correction on the feedback phase based on a specific formula to obtain the corrected Faraday phase shift; the Wilder constant compensation unit finds or calculates the corrected Wilder constant based on the weighted equivalent temperature, and then calculates the measured current.

[0080] Specifically, the waveplate error compensation unit is based on the waveplate temperature. Calculate waveplate error parameters and based on The feedback phase is corrected for nonlinear error to obtain the corrected Faraday phase shift. ,in This is the feedback phase output by demodulation module 10. Waveplate error parameters. Using a linear temperature model Obtain Reference temperature The waveplate error parameter values ​​obtained by calibration This is the temperature coefficient obtained through full-temperature calibration. The Wilder constant compensation unit is based on the weighted equivalent temperature. Find or calculate the corrected Wild constant The measured current is calculated using the following formula:

[0081] ,

[0082] in, This represents the number of turns of the sensing fiber 7.

[0083] Wilder constant With temperature Relationship The temperature coefficient of the Wilder constant of silica optical fiber is obtained through calibration. / °C, at At wavelength, the Wilder constant near room temperature is approximately In the temperature compensation module 12, the calibrated temperature can be... Relational storage can be used as a lookup table, or it can be fitted as a polynomial or linear function for real-time computation.

[0084] In this embodiment, the waveplate error compensation unit and the Wilder constant compensation unit work together to compensate for the temperature effects in the quarter-wave plate 6 region and the sensing fiber 7 region, respectively. The waveplate error compensation unit accurately calculates the waveplate error parameters using a linear temperature model, corrects the feedback phase, and eliminates the nonlinear error introduced by the non-ideality of the quarter-wave plate 6. The Wilder constant compensation unit corrects the Wilder constant based on the weighted equivalent temperature, taking into account the uneven temperature distribution along the sensing fiber 7. This independent compensation method does not interfere with each other, improves the overall compensation accuracy, and thus measures the plasma current more accurately. The technical advantage of this dual-temperature-zone independent compensation scheme is that the current calculation formula... In this process, the waveplate error correction of the numerator and the Wilder constant correction of the denominator are completely decoupled. The temperature in the quarter-wave plate region 6 (the first temperature zone) is relatively uniform, and the single-point temperature measurement of the waveplate temperature sensor 62 is sufficient to meet the requirements. The temperature distribution in the sensing fiber region 7 (the second temperature zone) is uneven. A distributed temperature measuring fiber 11 combined with magnetic field weighting is used to obtain a more accurate equivalent temperature. Each region adopts a compensation strategy that is most suitable for its physical characteristics, thus avoiding systematic errors caused by single temperature compensation.

[0085] Please refer to Figure 1 and Figure 3The fiber optic current sensor also includes an engineering integrated structure 13, which includes a metal protective tube 131, a high-temperature resistant insulating protective tube 133, and a cooling channel 134.

[0086] Specifically, the metal protective tube 131 is laid along the measurement path of the vacuum chamber, and both ends are connected to the vacuum chamber via welded flanges 132 to form a vacuum-sealed boundary. The metal protective tube 131 can be selected with an outer diameter of approximately Stainless steel tubing can be used, but copper tubing can also be selected for better thermal conductivity. The metal protective tube 131 circles the poloidal section once (or multiple times) inside the vacuum chamber, forming a closed (or nearly closed) loop surrounding the plasma. Both ends of the metal protective tube 131 pass through the vacuum chamber wall and are connected to the vacuum chamber wall via welded flanges 132. Welding is performed using TIG (tungsten inert gas) or laser welding processes to ensure that the weld quality meets the ultra-high vacuum sealing requirements (leakage rate). The welded flange 132 serves as a structural support, a vacuum sealing boundary, and an inlet / outlet for the cooling medium. The metal protective tube 131 also functions as an electromagnetic shield, reducing the interference of stray magnetic fields and electromagnetic pulses on the polarization state of the optical signal in the sensing fiber 7.

[0087] The metal protective tube 131 is directly connected to the vacuum chamber via a welded flange 132. The optical fiber passes through the tube, eliminating the need for an additional vacuum feedthrough sealing structure and simplifying the vacuum feedthrough design. Since the metal protective tube 131 is a continuous metal tube, the only channel connecting its interior to the vacuum chamber after welding and sealing is the tube itself. The tube is filled with a high-temperature resistant insulating protective tube 133 and the optical fiber, thus not creating a significant leakage path. The optical fiber exits from the metal protective tube 131 at the end face of the welded flange 132 and enters the optical fiber connection and demodulation system on the atmospheric side, where it connects with the acquisition device 15. If further improvement in sealing reliability is required, a partial sealing treatment can be applied to the tube cavity at the end face of the welded flange 132.

[0088] A high-temperature resistant insulating protective tube 133, made of PEEK, is inserted inside the metal protective tube 131. PEEK (polyetheretherketone) is a semi-crystalline polymer material with a continuous operating temperature up to 250°C (and short-term tolerance to temperatures above 300°C), meeting the baking temperature requirements of a vacuum chamber. It has excellent radiation resistance and is not easily degraded under the irradiation environment of a fusion device. It also has excellent electrical insulation properties, preventing the generation of induced current in a time-varying magnetic field. Furthermore, it has a low coefficient of friction, facilitating the insertion and extraction of optical fibers. The high-temperature resistant insulating protective tube 133 provides a secondary mechanical protective layer for the optical fiber, preventing direct contact between the fiber and the wall of the metal protective tube 131, thus avoiding micro-bending loss and stress birefringence caused by localized stress concentration. In one specific embodiment, the outer diameter of the high-temperature resistant insulating protective tube 133 is approximately... The inner diameter is approximately .

[0089] The sensing fiber 7 and the distributed temperature-sensing fiber 11 are both housed within the high-temperature resistant insulating protective tube 133. Since the two fibers are closely adjacent within the same protective tube, they experience identical temperature fields. The temperature distribution measured by the distributed temperature-sensing fiber 11 is the true temperature distribution of the environment where the sensing fiber 7 is located, eliminating temperature measurement deviations caused by the misalignment of the sensing fiber 7 and the temperature-sensing element. A cooling channel 134 is formed in the annular gap between the metal protective tube 131 and the high-temperature resistant insulating protective tube 133, used to introduce a cooling medium. The cooling medium can be water or gas, and the cooling channel 134 is compatible with both water-cooling and air-cooling modes. The cooling medium is introduced from the cooling medium inlet 1341 (which can be an air inlet or a water inlet) at one end of the welded flange 132, flows through the entire section of the metal protective tube 131, and exits from the cooling medium outlet 1342 (an air outlet or a water outlet) at the other end of the welded flange 132, forming a circulating cooling loop.

[0090] In water-cooled mode, circulating cooling water (or deionized water) is introduced, providing strong cooling capacity and suitable for conditions with high baking temperatures or large heat loads. In air-cooled mode, compressed air or nitrogen is introduced, resulting in a simpler structure and eliminating the risk of water leakage. This mode is suitable for situations where cooling water resources are inconvenient or as a backup solution for water-cooled systems. In one specific embodiment, the vacuum chamber baking temperature is 160°C, using air-cooled mode, with a compressed air flow rate of approximately... Measurements show that during the operation of the cooling system, the ambient temperature of the optical fiber inside the high-temperature resistant insulating protective tube 133 is stable below 80°C, which is far below the temperature resistance limit of the optical fiber coating material (polyimide coating, temperature resistance 300°C) and the high-temperature resistant insulating protective tube 133 material, ensuring the safety of the optical fiber and coating materials.

[0091] A quarter-wave plate 6 is located outside the vacuum chamber and encapsulated within an aluminum tube 61. The design considerations for placing the quarter-wave plate 6 outside the vacuum chamber include: the high temperature and irradiation environment inside the vacuum chamber may cause irreversible degradation of the optical performance of the quarter-wave plate 6; the waveplate temperature sensor 62 is easier to install outdoors and provides more accurate temperature measurements; and the outdoor temperature is relatively stable, resulting in less temperature drift in the waveplate error parameters and easier compensation. The aluminum tube 61 provides mechanical protection, and the high thermal conductivity of aluminum helps to homogenize the internal temperature distribution of the quarter-wave plate 6, enabling the waveplate temperature sensor 62 to accurately reflect the overall temperature of the quarter-wave plate 6. The sensing fiber 7 and the distributed temperature-measuring fiber 11 are located inside the vacuum chamber, surrounding the plasma along a poloidal path. The sensing fiber 7 can be removed and replaced from the high-temperature resistant insulating protective tube 133 without disassembling the welded flange 132 or damaging the weld seal, thus not affecting the vacuum seal boundary of the vacuum chamber. When the sensing fiber 7 or the distributed temperature measurement fiber 11 needs to be replaced due to irradiation aging, accidental damage, or other reasons, the operator only needs to pull out the old fiber from the pipe opening on the end face of the welding flange 132 and then insert the new fiber. The entire operation is completed on the atmospheric side, which reduces maintenance costs and downtime.

[0092] In this embodiment, the integrated engineering structure 13 integrates four functions: vacuum sealing, mechanical protection, active cooling, and temperature measurement. The metal protective tube 131 is connected to the vacuum chamber wall via a welded flange 132, ensuring vacuum integrity and providing electromagnetic shielding. The high-temperature resistant insulating protective tube 133 provides mechanical protection for the optical fiber, preventing damage. The cooling channel 134, through the introduction of a cooling medium, protects the optical fiber from high-temperature damage during the high-temperature baking process in the vacuum chamber. The sensing optical fiber 7 and the distributed temperature-measuring optical fiber 11 are laid along the same path, ensuring the accuracy of temperature measurement. This integrated design improves the engineering reliability of the sensor, enabling it to adapt to the extreme environment of the fusion device.

[0093] The working process of the integrated engineering structure 13 is as follows: During the normal operation of the tokamak device, the sensing fiber 7 senses the magnetic field generated by the plasma current in the sensing fiber loop within the vacuum chamber, and the distributed temperature measuring fiber 11 synchronously acquires the temperature distribution along the line. The cooling system can be turned on or off as needed. During the high-temperature baking stage in the vacuum chamber (baking temperature...), At 160°C, the cooling system is activated, and the cooling medium continuously circulates in the cooling channel 134 between the metal protective tube 131 and the high-temperature resistant insulating protective tube 133, carrying away the heat conducted from the vacuum chamber wall into the tube and controlling the ambient temperature of the optical fiber inside the high-temperature resistant insulating protective tube 133 within a safe range (below 80°C). After baking, the temperature drops to the normal operating temperature, and the sensor can continue to operate normally.

[0094] Using the above technical solution and engineering integration structure 13, this application has completed plasma current diagnostic verification on a tokamak fusion device.

[0095] During the laboratory calibration phase, a standard current source was used. Tests were conducted within the specified range. Without compensation, the nonlinear error of the quarter-wave plate 6 exhibited periodic oscillations with increasing current, significantly increasing at megaampere currents. The deterministic physical model of this application was adopted. After compensation, in Measurement accuracy is better than [a certain value] across the entire measurement range. This verifies the effectiveness and high accuracy of the waveplate error compensation model.

[0096] During the device operation phase, the sensor was installed on the poloidal section of the vacuum chamber. The vacuum chamber underwent constant temperature baking at 160°C, while the cooling system (air-cooled mode) operated continuously, stabilizing the fiber optic ambient temperature below 80°C. The sensor operated normally before and after baking, with no significant degradation in optical performance, confirming the high-temperature survivability of the engineering integrated structure 13.

[0097] In the plasma discharge experiment, the sensor measured in real time... The plasma current was measured at a certain level. The results were compared with the integral output of a Rokowski coil installed at the same location. The two showed good consistency and accurate waveform tracking, verifying the measurement capability and engineering reliability of this application in a real fusion device environment. This result indicates that the dual-temperature zone independent temperature compensation method and engineering integrated structure 13 of this application can meet the requirements of fusion devices for high-precision real-time diagnosis of plasma current.

[0098] Example 2

[0099] Please refer to Figure 2 This application provides a temperature compensation method for an optical fiber current sensor used for plasma current measurement. This method is applicable to the optical fiber current sensor described in Embodiment 1 and includes the following steps:

[0100] S1, acquire the waveplate temperature measured by the waveplate temperature sensor 62. The waveplate error parameters at the current temperature are calculated based on a pre-calibrated model relating waveplate error parameters to temperature. The model relating waveplate error parameters to temperature is as follows: ,in and The calibration was obtained by performing full-temperature calibration on the quarter-wave plate 6 at different temperatures. Full-temperature calibration means selecting multiple temperature points (e.g., 20°C, 30°C, 40°C, 50°C, 60°C) within the sensor's expected operating temperature range, applying a known standard current at each temperature point, and measuring the waveplate error parameters. The value of is used to fit the slope of the linear relationship. and intercept The technical effect of this step is that it establishes a quantitative model of how waveplate error parameters change with temperature, enabling accurate acquisition of the current waveplate error parameter values ​​at any operating temperature.

[0101] S2, Obtain the feedback phase output by demodulation module 10. Based on the waveplate nonlinear error model Calculate the corrected Faraday phase shift This step completes the temperature compensation for the first temperature region (region 6 of the quarter-wave plate). Its technical effect is that, through a deterministic physical model derived based on Jones matrix theory, the periodic nonlinear error introduced by the non-ideality of the quarter-wave plate 6 is separated and corrected from the feedback phase, resulting in a true Faraday phase shift. Due to the error term... It has an analytical form and can still be accurately corrected under high current (large Faraday rotation angle), overcoming the problem of failure of traditional empirical polynomial methods in the high current range.

[0102] S3, acquire the temperature distribution along the path of the sensing fiber 7 as measured by the distributed temperature measuring fiber 11. Combined with the magnetic field strength distribution along the sensing fiber 7 path The equivalent temperature is calculated using the weighted average formula:

[0103] ;

[0104] Magnetic field intensity distribution The magnetic field is calculated analytically or numerically based on the position of the conductor 16 under test and the geometric relationship between it and the sensing fiber 7. Specifically, the calculation of the magnetic field geometric model can be based on the Biot-Savart law or Ampère's circuital law, using the device's geometric parameters (such as the plasma's large and small radii, magnetic axis position, and coordinates of the fiber's winding path) for analytical or numerical solutions. For example, for a circular cross-section poloidal fiber, the distance between each point on the fiber and the plasma center varies along the path, resulting in a non-uniform distribution of the magnetic field strength along the fiber path. The technical effect of this step is that it physically couples the actual temperature field obtained from distributed temperature measurement with the magnetic field distribution to obtain a magnetic field-weighted equivalent temperature. Compared to simple arithmetic averaging or single-point temperature measurement, this more accurately reflects the weighted influence of the non-uniform temperature along the fiber on the Faraday rotation contribution.

[0105] S4, based on equivalent temperature The relationship between the pre-calibrated Wilder constant and temperature Calculate the Wilder constant at the current equivalent temperature. The temperature coefficient of the Wilder constant of silica optical fiber is approximately... / °C, at The Wilder constant near room temperature at the wavelength is approximately . The relationship can be obtained through calibration at different temperatures and stored as a lookup table or fitted as a function. This step completes the temperature compensation for the second temperature zone (sensing fiber 7 region), correcting the systematic measurement deviation of the Wilder constant caused by temperature changes.

[0106] S5, calculate the measured current according to the following formula:

[0107] ,

[0108] in, The number of turns of the sensing fiber 7 is used. This step unifies the compensation results of the first and second temperature zones into the current calculation formula to obtain a high-precision current measurement value after dual temperature compensation correction.

[0109] Steps S1 to S5 above are executed cyclically within each measurement cycle, updating the measurement results in real time. In actual operation, due to the different response characteristics of each measurement stage, different update strategies can be adopted: feedback phase. The acquisition and current calculation are performed at high frequencies (kilohertz levels) to ensure real-time tracking of rapid changes in plasma current; waveplate temperature Reading and Updates are performed at a low frequency (on the order of seconds) because the temperature changes slowly in region 6 of the quarter-wave plate; distributed temperature distribution. Acquisition and Updates are performed at a lower frequency (seconds to minutes), limited by the performance of the distributed temperature measurement system. During measurement cycles where distributed temperature data has not been updated, the previously acquired data is used. The value is calculated.

[0110] The implementation principle of this embodiment is as follows: This temperature compensation method solves the temperature influence problem of existing fiber optic current sensors during high-current measurement by separately compensating for the temperature of the quarter-wave plate 6 region and the sensing fiber 7 region. Steps one and two complete the temperature compensation for the quarter-wave plate 6 region (first temperature zone), correcting the nonlinear measurement error and its temperature drift introduced by the non-ideality of the quarter-wave plate 6; steps three and four complete the temperature compensation for the sensing fiber 7 region (second temperature zone), correcting the temperature dependence of the Wilder constant through distributed temperature-sensing fiber 11 and magnetic field weighting. The compensations for the two temperature zones are independent and uncoupled, and each adopts a compensation strategy best suited to its characteristics. Finally, the results of the two compensation steps are uniformly substituted into the current calculation formula to obtain a high-precision current measurement value.

[0111] The technical solution of this application is not limited to tokamak fusion devices, but can also be applied to other occasions that require high-precision current measurement in extreme environments such as strong magnetic fields, high temperatures, and vacuum, such as nuclear fusion devices, strong magnetic field scientific facilities, and high-voltage direct current transmission systems.

[0112] The specific embodiments described above do not constitute a limitation on the scope of protection of this application. Any other corresponding changes and modifications made based on the technical concept of this application should be included within the scope of protection of this application.

Claims

1. A fiber optic current sensor for plasma current measurement, comprising a light source (1), a coupler (2), a polarizer (3), a phase modulator (5), a sensing fiber (7), a reflector (8), a detector (9), and a demodulation module (10), wherein the sensing fiber (7) forms a sensing fiber loop around a conductor (16) under test; light emitted from the light source (1) enters the sensing fiber (7) after passing through the coupler (2), the polarizer (3), and the phase modulator (5), and is reflected at the reflector (8) and returns to the detector (9) along the same path; the demodulation module (10) demodulates the output signal of the detector (9) to obtain a feedback phase, characterized in that, Also includes: A quarter-wave plate (6) is disposed in the optical path between the phase modulator (5) and the sensing fiber (7); A waveplate temperature sensor (62) is disposed near the quarter-wave plate (6) for real-time acquisition of the waveplate temperature. ; a distributed temperature measuring optical fiber (11) laid along the laying path of the sensing optical fiber (7) for acquiring the temperature distribution along the line of the sensing optical fiber (7) ; Temperature compensation module (12), the temperature compensation module (12) according to the waveplate temperature The nonlinear error introduced by the quarter-wave plate (6) is compensated, and the temperature distribution is considered. and the magnetic field distribution along the path of the sensing fiber (7) Calculate the weighted equivalent temperature Based on the weighted equivalent temperature The Wilder constant of the sensing fiber (7) is corrected to obtain the measured current value.

2. The fiber-optic current sensor for plasma current measurement according to claim 1, characterized in that, The temperature compensation module (12) includes a waveplate error compensation unit and a Wilder constant compensation unit; The wave plate error compensation unit compensates for the wave plate error according to the wave plate temperature Computing wave plate error parameters And the feedback phase is nonlinearly error-corrected based on the following formula to obtain a corrected Faraday phase shift : ， wherein, is the feedback phase output by the demodulation module (10); The said Velt constant compensation unit is according to the said weighted equivalent temperature Finding or calculating the corrected Velt constant The measured current is calculated according to the following formula: ， wherein is the number of turns of the sensing fiber (7).

3. The fiber-optic current sensor for plasma current measurement according to claim 2, characterized in that, The wave plate error parameter By the following linear temperature model: ， wherein, Tref is a reference temperature the subscripted values of the waveplate error parameters, is a temperature coefficient obtained by full temperature calibration.

4. The fiber-optic current sensor for plasma current measurement according to claim 1, characterized in that, the weighted equivalent temperature is calculated by the following equation: ， in, The total length of the sensing fiber (7) is... The temperature distribution along the fiber path measured by the distributed temperature-measuring fiber (11) The magnetic field strength distribution along the path of the sensing fiber (7).

5. The fiber-optic current sensor for plasma current measurement according to claim 4, characterized in that, The magnetic field strength distribution The measured current conductor (16) and the radius of the sensing fiber (7) loop are calculated using a geometric model.

6. The optical fiber current sensor for plasma current measurement according to claim 1, characterized by, It also includes an engineering integration structure (13), which comprises: A metal protective tube (131) is laid along the measurement path of the vacuum chamber, and both ends are connected to the vacuum chamber by welding flanges (132) to form a vacuum sealing boundary; A high-temperature resistant insulating protective tube (133) is inserted inside the metal protective tube (131), and the sensing optical fiber (7) and the distributed temperature measuring optical fiber (11) are inserted together inside the high-temperature resistant insulating protective tube (133). A cooling channel (134) is formed in the annular gap between the metal protective tube (131) and the high-temperature resistant insulating protective tube (133) for introducing a cooling medium.

7. The fiber-optic current sensor for plasma current measurement according to claim 6, characterized in that The high-temperature resistant insulating protective pipe (133) is a PEEK pipe; the cooling medium is water or gas; the cooling channel (134) is compatible with both water cooling and air cooling modes; the cooling medium is introduced from one end of the welded flange (132) and led out from the other end of the welded flange (132) to form a circulating cooling circuit.

8. The fiber-optic current sensor for plasma current measurement according to claim 6, characterized in that, The quarter-wave plate (6) is disposed outside the vacuum chamber and encapsulated in an aluminum tube (61); the sensing fiber (7) and the distributed temperature measuring fiber (11) are disposed inside the vacuum chamber; the sensing fiber (7) can be extracted and replaced from the high-temperature resistant insulating protective tube (133) without damaging the vacuum seal boundary.

9. A temperature compensation method for a fiber-optic current sensor for plasma current measurement, characterized by, An optical fiber current sensor for plasma current measurement as described in any one of claims 1-8, comprising the following steps: Step 1: Obtain the waveplate temperature measured by the waveplate temperature sensor (62). The waveplate error parameters at the current temperature are calculated based on a pre-calibrated model relating waveplate error parameters to temperature. ; Step 2: Obtain the feedback phase output by the demodulation module (10) Based on the waveplate nonlinear error model Calculate the corrected Faraday phase shift ; Step three: obtaining the temperature distribution along the path of the sensing fiber (7) measured by the distributed temperature measurement fiber (11) in combination with the magnetic field intensity distribution along the path of the sensing fiber (7) calculating the equivalent temperature according to the weighted average formula: ； Step 4: Based on the equivalent temperature The relationship between the pre-calibrated Wilder constant and temperature Calculate the Wilder constant at the current equivalent temperature. ; Step 5: Calculate the measured current using the following formula: 。 10. The temperature compensation method of the optical fiber current sensor for plasma current measurement according to claim 9, characterized by, In step one, the relationship model between the waveplate error parameters and temperature is as follows: ,in and The quarter-wave plate (6) was obtained by full-temperature calibration at different temperatures; In the third step, the magnetic field intensity distribution The position of the current-carrying conductor (16) is determined by the analytical or numerical model according to the position of the sensing fiber (7) and the wrapping geometry.

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