JP2006078494A - Polarimeter for measuring electron density of tokamak device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To measure polarization angle rotation for measuring electron density of a tokamak device, and to stably measure the electron density of the tokamak device with high reliability. <P>SOLUTION: A light source for generating a single wavelength of light is arranged in an outside of the tokamak device. The single wavelength of light gets incident into plasma inside the tokamak device, by the light source. The wavelength has a length of a wavelength of a Faraday rotation angle due to the plasma exhibiting the Faraday rotation angle larger than a measured polarization angle resolution. The single wavelength of light emitted from the plasma is detected to measure the Faraday rotation angle by a polarization detecting/measuring means. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a polarimeter, and in particular, for measuring the electron density of a polarimeter and a tokamak device that can measure the Faraday rotation of the object to be measured when Faraday rotation by an object other than the object to be measured is included in the measurement. It relates to a suitable polarimeter.

  In the operation of tokamak devices, it is essential to always measure the electron density with high reliability. For this purpose, density measurement by interferometry has generally been performed in a number of tokamak devices. The laser interferometer for measuring the electron density of a large tokamak has a weak point that comes from its measurement principle, that is, a phenomenon called “fringe jump” when it breaks due to interference conditions. Once a “fringe jump” occurs, the reliability of the density signal will then be reduced. In the operation of the tokamak, the plasma electron density is usually feedback controlled and maintained at a predetermined value. In many cases, a laser interferometer or a microwave interferometer is used as an electron density monitor for generating a feedback reference signal. In general, since interferometers follow the time development from the time of plasma generation, if for some reason a fringe jump occurs or noise is mixed in during the process, the reliability of the subsequent data is lowered. Since reliable feedback control of electron density requires an accurate density signal, a more stable system should be developed.

In an interferometer, a time history of the phase shift signal up to the tense in question is usually required to obtain data relating to that tense. In contrast, the Faraday rotation angle measured at any sampling time in the polarimeter can individually provide the data, ie the product of the electron density and the magnetic field parallel to the laser beam. Despite a few studies aimed at CO ₂ laser polarization measurements along the vertical code, only preliminary data were obtained. Furthermore, there were no studies on actual tangential code polarimetry in current tokamaks. Therefore, demonstration of the effectiveness of toroidal tangential Faraday rotation was needed to establish density measurements for the International Thermonuclear Experimental Reactor (ITER). For reference, FIG. 7 shows a tangent code of JT-60U which is one of the tokamak devices.

  As a method of measuring polarization using a laser beam as a light source in a tokamak device, conventionally, measuring the orthogonal component intensity of the polarization angle by heterodyne light interference detection, using a clockwise and counterclockwise circularly polarized laser as the laser beam, There are those that modulate the polarization angle of laser light with an electro-optic modulator. However, under the conditions where the incident laser beam intensity changes due to the displacement of the laser beam axis as expected in a large tokamak and the laser beam quality decreases due to degradation of optical components, the above method does not provide reliable measurements. It may be difficult, and it may be difficult to achieve compatibility with interference measurement.

  In an actual tokamak device, the Faraday rotation of the laser beam is considered to be added by the vacuum window, that is, the vacuum sealed window in addition to the plasma, and the Faraday rotation by the vacuum window is not negligible for measurement. May have a size. In this case, the conventional polarimeter cannot measure the Faraday rotation only due to the plasma.

  An object of the present invention is to provide a polarimeter for measuring electron density of a tokamak device.

  In order to solve the above problems, a polarimeter for measuring electron density of a tokamak device according to the present invention is a light source that generates light of one wavelength, and is disposed outside the tokamak device, and transmits the light of the one wavelength. The light is incident on the plasma in the tokamak device, and the wavelength is a Faraday rotation angle by the plasma and has a length of a wavelength that exhibits a Faraday rotation angle larger than the resolution of the measured polarization angle, and is emitted from the plasma. And a polarization detecting / measuring means for detecting the polarization of the light having the one wavelength and measuring the Faraday rotation angle.

  According to the present invention having the above-described configuration, the polarization angle rotation can be measured to measure the electron density of the tokamak device, and the electron density of the tokamak device can be stably measured with high reliability.

  In one preferred embodiment of the polarimeter for measuring electron density of the tokamak device of the present invention, the object to be measured can be obtained even when there is an object to be polarized other than the object to be measured by using two different wavelengths as the wavelength of the light source. The polarization generated by the object can be measured.

Furthermore, in another preferred embodiment of the polarimeter for measuring the electron density of the tokamak device of the present invention, the following effects can be obtained by using the photoelastic modulation means for detecting the polarization.
(1) Since there is no optical interference process in polarization measurement, high laser light quality required for optical interference becomes unnecessary.

(2) Since the modulation of the laser beam is not required, the configuration is simplified and compatibility with the interferometer can be easily achieved.
(3) When a pair of photoelastic modulators is used, two orthogonal components of the polarization angle can be detected by a single photodetector, and the polarization angle can be measured based on the signal ratio. It is possible to avoid the influence of fluctuations in the laser light intensity and the propagation axis. In other words, stable rotation angle detection can be performed without being affected by fluctuations in the received light intensity of the laser and fluctuations in the propagation axis due to fluctuations in laser output, mechanical vibration displacement of the mirror, and the like.

  (4) When used in a tokamak device, the plasma electron density can be measured by measuring the Faraday rotation angle of the laser light plasma propagating in the toroidal tangential direction. In particular, in ITER, a polarimeter using a carbon dioxide laser beam as a light source can measure more stably than a general interferometer as an electron density monitor.

  (5) The polarimeter of the present invention is less susceptible to external noise than the interferometric method.

First, the two-wavelength polarimeter (also referred to as a two-color light polarimeter) of the present invention will be described with reference to an example of measuring polarization by plasma in a tokamak device.
In the plasma, the Faraday rotation angle α _p of the linearly polarized laser beam is approximately expressed as follows.

Where e is the charge of the electron, m _e is the mass of the electron, ε ₀ is the vacuum dielectric constant, c is the speed of light in vacuum, k is the wave number of the laser beam (2π / λ), and λ is the wavelength of the laser. , n _e is the electron density, B‖ (hereinafter this symbol denote the symbols have the ‖ to B and subscript.) represents respectively a parallel magnetic field strength with respect to the beam propagation direction, integral laser beam It is executed along the propagation path.

  Usually, the measured rotation angle includes Faraday rotation not only by the plasma but also by the vacuum window.

Here, V is the Verdet constant of the window material, B‖ _w is parallel magnetic field strength relative to the beam propagation in the window, d _w respectively represent the effective thickness of the window. If α _w cannot be ignored as a significant offset component, α _w must be excluded from the measured rotation angle. For this purpose, a two-wavelength laser polarimeter was devised for the rotation angles α ₁ and α ₂ measured with two polarimeters with different wavelength lasers and written as:

Here, subscripts 1 and 2 indicate different wavelengths. The integral in equation (3) is easily obtained as follows.

Here, R _V is the ratio of Verde constants R _V = V ₁ / V ₂ . Thus, the Faraday rotation component at the vacuum window is eliminated using the dual wavelength polarimeter configuration. When the wavelength dependence of the Verde constant, ie V ₂ ≈ (k ₂ / k ₁ ) ² V _1, is valid for use, equation (4) can be written as:

The effective resolution in the above two-wavelength configuration is evaluated by considering the signal to noise ratio S / N. From equation (5), S / N is determined by the precision of the last parenthesis as follows.

Here, r _α is the original resolution of the rotation angle of each polarimeter, x is the wavelength ratio k ₂ / k ₁ , and r _α ^eff is the effective resolution of the rotation angle in the two-wave polarimeter. . Therefore, the effective resolution is smaller than the original resolution by a factor of (1-1 / x ² ).

  As described above, by using two different wavelength lasers, the polarization generated by the vacuum window in the tokamak device is excluded from the measurement result, and only the polarization by the plasma can be measured. As is clear from the above description, the light source used in the present invention is not limited to a laser, and any type of light source may be used. Further, the object to be measured and the other polarizing object are not limited to the plasma and the vacuum window, respectively, and it is obvious that the object to be polarized may be used.

Next, a preferred embodiment of a polarimeter for measuring electron density of a tokamak device according to the present invention will be described.
In accordance with the results shown above for the dual wavelength polarimeter of the present invention, a dual wavelength laser polarimeter was studied against JT-60U and ITER. Table 1 shows typical parameters for JT-60U and ITER, including several parameters for the vacuum window.

The view of Table 1 is as follows. R ₀ is the main radius of the center of the vacuum vessel, and α _p is the secondary radius of the plasma. _ne is the electron density and B _t0 is the toroidal magnetic field at R ₀ . R _w is the major radius of the position of the vacuum window, B‖ _w are parallel toroidal magnetic field to the laser beam at R _w, d _w is the vacuum window experiencing the laser beam therethrough thickness That's it. For JT-60U, typical plasma parameters and current two-wavelength CO ₂ laser interferometer geometry are selected. For ITER, a toroidal tangent code with a tangent to _R0 is considered. R _w and d _w are each selected to be the appropriate radius and the same thickness for JT-60U.

  Now consider the case where the vacuum window is oriented so that the normal of the vacuum window is almost parallel to the tangential beam path. In this case, the toroidal field should be considered for Faraday rotation in the vacuum window rather than the horizontal field. It is not discussed when the vacuum windows are oriented so that their normals are radial, despite the negligible toroidal field component. This is because the folding mirror must be installed inside the vacuum vessel to realize the tangential cord. In actual equipment, damage to these mirrors can be significant due to tokamak operation. For consideration of polarimeter wavelength combinations, Table 2 shows the Faraday rotation angles evaluated for several lasers with wavelengths ranging from the visible to the far infrared.

In Table 2, λ is the wavelength of the laser light, α _p is the Faraday rotation angle by the plasma with respect to the round trip path of laser propagation in the plasma, and α _w is the Faraday rotation angle by the material of the vacuum window. for the evaluation of alpha _p, spatial uniform density distribution of the plasma is assumed. for the evaluation of alpha _w, it is assumed that the lead sulfide (ZnS) is a material of the vacuum window. Here, it is assumed that the Verde constant of ZnS is proportional to λ- ² .

Based on experience with a two-wavelength CO ₂ laser interferometer in JT-60U, two different wavelengths of CO ₂ laser of 10.6 and 9.27 μm are considered. Table 2 is used to exclude YAG green lasers and YAG-near infrared lasers. This is because their Faraday rotation angle in the plasma is substantially small, while their Faraday rotation angle in the vacuum window is too large to be easily measured. A CH ₃ OH laser is also excluded because its Faraday rotation in the plasma is too large to measure. CO lasers may be used, but the danger of handling CO gas is undesirable. Moderate rotation angles both in the plasma and in the vacuum window are shown for _two different CO ₂ wavelengths of 10.6 and 9.27 μm. They therefore provide the most preferred combination for the polarimeter. Naturally, this polarimeter It should be called two-wavelength CO ₂ laser polarimeter The polarimeter has _two laser interferometers compatible 2-wavelength CO. When ˜100 S / N of a two-wavelength CO ₂ polarimeter is required for ITER, by using equation (5), r _α ^eff should be better than 0.42 °, or r _α is Should be better than 0.1 °.

Polarization measurements for large tokamaks should not be sensitive to any changes in the measuring laser beam at the detector. However, in large devices, the intensity of the measurement laser beam at the detector can be due to several reasons, such as refraction of the laser beam in the plasma, displacement and vibration of the mirror mounting structure, laser It may change due to reduction in transmission of the guide optical system, fluctuation of the output power of the oscillator, and the like. Displacement and vibration of the mirror mounting structure also causes a shift of the propagation beam axis of the measuring laser. To account for these problems and avoid errors such as fringe jumping, a conventional polarimeter that measures the two orthogonal components of the plane of polarization of the interference signal is not appropriate. Polarization modulation techniques using electro-optic modulators also appear to be disadvantageous in this situation. Since the rotation angle obtained by the above technique is directly related to the intensity of the measuring laser beam, the accuracy of the data is reduced when the intensity of the laser beam changes significantly. The inventor has therefore devised polarization detection using a pair of photoelastic modulators (PEMs) for stable polarization measurements on large tokamaks. FIG. 1 shows a schematic diagram of a CO ₂ laser polarimeter using a PEM according to a preferred embodiment of the present invention. The polarimeter comprises two photoelastic modulators (PEM 1 and PEM 2) 10 and 12, a polarizer 14, and an HgCdTe detector 16. The polarimeter has a light source, in this case a CO ₂ laser, which is not shown in FIG. Each PEM 10 and 12 is driven by a different drive frequency of f ₁ (37 kHz) and f ₂ (50 kHz), both PEMs 10 and 12 being 45 ° and 0 ° with respect to the plane of polarization of the incident CO ₂ laser beam. Are aligned at different polarization coordination angles. As described above, one feature of the present embodiment is that two orthogonal components of the polarization angle can be measured with one detector by combining two photoelastic modulators having different driving frequencies by rotating 45 degrees around the optical axis. One. The detected signal has two lock-in amplifiers 18 and 19 with different lock-in frequencies corresponding to the frequency doubled with respect to the driving frequency of PEM1 and PEM2, ie 2f ₁ (74 kHz) and 2f ₂ (100 kHz). Is analyzed.

The rotation angle of the plane of polarization of the incident CO ₂ laser beam is simply determined as follows.

Here, α is the rotation angle of the polarization plane of the incident laser, and V _out1 and V _out2 are the output voltages of the two lock-in amplifiers. This configuration of the polarimeter is largely the same as that used in the motional Stark effect polarimeter for plasma current distribution measurements in the tokamak, except for the detectable wavelength of light. That is, although this method has a track record in measuring the motional Stark effect in the visible region, it is necessary to newly evaluate the performance of the carbon dioxide laser in the infrared wavelength region (-10 μm). Therefore, when a calibration test of the polarization angle detection unit was performed, a polarization angle resolution within 0.1 degrees was obtained by using an appropriate approximate function in the range from the Faraday rotation angle assumed to ITER to about 40 degrees. I found out that With the configuration shown in FIG. 1, it can be expected that the Faraday rotation angle can be measured with sufficient accuracy. Since a single detector provides two orthogonal components of the polarization angle of the laser beam, the polarimeter is essentially insensitive to changes in the propagation axis and intensity of the measuring laser beam within the modulation frequency of PEM1. . A pair of polarimeters are used in a two wavelength CO ₂ laser polarimeter.

Therefore, the polarimeter of the present invention produces the following effects by adopting the photoelastic modulator. That is,
(1) Since there is no optical interference process in polarization measurement, high laser light quality required for optical interference becomes unnecessary.

(2) Since the modulation of the laser beam is not required, the configuration is simplified, and compatibility with the interferometer can be easily achieved as will be described later.
(3) When a pair of photoelastic modulators is used, two orthogonal components of the polarization angle can be detected by a single photodetector, and the polarization angle can be measured based on the signal ratio. It is possible to avoid the influence of fluctuations in the laser light intensity and the propagation axis. That is, it is possible to detect a stable rotation angle that is not affected by fluctuations in the received light intensity of the laser and fluctuations in the propagation axis due to fluctuations in laser output, mechanical vibration displacement of the mirror, and the like.

  (4) When used in a tokamak device, the plasma electron density can be measured by measuring the Faraday rotation angle of the laser light plasma propagating in the toroidal tangential direction. In particular, in ITER, a polarimeter using a carbon dioxide laser beam as a light source can measure more stably than a general interferometer as an electron density monitor.

(5) The polarimeter of the present invention is less susceptible to external noise than the interferometric method.
Next, it will be described that the polarimeter for measuring electron density of the tokamak device of the present invention is suitable for use with an interferometer.

  According to the prior art, as described above, the following three methods can be considered as a method for simultaneously measuring polarization and interference. One is to use two interferometers whose polarization angles are orthogonal to each other to measure interference and to measure polarization by causing interference. The second is to measure the polarization at the same time as the interference by modulating the linearly polarized light in the clockwise and counterclockwise circular polarization to cause interference. The third is to change the polarization angle of linearly polarized light with time to cause interference and measure the polarization simultaneously with the interference. In these methods, it is difficult to cause interference, circular polarization modulation, or changing the polarization angle with time, and it is not easy to measure polarization and interference simultaneously.

  Using the polarimeter for electron density measurement of the tokamak device of the present invention, it is possible to measure interference and polarization simultaneously without interfering two waves, modulating the circular polarization, or changing the polarization angle with time. A preferred embodiment of the present invention which can be made is shown in FIG. The interference measurement system is known. In FIG. 2, reference numerals 20 and 22 are carbon dioxide lasers that are light sources, and the two carbon dioxide lasers have different wavelengths, 9.27 μm and 10.6 μm. The carbon dioxide laser 20 is used as a light source for both interference and polarized light, and the carbon dioxide laser 22 is used as a light source for interference measurement. Reference numeral 24 denotes a polarization detection unit, which includes two photoelastic modulators (PEM1 and PEM2) 50 and 52, a polarizer (P) 54, a filter 56, and a detection unit 58. The photoelastic modulators 50 and 52 are the same as the photoelastic modulators 10 and 12 in the embodiment shown in FIG. The detection unit 58 includes the same components as the HgCdTe detector 16 and the two lock-in amplifiers 18 and 20 in the embodiment shown in FIG. The filter 56 transmits the 9.27 μm laser beam and blocks the 10.6 μm laser beam. Reference numeral 26 is an interference detection unit, which acts as a reflector for only two detectors 60 and 62, two filters 64 and 66, a plurality of total reflection mirrors TM, and 9.27 μm. It has a plurality of dichroic mirrors DM and a plurality of semi-reflective mirrors HM. Filter 64 transmits a 10.6 μm laser beam and blocks 9.27 μm, while filter 66 is vice versa. The components provided between the carbon dioxide lasers 20 and 22 and the polarization detector 24 and the interference detector 26 are as follows. The components indicated by the reference symbols BE1 to BE3 are beam expanders for changing the laser beam parameters so that they are propagated to the delay optical unit, and the reference symbols AOM1 to AOM2 are frequencies for heterodyne detection. An acousto-optic modulator used as a shifter, reference symbol TM is a total reflection mirror, reference symbol HM is a semi-reflection mirror, and reference symbol MD is a plurality of dichroic acting as a reflector for 9.27 μm only. A mirror, and reference symbol L is a lens. A beam splitter (BS) 28 provided after the lens L is specially provided for simultaneously measuring polarization and interference, and is one of the features of the embodiment of the present invention shown in FIG. It is. The ratio of transmission and reflection of the beam splitter 28 is set to 2: 8. The reference sign Vis-HeNe is a visible HeNe laser, which is used to align the optical components, but not directly to measure polarization and interference.

  The laser beams emitted at the respective wavelengths from the carbon dioxide lasers 20 and 22 are incident on a plurality of total reflection mirrors TM, semi-reflection mirrors HM, and dichroic mirrors DM through beam expanders BE1 and BE2. The laser beam is divided into two laser beams by a mirror and used as measurement light and reference light that pass through the delay optical unit and the plasma via frequency shifters AOM1 and AOM2, respectively. In FIG. 2, the flow of the laser beam indicated by the solid line indicates the measurement light, and the flow of the laser beam indicated by the broken line indicates the reference light. About 80% of the two-wavelength laser beam that has passed through the delay optical section and the plasma is reflected by the beam splitter (BS) 28 behind the lens L and is incident on the interference detection section 26, while the two-wavelength laser beams are incident on the interference detection section 26. The reference light, which is a laser beam, is incident on the interference detection unit 26 by the total reflection mirror TM in front of the lens L. The measurement light and the reference light incident on the interference detection unit 26 are converted into a laser beam having only one wavelength by the filters 64 and 66 and are incident on the detectors 60 and 62. The measurement light of each wavelength is detected by each detector. The interference signal of the reference light is detected, and the interference fringe amount is obtained based on the result. Note that the measurement using two different wavelengths is for eliminating the influence on the interference due to the position change during the measurement of the system under measurement. In principle, for example, there is no such a change or If negligible, one wavelength, i.e. one light source and one detector.

  In the polarization measurement, only the measurement light that has passed through the plasma is used, so that all the reference light is incident on the interference detection unit 26 by the total reflection mirror TM in front of the lens L, and only the measurement light is a beam splitter after the lens L. The laser beam having a wavelength of 9.27 μm of the measurement light is reflected by the dichroic mirror DM and is incident on the polarization detector 24. In the polarization detector 24, the polarization is measured in the same manner as the embodiment shown in FIG. The above-described operation makes it easy to measure polarization and interference at the same time, without using any of the two methods that interfere with conventional methods, such as interference between two waves, circular polarization modulation, or changing the polarization angle over time. it can.

  In the embodiment shown in FIG. 2, an example in which one wavelength is used for the polarization measurement has been shown. However, when two wavelengths are used, the polarization detection unit 24 further uses, for example, 9.27 μm illustrated for 10.6 μm. Obviously, it is sufficient to provide a series of the same.

  Table 3 shows the result of the trial calculation of the Faraday rotation angle by the plasma of the tokamak device to be measured by the measurement system shown in FIG. It is assumed that the plasma has a uniform density distribution in space.

3 and 4 show the results of calibration using the polarization detector 24 alone. FIG. 3 is for confirming the linearity, and it can be seen that the following expression is obtained as the fitting function 1 and shows a good linearity.

Measurement angle = 0.99 x set angle + 0.53 (degrees)

FIG. 4 is for confirmation of accuracy, and the following equation is obtained as the fitting function 2, and accuracy of 0.1 degrees or less can be expected by this appropriate fitting function.

Measurement angle = 0.95 × setting angle + 6.7 × 10 ⁻⁴ × setting angle ²
+ 9.2 × 10 ⁻⁶ × set angle ³ +0.74 (degrees)

FIG. 5 shows a measurement result of the tangential Faraday rotation angle (change in polarization angle) measured by the measurement system shown in FIG. From this figure, it can be seen that the change in the polarization angle is in good agreement with the line density waveform of the tangential carbon dioxide laser interferometer of the same line of sight, thus reflecting the electron density and correctly measuring the Faraday rotation by the plasma. Moreover, it can be said from the figure that the resolution is about 0.1 degree, and the target value is achieved.

  FIG. 6 shows various measurement waveforms measured by the measurement system shown in FIG. (E) of the figure shows that the polarization angle of the tangential carbon dioxide laser beam was successfully detected for the first time, and that the Faraday rotation that seems to be caused by the vacuum window can be detected. In addition, it is confirmed that stable measurement can be performed even if the incident laser beam intensity fluctuates, and further, it is confirmed that polarized light can be measured simultaneously with interference, so that it is possible to measure the electron density of plasma with high accuracy. It is.

FIG. 1 is a schematic diagram of a CO ₂ laser polarimeter using a photoelastic modulator PEM according to a preferred embodiment of the present invention. FIG. 2 is a diagram showing a preferred embodiment of the present invention that enables simultaneous measurement of interference and polarization. FIG. 3 is a diagram showing a result of calibration for confirming linearity with the polarization detection unit 24 alone shown in FIG. FIG. 4 is a diagram showing the result of calibration for confirming the accuracy of the polarization detector 24 shown in FIG. 2 alone. FIG. 5 is a diagram showing measurement results of the tangential Faraday rotation angle (change in polarization angle) measured by the measurement system shown in FIG. FIG. 6 is a diagram showing various measurement waveforms measured by the measurement system shown in FIG. FIG. 7 is a diagram showing a tangent code of JT-60U which is one of the tokamak devices.

Explanation of symbols

10, 12, 50, 52 Photoelastic modulator 14, 54 Polarizer 16 HgCdTe detector 18, 19 Lock-in amplifier 20, 22 Carbon dioxide laser 24 Polarization detector 26 Interference detector 28 Beam splitter 56, 62, 64 Filter 58 detection unit

Claims (8)

A light source that generates light of one wavelength, and is disposed outside the tokamak device, and the light of the one wavelength is incident on plasma in the tokamak device, and the wavelength is a Faraday rotation angle by the plasma. The light source having a wavelength length exhibiting a Faraday rotation angle greater than the measured polarization angle resolution;
A polarimeter for measuring electron density of a tokamak device, comprising: polarization detection / measurement means for detecting polarization of light of the one wavelength emitted from the plasma and measuring the Faraday rotation angle. There is an unnecessary object other than the plasma that causes Faraday rotation,
The light of the one wavelength propagates through the plasma and unnecessary materials and is emitted,
The one wavelength also has a length of wavelength that exhibits a Faraday rotation angle greater than its measured polarization angle resolution due to the unwanted Faraday rotation angle,
The polarization detection / measurement means detects the polarization of the light having the one wavelength that has been propagated through the plasma and the unnecessary object, and measures the Faraday rotation angle due to the plasma and the unnecessary object.
And further comprising another light source that generates light of one wavelength different from the one wavelength,
The another light source is disposed outside the tokamak device, and the light of one different wavelength is incident on the plasma and unwanted objects to propagate,
The one different wavelength has a length of a wavelength that exhibits a Faraday rotation angle by each of the plasma and the unwanted material and is larger than their measured polarization angle resolution,
Another polarization detection / measurement means for detecting the polarization of the light of one different wavelength emitted through the plasma and the unwanted object and measuring the Faraday rotation angle due to the plasma and the unwanted object;
The polarization for electron density measurement of the tokamak device according to claim 1, further comprising: an arithmetic unit that calculates a Faraday rotation angle by the plasma using two Faraday rotation angles measured by both of the polarization detection / measurement units. Total.   The polarimeter for measuring electron density of a tokamak device according to claim 1 or 2, wherein the light source is laser light. The polarimeter for measuring the electron density of the tokamak device according to claim 3, wherein the laser light is CO ₂ laser light.   Interference between the reference light that is a part of the light of one wavelength and the remaining light of the light of the one wavelength that has passed through the plasma or the plasma and unwanted objects 5. The polarimeter for measuring the electron density of the tokamak device according to claim 1, further comprising interference detecting means for measuring the polarization simultaneously with the polarization. A light splitting unit that splits the light of one wavelength, generates the reference light and measurement light, enters the delay optical unit that delays the reference light, and causes the measurement light to enter the plasma;
The measurement light is divided into two, one of the divided measurement lights is incident on the polarization detection / measurement means for measuring the Faraday rotation angle, and the other measurement light divided is applied to the interference detection means. Measuring beam splitting means to be incident;
6. The polarimeter for measuring electron density of a tokamak device according to claim 5, further comprising means for causing the reference light that has passed through the delay optical unit to enter the interference detection means.   The polarimeter for measuring an electron density of a tokamak device according to any one of claims 1 to 6, wherein the polarization detection unit includes a photoelastic modulation unit.   The polarimeter for measuring electron density of a tokamak device according to claim 7, wherein a pair of the photoelastic modulation means is provided.

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