- Email: [email protected]
Status and future developments of R&D on fiber optics current sensor for ITER
Fusion Engineering and Design xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Status and future developments of R&D on fiber optics current sensor for ITER ⁎
Andrei Gusarova, , Willem Leysena, Marc Wuilpartb, Patrice Mégretb a b
SCK-CEN Belgian Nuclear Research Center, Boeretang 200, B-2400 Mol, Belgium University of Mons, Bd Dolez 31, B-7000 Mons, Belgium
A R T I C LE I N FO
A B S T R A C T
Keywords: ITER diagnostics Plasma current measurements Fiber-optic current sensor (FOCS) JET
Successful operation of ITER will rely on the use of a large set of magnetic diagnostics. Fundamental parameters such as plasma position, shape, and current are required for real-time plasma control and machine protection. For operating tokamaks such measurements are successfully performed using inductive sensors. In ITER and later in DEMO, the presence of strong radiations combined with steady-state operation creates a difficult problem: the useful signal is affected by the integration of radiation-induced noise. An attractive alternative for plasma current measurement consists in using Fiber Optic Current Sensor (FOCS). However, combined effects of radiation, elevated temperatures, vibrations together with the requirement of vacuum compatibility and installation constrains present a significant challenge for the ITER FOCS system. This paper describes recent results of the ITER FOCS R&D intended to demonstrate that the system can be installed on the tokamak and its performance can satisfy the required criteria. We emphasize that the choice of appropriate fibers is critical. Our simulations show that the spun fibers allow satisfying the target performance provided that the fiber beat length over spun period ratio is above a given value. This conclusion is confirmed by the experimental results obtained on JET.
1. Introduction Successful operation of ITER will rely on the use of a large set of magnetic diagnostics [1]. From data obtained with those systems fundamental parameters such as plasma position, shape, and current will be derived to allow for real-time plasma control and machine protection. For operating tokamaks such measurements are successfully performed using inductive sensors, where the signal (voltage) is proportional to the derivative of the magnetic flux through the sensor. To find the plasma current the integration step is necessary. Currently used analog integrators have known intrinsic problems. Two most important are the integrator drift, which introduces an absolute error increasing with the integration time, and saturation of the integrator in the case of fast flux variation, for example during disruptions. It was demonstrated during Tore Supra long pulse operations that the performance of existing integrators can be improved to satisfy conditions of 1000 s pulse duration [2]. It was also suggested that operating during ITER-relevant times of ∼3000 s might be possible. However, in Tore Supra nuclear radiation levels are much lower as compared to ITER. The presence of strong radiations combined with steady-state operation creates a difficult problem: the useful signal is derived from integration of a weak
⁎
signal mixed with radiation-induced noise generated by radiation-induced short and long-term spurious electrical effects like RIEMF, RIC, etc. With ITER performance improvement, which means longer and more stable plasma operation and higher energy output, the problem may also worsen. It is therefore reasonable to look at other types of sensors, which can give the plasma current without the integration step. Currently, two system are considered for the ITER installation: Hall probes and fiber optics current sensor (FOCS). Initially installation of micro electro-mechanical sensors was also foreseen but it was cancelled following preliminary R&D results. The status of the Hall sensors was recently reviewed in [3]. In the present paper we will address the ITER FOCS. FOCS operation is based on the detection of the Faraday rotation experienced by polarized light in an optical fiber. FOCS signal is proportional to the plasma current and therefore its sensitivity is independent on the plasma discharge duration. FOCS has a broad frequency band-width from DC up to MHz, low weight, small size, simple coupling with fiber-optic data links. It is also important that for the modern radiation-hard optical fibers the radiation-induced absorption levels are compatible with the requirements for signal transmission for the full ITER lifetime.
Corresponding author. E-mail address: [email protected] (A. Gusarov).
https://doi.org/10.1016/j.fusengdes.2018.03.001 Received 22 September 2017; Received in revised form 25 February 2018; Accepted 1 March 2018 0920-3796/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Gusarov, A., Fusion Engineering and Design (2018), https://doi.org/10.1016/j.fusengdes.2018.03.001
Fusion Engineering and Design xxx (xxxx) xxx–xxx
A. Gusarov et al.
2. FOCS implementation in ITER The ITER FOCS system will use optical fibers placed around the Vacuum Vessel (VV) to measure the ITER plasma current. The fibers will be inserted into stainless steel tubes which provide a boundary between vacuum and non-vacuum, and also protect the fibers from mechanical damage. These tubes will be fixed with clamps on the outer VV surface. To allow easy monitoring of the system leak tightness, the tubes will be filled with gas which is not used in other systems. Currently neon is most probable candidate. In total, there are 3 FOCS locations shifted toroidally by 120°, in “ports” 5, 11 and 17, Fig. 1. Each sector has a single stainless steel tube containing a bundle of several optical fibers. A bridge will guide the FOCS tubes from the VV to the cryostat. The FOCS tubes will require penetrations through the cryostat wall to the FOCS interface. Steel tubes placed around the VV is a very interesting feature of the ITER FOCS. It allows for FOCS fiber replacement using blow-in technology. This significantly reduces the requirements for the fiber radiation hardness, because it is sufficient that the fibers survive only a fraction of the radiation load expected during the ITER operation time. This can be compared with the case of Hall probes which are welded on the VV and must survive the whole ITER lifetime. The replacability of the fibers also means that the system can be immediately upgraded if new fiber sensing technologies appear, for example to perform distributed magnetic field measurements. The commercial blow-in technology relies on several parameters like air pressure, size of the fiber tip, and fiber-tube-air friction, e.g. http://hexatronic.com, https://www.condux.com/. Often special fiber coatings and materials for guiding tubes are used in such installations with additional limitation on the bending radii. For the case of ITER all these parameters are defined by specific requirements, like vacuum compatibility, radiation hardness, installation procedures. A theoretical prediction on how the blow-in parameters should be selected to allow ITER installation is hardly possible or at least very complicated. Therefore a dedicated set-up has been installed at SCK·CEN with a tube loop geometry identical to that of ITER, including spiral penetration through the cryostat, to demonstrate the possibility of the ITER FOCS fiber replacement, Fig. 2. According to the performed tests with the prototype stainless steel tube with 6/4 mm outer/inner diameter the air pressure should be adjusted in a range of 3–4 bar, depending on the coating type. At higher pressures fiber can be broken. No special fiber termination is necessary. The fiber routing from the cryostat interface to the diagnostic building can be provided using standard fiber technology, which still must be compatible with the ITER requirements. The fiber signal will be analyzed in the cubicle area using FOCS data acquisition system and then sent to the CODAC system.
Fig. 2. SCK-CEN fiber blow-in test installation.
3. ITER FOCS environment 3.1. Radiation load Radiation environment in ITER strongly depends on the position of interest. The FOCS will be installed on the VV external skin, where the nuclear loads are reduced thanks to the shielding by the VV. Table 1 gives radiation flux estimations relevant for FOCS. The wide range of the flux values for each flux component is related with the variations along the fiber path around the VV. Most significant differences are between inner and outer sides of the torus. As a result the difference in the radiation loads in two locations can be larger than the two orders of magnitude, see Table 1. For γ-radiation the estimated average dose rate is ∼0.5 Gy/s [4], i.e. 8.5 MGy total dose for 4700 h of operation. A 20% margin should be added in the design. 3.2. Thermal loads The FOCS will be in thermal equilibrium with the VV, which will completely define the FOCS temperature environment. Table 2 summarizes possible temperature conditions for the FOCS. One end the FOCS bridge is attached to the vacuum vessel and is at the vessel temperature. The other end of the bridge comes out of the thermal shield and is at room temperature. From the cryostat vacuum the bridge is isolated with a thermal shield actively cooled with helium and maintained at 80–100 K. The ducts of vacuum vessel cooling may be placed in the same shield. Therefore, the temperature of the FOCS bridge can be in a range from 80 to 383 K. 4. FOCS performance: modelling and experimental results In the ideal case of a perfect no-birefringent fiber the magnetic fieldinduced circular birefringence is the only effect which influence the light polarization state of FOCS. The rotation angle θ of a linearly polarized wave propagating in such a fiber is defined as
θ=
∮
VB (l) dl,
(1)
where V is the Verdet constant, B is the component of magnetic field along the fiber, and the integration is performed along the fiber contour. In case when a fiber makes a loop around current, application of the Ampere theorem give a simple linear relationship between the enclosed current I and the rotation angle:
Fig. 1. CAD model of FOCS (from ITER database for CATIA models, 55 A8_FOCS in Enovia tree). The stainless steel FOCS tube is fixed on the external surface of the VV using special clapms.
θ = μVI, 2
(2)
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Table 1 Neutronic loads at the FOCS location [4]. Fp–pulse fluence, duration 3000 s. FL–life fluence, 4700 h operation time.
Flux, 1/cm2s Fp, n/cm2, FL, n/cm2
< 0.1 MeV
14 MeV
Total n
1.7 × 108 to 3.3 × 1010 < 1014 < 5.6 × 1017
4.3 × 106 to 2.6 × 109 < 7.8 × 1012 < 4.4 × 1016
3.8 × 108 to 7.7 × 1010 < 2.3 × 1013 < 1.3 × 1018
Table 2 Thermal loads for the ITER FOCS. Scenario
Value
Comment
Operation
383 K 80–383 K
Survival
493 K 70 K 5.5 mW/cm3
Normal operation at 373 K + 10 K margin Variations related to the FOCS bridge, temperature of the part around vacuum vessel is practically constant Baking at 473 K + 20 K margin Coolant leak from thermal shield Heating of the FOCS tubes and the supporting structure.
Nuclear heating
where μ is the permeability of fiber material. In a real situation the FOCS fiber has its own linear and circular birefringence, making the output in general case elliptically polarized; the simple Eq. (2) is not applicable to define the enclosed current. The fiber birefringence depends on both internal properties and external perturbations, the latter are in general not known. As a result, application of Eq. (2) gives an error of the current estimation. The error can be reduced by the proper system design. In particular, the use of a FOCS operating in the reflection mode with the Faraday mirror [5] and installation of spun fibers [6] have been proposed to compensate the detrimental effect of the linear birefringence. ITER sets requirement on magnetic diagnostic system, for which FOCS must also satisfy. For plasma current measurements it is an absolute accuracy better than 10 kA for currents up to 1 MA and a relative accuracy better than 1% for bigger currents. Recently a number of numerical simulations has been performed to verify that the FOCS can indeed satisfy these requirements. The merit of using the Faraday mirror was analyzed in [7]. That work emphasized that compensation of the linear birefringence is not efficient when a non-reciprocal effect such as the Faraday rotation is present simultaneously with the linear birefringence. The idea of using FM is based on the assumption that by exchanging the waves propagating along the slow and the fast axis with the Faraday mirror the phase difference accumulated during propagation to the mirror can be compensated on the way back, which is obviously not the case with the additional nonreciprocal effect. The effect of fiber type on the FOCS performance was investigated in [8]. FOCS systems with the geometrical characteristics corresponding the ITER implementation and with low-birefringent and spun fibers were compared. It was concluded, that the use of a low-birefringent fiber with a realistic beat length does not allow to satisfy the ITER requirement, while for the spun fiber the ratio of the beat length of unspun fiber to the spun period must be above a minimal value. This value depends on the FOCS configuration and should be at least greater than 10. If possible temperature variations are taken into account, see Table 2, this value should be increased up to 20 [9]. Vibrations of the VV during ITER operation, for example related to coolant flow, result in additional induced birefringence in the FOCS fiber. In the case of the FOCS reflection scheme including a spun fiber and a Faraday mirror the induced error remains acceptable regarding the ITER requirements [10]. There are several experimental demonstrations of plasma current measurements using FOCS. Three FOCS using low-birefringence and standard SMF-28 optical fibers have been installed and tested on the
Fig. 3. Comparison of plasma current measurements at JET using FOCS and external Rogowski coils, Shot 91600. The inset shows data near the maximal current.
Tore Supra Tokamak to measure plasma currents above 1MA [11]. The results have shown that the linear birefringence must be properly taken into account to reconstruct the plasma current. A very good linearity with respect to the plasma current and long term stability of the measurements were demonstrated. More recently plasma current measurements using FOCS were performed at JET. The system used an acrylate coated low-birefringence spun fiber, Fibercore SLB 1250 (8.9/125)-5 and operated in the transmission mode. This fiber has the ratio of the beat length of unspun fiber to the spun period of ∼100, so that the criterion discusses above is definitely satisfied. The measured current was a sum of the plasma current, the divertor coil current, and currents in conductive structures such as the vacuum vessel, divertor support structure and the PF coil. The external Rogowski coil installed at JET were expected to measure the same current and could be therefore used as a reference to which the FOCS signal can be compared. In was found that before any plasma discharges (∼5 s) the Rogowski coil measured already a significant current. It is believed that this current was induced by “crosstalk” with the toroidal field coils. We used therefore an empiric factor to correct JET data to have a fair comparison with the FOCS. After this correction the FOCS measurements are in a very good agreement with the JET data, Fig. 3. 5. Discussion and conclusions We have presented the current status the ITER FOCS R&D. Simulations and experimental results show that correct selection of the FOCS fiber is critical to achieve the necessary performance. This choice must also take into account radiation and thermal loads. For example, the fiber coating must be compatible with long-term high-temperature exposure. There are different options, with polyimide coating looking as most reasonable choice thanks to its high temperature and radiation tolerance. However, this coating is known to influence fiber polarization properties. It was found during measurements at JET that with a polyimide coated fiber the performance was significantly worse than that for the identical fiber with acrylate coating. The impact of this effect on the ITER FOCS performance requires due assessment. 3
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The FOCS fiber must sustain the radiation loads. Most known effect of radiation in optical fibers is the radiation-induced absorption (RIA). FOCS operation is based on the detection of polarization state changes and formally is independent of RIA. However, a transmission decrease results in a signal-to-noise ratio degradation, while FOCS must satisfy the ITER performance requirements. Therefore, the RIA must remain limited. This means that pure silica core fibers are best candidates. A problem here is that it may be difficult to obtain the necessary fiber performance when using only un-doped fibers. The possibility for fiber replacement is very important in this case: it allows finding a compromise between the ultimate radiation hardness and the FOCS functional requirements. The simulations discussed in the previous section assume reflective FOCS configuration with the Faraday mirror placed in the cubicle area. For non-ideal Faraday mirror the long lead fiber results in the performance degradation. It may be beneficial to place the Faraday mirror close to the vacuum vessel. In this case, it would be exposed to radiation and the effect of radiation needs to be investigated. The accurate knowledge of the Verdet constant is critical to provide the absolute current measurements. In general, it may be influenced by radiation. No change was detected with off-line measurements after gamma-irradiation up to 5.4 MGy and neutron irradiation up to ∼1015 n/cm2 (E > 0.1 MeV) [12]. However, the total neutron fluence in those tests was small compared to the fluence expected in ITER and the tests were performed off-line. Standard fibers were tested, while the current choice is the spun fibers, which are more radiation sensitive. Therefore, the conclusion should be confirmed with new reactor tests on relevant fibers and preferably with on-line measurements. Irradiation at JET could provide an assessment of transient effect, because the dose-rates during future D-T campaign will be fully relevant for ITER. FOCS measures current through the fiber contour. In ITER, the fiber is placed on the outer VV surface and it is therefore a sum of two contributions: the plasma current and the current induced in the two shells of the VV. Measurements and/or modelling of the current flowing in the VV is required to reconstruct the plasma equilibrium state. The
situation is basically the same as that of the ex-vessel partial Rogowski arrays or pick-up coils, and any other steady-state ex-vessel sensors. Acknowledgments This work has been carried out under support from the Belgian Federal Government, also within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014–2018 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the organizations mentioned above. References [1] A.J.H. Donné, et al., Diagnostics, Nuclear Fusion vol. 47, (2007), p. S337 Chapter 7. [2] P. Spuig, et al., An analog integrator for thousand second long pulses in Tore Supra, Fusion Eng. Des. 68 (2003) 953–957. [3] M. Kocan, et al., Final design of the ITER outer vessel steady-state magnetic sensors, Fusion Eng. Des. 123 (2017) 936–939. [4] G. Vayakis, et al., Nuclear technology aspects of ITER vessel-mounted diagnostics, J. Nucl. Mater. 417 (2011) 780–786. [5] P. Drexler, P.F. Fiala, Utilization of Faraday mirror in fiber optic current sensors, Radioengineering 17 (2008) 101–107. [6] R.I. Laming, D.N. Payne, Electric current sensors employing spun highly birefringent optical fibers, J. Lightw. Technol. 7 (1989) 2084–2094. [7] M. Aerssens, et al., Faraday Effect Based Optical Fiber Current Sensor for Tokamaks, ANIMMA-2011, (2011) Ghent, Belgium. [8] M. Aerssens, et al., Influence of the optical fiber type on the performances of fiberoptics current sensor dedicated to plasma current measurement in ITER, Appl. Opt. 54 (2015) 5983–5991. [9] M. Wuilpart, et al., Study of a fibre optics current sensor for the measurement of plasma current in ITER, in: E. Lewis (Ed.), Sixth European Workshop on Optical Fibre Sensors, 2016, Proc. SPIE vol. 9916, (2018) 99160L–99164L. [10] F. Descamps, et al., Simulation of vibration-induced effect on plasma current measurement using a fiber optic current sensor, Opt. Express 22 (2014) 14666–14680. [11] P. Moreau, et al., Test of fiber optic based current sensors on the Tore Supra tokamak, Fusion Eng. Des. 86 (2011) 1222–1226. [12] B. Brichard, Final report on the irradiation testing of a fibre-optic sensor suitable for plasma-current measurement, SCK-CEN (2009) 30.
4
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Status and future developments of R&D on fiber optics current sensor for ITER ⁎
Andrei Gusarova, , Willem Leysena, Marc Wuilpartb, Patrice Mégretb a b
SCK-CEN Belgian Nuclear Research Center, Boeretang 200, B-2400 Mol, Belgium University of Mons, Bd Dolez 31, B-7000 Mons, Belgium
A R T I C LE I N FO
A B S T R A C T
Keywords: ITER diagnostics Plasma current measurements Fiber-optic current sensor (FOCS) JET
Successful operation of ITER will rely on the use of a large set of magnetic diagnostics. Fundamental parameters such as plasma position, shape, and current are required for real-time plasma control and machine protection. For operating tokamaks such measurements are successfully performed using inductive sensors. In ITER and later in DEMO, the presence of strong radiations combined with steady-state operation creates a difficult problem: the useful signal is affected by the integration of radiation-induced noise. An attractive alternative for plasma current measurement consists in using Fiber Optic Current Sensor (FOCS). However, combined effects of radiation, elevated temperatures, vibrations together with the requirement of vacuum compatibility and installation constrains present a significant challenge for the ITER FOCS system. This paper describes recent results of the ITER FOCS R&D intended to demonstrate that the system can be installed on the tokamak and its performance can satisfy the required criteria. We emphasize that the choice of appropriate fibers is critical. Our simulations show that the spun fibers allow satisfying the target performance provided that the fiber beat length over spun period ratio is above a given value. This conclusion is confirmed by the experimental results obtained on JET.
1. Introduction Successful operation of ITER will rely on the use of a large set of magnetic diagnostics [1]. From data obtained with those systems fundamental parameters such as plasma position, shape, and current will be derived to allow for real-time plasma control and machine protection. For operating tokamaks such measurements are successfully performed using inductive sensors, where the signal (voltage) is proportional to the derivative of the magnetic flux through the sensor. To find the plasma current the integration step is necessary. Currently used analog integrators have known intrinsic problems. Two most important are the integrator drift, which introduces an absolute error increasing with the integration time, and saturation of the integrator in the case of fast flux variation, for example during disruptions. It was demonstrated during Tore Supra long pulse operations that the performance of existing integrators can be improved to satisfy conditions of 1000 s pulse duration [2]. It was also suggested that operating during ITER-relevant times of ∼3000 s might be possible. However, in Tore Supra nuclear radiation levels are much lower as compared to ITER. The presence of strong radiations combined with steady-state operation creates a difficult problem: the useful signal is derived from integration of a weak
⁎
signal mixed with radiation-induced noise generated by radiation-induced short and long-term spurious electrical effects like RIEMF, RIC, etc. With ITER performance improvement, which means longer and more stable plasma operation and higher energy output, the problem may also worsen. It is therefore reasonable to look at other types of sensors, which can give the plasma current without the integration step. Currently, two system are considered for the ITER installation: Hall probes and fiber optics current sensor (FOCS). Initially installation of micro electro-mechanical sensors was also foreseen but it was cancelled following preliminary R&D results. The status of the Hall sensors was recently reviewed in [3]. In the present paper we will address the ITER FOCS. FOCS operation is based on the detection of the Faraday rotation experienced by polarized light in an optical fiber. FOCS signal is proportional to the plasma current and therefore its sensitivity is independent on the plasma discharge duration. FOCS has a broad frequency band-width from DC up to MHz, low weight, small size, simple coupling with fiber-optic data links. It is also important that for the modern radiation-hard optical fibers the radiation-induced absorption levels are compatible with the requirements for signal transmission for the full ITER lifetime.
Corresponding author. E-mail address: [email protected] (A. Gusarov).
https://doi.org/10.1016/j.fusengdes.2018.03.001 Received 22 September 2017; Received in revised form 25 February 2018; Accepted 1 March 2018 0920-3796/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Gusarov, A., Fusion Engineering and Design (2018), https://doi.org/10.1016/j.fusengdes.2018.03.001
Fusion Engineering and Design xxx (xxxx) xxx–xxx
A. Gusarov et al.
2. FOCS implementation in ITER The ITER FOCS system will use optical fibers placed around the Vacuum Vessel (VV) to measure the ITER plasma current. The fibers will be inserted into stainless steel tubes which provide a boundary between vacuum and non-vacuum, and also protect the fibers from mechanical damage. These tubes will be fixed with clamps on the outer VV surface. To allow easy monitoring of the system leak tightness, the tubes will be filled with gas which is not used in other systems. Currently neon is most probable candidate. In total, there are 3 FOCS locations shifted toroidally by 120°, in “ports” 5, 11 and 17, Fig. 1. Each sector has a single stainless steel tube containing a bundle of several optical fibers. A bridge will guide the FOCS tubes from the VV to the cryostat. The FOCS tubes will require penetrations through the cryostat wall to the FOCS interface. Steel tubes placed around the VV is a very interesting feature of the ITER FOCS. It allows for FOCS fiber replacement using blow-in technology. This significantly reduces the requirements for the fiber radiation hardness, because it is sufficient that the fibers survive only a fraction of the radiation load expected during the ITER operation time. This can be compared with the case of Hall probes which are welded on the VV and must survive the whole ITER lifetime. The replacability of the fibers also means that the system can be immediately upgraded if new fiber sensing technologies appear, for example to perform distributed magnetic field measurements. The commercial blow-in technology relies on several parameters like air pressure, size of the fiber tip, and fiber-tube-air friction, e.g. http://hexatronic.com, https://www.condux.com/. Often special fiber coatings and materials for guiding tubes are used in such installations with additional limitation on the bending radii. For the case of ITER all these parameters are defined by specific requirements, like vacuum compatibility, radiation hardness, installation procedures. A theoretical prediction on how the blow-in parameters should be selected to allow ITER installation is hardly possible or at least very complicated. Therefore a dedicated set-up has been installed at SCK·CEN with a tube loop geometry identical to that of ITER, including spiral penetration through the cryostat, to demonstrate the possibility of the ITER FOCS fiber replacement, Fig. 2. According to the performed tests with the prototype stainless steel tube with 6/4 mm outer/inner diameter the air pressure should be adjusted in a range of 3–4 bar, depending on the coating type. At higher pressures fiber can be broken. No special fiber termination is necessary. The fiber routing from the cryostat interface to the diagnostic building can be provided using standard fiber technology, which still must be compatible with the ITER requirements. The fiber signal will be analyzed in the cubicle area using FOCS data acquisition system and then sent to the CODAC system.
Fig. 2. SCK-CEN fiber blow-in test installation.
3. ITER FOCS environment 3.1. Radiation load Radiation environment in ITER strongly depends on the position of interest. The FOCS will be installed on the VV external skin, where the nuclear loads are reduced thanks to the shielding by the VV. Table 1 gives radiation flux estimations relevant for FOCS. The wide range of the flux values for each flux component is related with the variations along the fiber path around the VV. Most significant differences are between inner and outer sides of the torus. As a result the difference in the radiation loads in two locations can be larger than the two orders of magnitude, see Table 1. For γ-radiation the estimated average dose rate is ∼0.5 Gy/s [4], i.e. 8.5 MGy total dose for 4700 h of operation. A 20% margin should be added in the design. 3.2. Thermal loads The FOCS will be in thermal equilibrium with the VV, which will completely define the FOCS temperature environment. Table 2 summarizes possible temperature conditions for the FOCS. One end the FOCS bridge is attached to the vacuum vessel and is at the vessel temperature. The other end of the bridge comes out of the thermal shield and is at room temperature. From the cryostat vacuum the bridge is isolated with a thermal shield actively cooled with helium and maintained at 80–100 K. The ducts of vacuum vessel cooling may be placed in the same shield. Therefore, the temperature of the FOCS bridge can be in a range from 80 to 383 K. 4. FOCS performance: modelling and experimental results In the ideal case of a perfect no-birefringent fiber the magnetic fieldinduced circular birefringence is the only effect which influence the light polarization state of FOCS. The rotation angle θ of a linearly polarized wave propagating in such a fiber is defined as
θ=
∮
VB (l) dl,
(1)
where V is the Verdet constant, B is the component of magnetic field along the fiber, and the integration is performed along the fiber contour. In case when a fiber makes a loop around current, application of the Ampere theorem give a simple linear relationship between the enclosed current I and the rotation angle:
Fig. 1. CAD model of FOCS (from ITER database for CATIA models, 55 A8_FOCS in Enovia tree). The stainless steel FOCS tube is fixed on the external surface of the VV using special clapms.
θ = μVI, 2
(2)
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Table 1 Neutronic loads at the FOCS location [4]. Fp–pulse fluence, duration 3000 s. FL–life fluence, 4700 h operation time.
Flux, 1/cm2s Fp, n/cm2, FL, n/cm2
< 0.1 MeV
14 MeV
Total n
1.7 × 108 to 3.3 × 1010 < 1014 < 5.6 × 1017
4.3 × 106 to 2.6 × 109 < 7.8 × 1012 < 4.4 × 1016
3.8 × 108 to 7.7 × 1010 < 2.3 × 1013 < 1.3 × 1018
Table 2 Thermal loads for the ITER FOCS. Scenario
Value
Comment
Operation
383 K 80–383 K
Survival
493 K 70 K 5.5 mW/cm3
Normal operation at 373 K + 10 K margin Variations related to the FOCS bridge, temperature of the part around vacuum vessel is practically constant Baking at 473 K + 20 K margin Coolant leak from thermal shield Heating of the FOCS tubes and the supporting structure.
Nuclear heating
where μ is the permeability of fiber material. In a real situation the FOCS fiber has its own linear and circular birefringence, making the output in general case elliptically polarized; the simple Eq. (2) is not applicable to define the enclosed current. The fiber birefringence depends on both internal properties and external perturbations, the latter are in general not known. As a result, application of Eq. (2) gives an error of the current estimation. The error can be reduced by the proper system design. In particular, the use of a FOCS operating in the reflection mode with the Faraday mirror [5] and installation of spun fibers [6] have been proposed to compensate the detrimental effect of the linear birefringence. ITER sets requirement on magnetic diagnostic system, for which FOCS must also satisfy. For plasma current measurements it is an absolute accuracy better than 10 kA for currents up to 1 MA and a relative accuracy better than 1% for bigger currents. Recently a number of numerical simulations has been performed to verify that the FOCS can indeed satisfy these requirements. The merit of using the Faraday mirror was analyzed in [7]. That work emphasized that compensation of the linear birefringence is not efficient when a non-reciprocal effect such as the Faraday rotation is present simultaneously with the linear birefringence. The idea of using FM is based on the assumption that by exchanging the waves propagating along the slow and the fast axis with the Faraday mirror the phase difference accumulated during propagation to the mirror can be compensated on the way back, which is obviously not the case with the additional nonreciprocal effect. The effect of fiber type on the FOCS performance was investigated in [8]. FOCS systems with the geometrical characteristics corresponding the ITER implementation and with low-birefringent and spun fibers were compared. It was concluded, that the use of a low-birefringent fiber with a realistic beat length does not allow to satisfy the ITER requirement, while for the spun fiber the ratio of the beat length of unspun fiber to the spun period must be above a minimal value. This value depends on the FOCS configuration and should be at least greater than 10. If possible temperature variations are taken into account, see Table 2, this value should be increased up to 20 [9]. Vibrations of the VV during ITER operation, for example related to coolant flow, result in additional induced birefringence in the FOCS fiber. In the case of the FOCS reflection scheme including a spun fiber and a Faraday mirror the induced error remains acceptable regarding the ITER requirements [10]. There are several experimental demonstrations of plasma current measurements using FOCS. Three FOCS using low-birefringence and standard SMF-28 optical fibers have been installed and tested on the
Fig. 3. Comparison of plasma current measurements at JET using FOCS and external Rogowski coils, Shot 91600. The inset shows data near the maximal current.
Tore Supra Tokamak to measure plasma currents above 1MA [11]. The results have shown that the linear birefringence must be properly taken into account to reconstruct the plasma current. A very good linearity with respect to the plasma current and long term stability of the measurements were demonstrated. More recently plasma current measurements using FOCS were performed at JET. The system used an acrylate coated low-birefringence spun fiber, Fibercore SLB 1250 (8.9/125)-5 and operated in the transmission mode. This fiber has the ratio of the beat length of unspun fiber to the spun period of ∼100, so that the criterion discusses above is definitely satisfied. The measured current was a sum of the plasma current, the divertor coil current, and currents in conductive structures such as the vacuum vessel, divertor support structure and the PF coil. The external Rogowski coil installed at JET were expected to measure the same current and could be therefore used as a reference to which the FOCS signal can be compared. In was found that before any plasma discharges (∼5 s) the Rogowski coil measured already a significant current. It is believed that this current was induced by “crosstalk” with the toroidal field coils. We used therefore an empiric factor to correct JET data to have a fair comparison with the FOCS. After this correction the FOCS measurements are in a very good agreement with the JET data, Fig. 3. 5. Discussion and conclusions We have presented the current status the ITER FOCS R&D. Simulations and experimental results show that correct selection of the FOCS fiber is critical to achieve the necessary performance. This choice must also take into account radiation and thermal loads. For example, the fiber coating must be compatible with long-term high-temperature exposure. There are different options, with polyimide coating looking as most reasonable choice thanks to its high temperature and radiation tolerance. However, this coating is known to influence fiber polarization properties. It was found during measurements at JET that with a polyimide coated fiber the performance was significantly worse than that for the identical fiber with acrylate coating. The impact of this effect on the ITER FOCS performance requires due assessment. 3
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The FOCS fiber must sustain the radiation loads. Most known effect of radiation in optical fibers is the radiation-induced absorption (RIA). FOCS operation is based on the detection of polarization state changes and formally is independent of RIA. However, a transmission decrease results in a signal-to-noise ratio degradation, while FOCS must satisfy the ITER performance requirements. Therefore, the RIA must remain limited. This means that pure silica core fibers are best candidates. A problem here is that it may be difficult to obtain the necessary fiber performance when using only un-doped fibers. The possibility for fiber replacement is very important in this case: it allows finding a compromise between the ultimate radiation hardness and the FOCS functional requirements. The simulations discussed in the previous section assume reflective FOCS configuration with the Faraday mirror placed in the cubicle area. For non-ideal Faraday mirror the long lead fiber results in the performance degradation. It may be beneficial to place the Faraday mirror close to the vacuum vessel. In this case, it would be exposed to radiation and the effect of radiation needs to be investigated. The accurate knowledge of the Verdet constant is critical to provide the absolute current measurements. In general, it may be influenced by radiation. No change was detected with off-line measurements after gamma-irradiation up to 5.4 MGy and neutron irradiation up to ∼1015 n/cm2 (E > 0.1 MeV) [12]. However, the total neutron fluence in those tests was small compared to the fluence expected in ITER and the tests were performed off-line. Standard fibers were tested, while the current choice is the spun fibers, which are more radiation sensitive. Therefore, the conclusion should be confirmed with new reactor tests on relevant fibers and preferably with on-line measurements. Irradiation at JET could provide an assessment of transient effect, because the dose-rates during future D-T campaign will be fully relevant for ITER. FOCS measures current through the fiber contour. In ITER, the fiber is placed on the outer VV surface and it is therefore a sum of two contributions: the plasma current and the current induced in the two shells of the VV. Measurements and/or modelling of the current flowing in the VV is required to reconstruct the plasma equilibrium state. The
situation is basically the same as that of the ex-vessel partial Rogowski arrays or pick-up coils, and any other steady-state ex-vessel sensors. Acknowledgments This work has been carried out under support from the Belgian Federal Government, also within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014–2018 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the organizations mentioned above. References [1] A.J.H. Donné, et al., Diagnostics, Nuclear Fusion vol. 47, (2007), p. S337 Chapter 7. [2] P. Spuig, et al., An analog integrator for thousand second long pulses in Tore Supra, Fusion Eng. Des. 68 (2003) 953–957. [3] M. Kocan, et al., Final design of the ITER outer vessel steady-state magnetic sensors, Fusion Eng. Des. 123 (2017) 936–939. [4] G. Vayakis, et al., Nuclear technology aspects of ITER vessel-mounted diagnostics, J. Nucl. Mater. 417 (2011) 780–786. [5] P. Drexler, P.F. Fiala, Utilization of Faraday mirror in fiber optic current sensors, Radioengineering 17 (2008) 101–107. [6] R.I. Laming, D.N. Payne, Electric current sensors employing spun highly birefringent optical fibers, J. Lightw. Technol. 7 (1989) 2084–2094. [7] M. Aerssens, et al., Faraday Effect Based Optical Fiber Current Sensor for Tokamaks, ANIMMA-2011, (2011) Ghent, Belgium. [8] M. Aerssens, et al., Influence of the optical fiber type on the performances of fiberoptics current sensor dedicated to plasma current measurement in ITER, Appl. Opt. 54 (2015) 5983–5991. [9] M. Wuilpart, et al., Study of a fibre optics current sensor for the measurement of plasma current in ITER, in: E. Lewis (Ed.), Sixth European Workshop on Optical Fibre Sensors, 2016, Proc. SPIE vol. 9916, (2018) 99160L–99164L. [10] F. Descamps, et al., Simulation of vibration-induced effect on plasma current measurement using a fiber optic current sensor, Opt. Express 22 (2014) 14666–14680. [11] P. Moreau, et al., Test of fiber optic based current sensors on the Tore Supra tokamak, Fusion Eng. Des. 86 (2011) 1222–1226. [12] B. Brichard, Final report on the irradiation testing of a fibre-optic sensor suitable for plasma-current measurement, SCK-CEN (2009) 30.
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