Fibre-optic current sensors for ITER and WEST

Miazin Anton

Promoter

Wuilpart Marc, (UMONS), marc.WUILPART@umons.ac.be

SCK•CEN Mentor

Goussarov Andrei
andrei.goussarov@sckcen.be
+32 14 33 85 06

SCK•CEN Co-mentor

Leysen Willem
willem.leysen@sckcen.be
+32 14 33 34 43

Expert group

Irradiations and Experiments

PhD started

2016-11-16

Short project description

The main purpose of this PhD project is the development of Fiber Optics Current Sensor (FOCS) prototype suitable for performing measurements in future thermonuclear fusion reactors, considering WEST and ITER tokamaks as milestones on this path. Solution of this task requires innovations in fiber-optics current sensing combined with the adaptation of the FOCS engineering design, making is suitable for the use during Burning Plasma eXperiments (BPX). Based on experimental work and numerical simulations, an innovative FOCS design will be proposed. The design must take into account and mitigate the impact of the harsh environment on the sensors performance characteristics. Perturbations related with high temperatures, vibrations, and radiation combined with the need to measure extreme currents, far beyond currents measured in any other application, make this task extremely challenging. The proposed design will be validated by comparing the modelling results with measurements on the FOCS test bed. The final step of the validation will be performing current measurements on tokamaks with an ITER-representative environment.

In magnetic fusion reactors the plasma current is a fundamental parameter required for the control of the reactor operation. It should typically be measured with an accuracy of 1%. Nowadays, the plasma current is mainly measured by using inductive coils (ex. Pick-up coils and/or Rogowski coils) placed around the reactor [1]. Their functioning principle requires an integration step, which makes the sensor insensitive to stationary currents. The usage of this type of sensor in future BPX is therefore extremely challenging. Already for ITER, which will operate still in a quasi-stationary mode (plasma pulses of typically 3000 s) development of dedicated integrators with a very low drift appears to be an extremely difficult task.

An alternative to the traditional inductive sensors consists in exploiting the Faraday effect in an optical fibre when it is subject to a magnetic field: the presence of a magnetic field aligned with the fibre axis induces a circular birefringence. As a consequence of this circular birefringence, an input state of polarization rotates by an angle θ proportional to the integral of the magnetic field component parallel to the fibre along the fibre length. It follows from the Ampere theorem, that if the fibre forms a loop around a current I, the rotation θ is proportional to I. Thus, this approach provides a direct measurement of the current without requiring any integration step [2], which is of high interest in the context of ITER.

Commercial FOCSs are successfully used in power industry for decades. Unfortunately, those sensors could not be immediately installed in fusion devices. For example, one of the FOCS issue is the unavoidable presence of linear birefringence in the fibre, which competes with the useful current-induced circular birefringence. Solutions of this problem implemented in commercial devices are not suitable for fusion installations due to restrictions imposed by the presence of radiation, high temperatures, and vacuum compatibility. Another problem is the need to measure currents in the order of 20 MA, while the operation of commercial devices is limited by 800 kA. The high currents makes the available detection schemes unsuitable and can also bring into play new effects. This list can be extended. This list can be extended. However, advantages which FOCS brings for ITER warrant search of their solution.

In that context, a collaboration work between the University of Mons, the SCK•CEN and the IRFM (CEA Cadarache) has started in the frame of a PhD thesis that will end in September 2015. One of the main result of it is the determination of the best type of fibre that should be used in order to minimize the linear birefringence detrimental effect in the context of ITER monitoring. Existing studies have shown that the radiation effects on fibre attenuation are sufficiently limited in the ITER environment [3]. Within this collaboration framework, it has been demonstrated that a FOCS located around the tokamak vacuum vessel are suitable to measure the plasma current [4].

The proposed new PhD work can be considered as a continuation of the previous work. For the fulfilment of the PhD the collaboration with the IRFM is extremely beneficial since it allows participating in the WEST project. WEST (Tungsten (W) Environment in Steady-state Tokamak) project consists of a major modification of the Tore Supra tokamak. With WEST, Tore Supra will become a test platform for a key component of ITER: the tungsten divertor. To achieve this goal, Tore Supra must be re-configured: a tungsten divertor will be added and poloïdal field coils will be installed in the vacuum chamber. The measurement of the current plasma and the quantification of the current circulating in the conductive elements installed in the chamber are required in order to ensure a precise measurement of the shape and the position of the plasma. In the context of WEST, fibre-optics sensors are also of high interest since the size of optical fibres is such that it could fit in the small space available for the sensor.

[1] Ziegler et al., IEEE Sensors Journal, Vol. 9, N°4, pp. 354-376, 2009.

[2] Aerssens et al., Proceedings Photonics Europe 2012, SPIE 8439, papier 84390D-1.

[3] Brichard, Fusion Technology Program, Physics Integration Diagnostics, EFDA TW5-IRRCER, 2006.

[4] Ph. Moreau, B. Brichard et al. Fusion Engineering and Design 86 (2011) 1222–1226

Objective

Standard FOCS: Improvement of the performance

The standard FOCS technique consists in measuring the polarization rotation undergone by an input light after propagation in an optical fibre loop surrounding the current to be measured. The current can be retrieved from the rotation angle. Practically, this task is not straightforward due to the presence of linear birefringence. The linear birefringence must be properly accounted for. Attempts to perform this task have already been made in the frame of a collaborative work between the University of Mons, the SCK-CEN and the CEA [3]. The use of different fibre types has been investigated and it was concluded that spun fibres are the best candidates for such a measurement [5]. Experiments also confirmed that vibration effects can generate large extra measurement error [6]. Nevertheless, the design of a FOCS suitable for operation in an ITER-like environment remains at an initial stage. The goal of the present PhD project is to bring new insights and innovations required to make the FOCS suitable for tokamak diagnosis. In particular the following tasks will be considered:

  • Identification on all possible nuisance parameters such as: temperature, vibration, bending, micro bending, fibre birefringence, link fibre, non-ideal faraday mirror, wavelength stability, radiation, coating, which may influence the measurement accuracy.
  • Experimental work to qualify the identified nuisance parameters.
  • Development of a modelling that includes the identified nuisance parameters and study of its effect on the measurement accuracy.
  • Development of the current reconstruction algorithms in case of a strong linear birefringence and extreme currents.
  • Perform simulations for different fibre types, including those most recently appeared on the market.
  • Development and characterisation of the FOCS test-bed at SCK•CEN.
  • Validation of the modelling by using the test bed.
  • Extrapolation of the experimental work and the modelling to ITER and WEST conditions.

 

Advanced FOCS: fibre optic magnetic measurements

The FOCS functionality can be extended by using a reflectometry technique called Polarization Optical Time Domain Reflectometry (POTDR). An advantage of this novel approach is the possibility to get rid of the link fibre influence (fibre installed between the interrogating device and the sensing fibre). In addition, this technique may allow local magnetic field measurement, which could be used on ITER to cross validate the standard pick-up coil sensors. In the frame of the current collaborative work, an attempt to perform such measurement has already been made on the Tore Supra tokamak. It was concluded that it should be possible to evaluate the plasma current by performing a frequency analysis of the POTDR trace. However, theoretical predictions and experimental data were inconsistent and the analysis of the disagreement has to be performed. The possibility to use another technique – Polarization Optical Frequency Domain Reflectometry (POFDR) will also be studied. POFDR provides the same measurement trace as the POTDR but offers a higher spatial resolution.

To pursue the work on advanced FOCS in the frame of the PhD project, it is proposed to:

  • Develop a new algorithm to apply on the POTDR/POFDR trace in order to determine the plasma current with a sufficient accuracy. The idea is to exploit the fact that every point of the trace contains information about the current. It could also be useful to optimise the number of fibre turns around the reactor.
  • Determine by simulation the best fibre type to use for the POTDR/POFDR measurement.
  • Study the possibility to measure the profile of the magnetic field around the vacuum chamber thanks to the local feature of POTDR/POFDR measurements.

 

Plasma current measurement for the WEST:

The objective of FOCS for WEST is the monitoring of the currents circulating in the conductive elements installed in the WEST vacuum chamber (typically 2 to 100 kA; plasma current is ranging from 400 to 1000 kA). The available space is such that only fibre-optic sensors can be envisaged. For this application and in the frame of the PhD project, we propose to apply both the standard FOCS and POTDR/POFDR systems.

A fibre will also be placed around the torus in order to measure the plasma current. The measured current will correspond to the sum of the plasma current, the currents circulating in the poloïdal field coils and the current circulating in passive conducting structures. The knowledge of poloïdal field coils currents obtained thanks to the sensors placed around them will allow deducing the plasma current circulating in the torus. Let us note that the fibres will already been installed on WEST before the beginning of the new PhD thesis. Finally, it is important to mention that all the developments performed for WEST will be transposable to ITER.