Plasma current measurement at JET using polarimetry-based fibre optic current sensor

doi:10.1016/j.fusengdes.2020.111754

Fusion Engineering and Design

Volume 160, November 2020, 111754

https://doi.org/10.1016/j.fusengdes.2020.111754 Get rights and content

Highlights

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Fibre Optic Current Sensor (FOCS) was installed on JET.
•
A good agreement between the FOCS and the reference Continuous External Rogowski coils measurements was observed.
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The sensor response is linear and is not influenced by magnetic crosstalk effects.
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The FOCS performance characteristics are compatible with the ITER requirements.

Abstract

The ITER Fibre Optic Current Sensor (FOCS) is a dc coupled current measurement system that will provide back-up ITER plasma current measurements using a sensing fibre located on the outer surface of the vacuum vessel. To address the impact of a harsh tokamak environment on the sensor performance we installed a FOCS system on JET and performed current measurements in various machine operating scenarios for currents up to 3 MA. Me have observed a good agreement between the FOCS and the reference Continuous External Rogowski (CER) coils data. The linearity of the FOCS response was demonstrated. The FOCS has performance characteristics compatible with the ITER requirements and in contrast to the CER is non-sensitive to magnetic crosstalk effects.

Introduction

For tokamaks plasma current measurements are essential for providing real-time plasma control and the machine protection function. At JET and other tokamaks this task is performed using standard electromagnetic sensors, arrays of different types of coils [1]. These sensors share a common feature – the need of the time integration step, which sometimes may result in the output drift [2]. The importance of this feature grows with the increase of the plasma operation time. Significant improvements have been made in the integrator design to allow for steady-state plasma measurements during shots with an ITER-relevant duration [3]. However, there are indications that in future burning plasma machines, the performance of electromagnetic sensors may degrade due to a measurement drift caused by the combined effect of steady-state operation and the presence of strong nuclear radiation, which induces spurious currents [4]. To mitigate this risk alternative solutions for plasma current measurements must be implemented. The Fibre Optic Current Sensor (FOCS) is one of such systems. The FOCS operation is based on the detection of the polarisation rotation as a result of magneto-optic interaction, the Faraday effect [5]. The rotation angle is directly proportional to the enclosed current, which makes the system free of the mentioned inherent problem of the electromagnetic sensors.

Installation of FOCS is included in the ITER design as a back-up option [6]. It should be operational for Pre-Fusion Power Operation 1 and must satisfy the ITER performance requirements: absolute error below 10 kA for currents lower than 1 MA and relative error less than 1% for higher currents. It may appear that FOCS implementation at ITER is straightforward, as such sensor systems are used in the electrical power industry and commercial systems are available off the shelf [7]. However, commercial designs are not appropriate for the ITER environment, which requires vacuum compatibility, tolerance with respect to the presence of strong radiation fields and elevated temperatures. Measurement of currents up to 15 MA is not considered for any other industrial application. Therefore, an assessment of the FOCS in a relevant environment in combination with multi-MA range currents is necessary. From this point of view JET is a nearly ideal testbed. JET is equipped with a large number of electro-magnetic sensors distributed all over the torus, both in-vessel and ex-vessel, which can provide accurate reference data for the FOCS. Achievable plasma currents in a range up to 4.5 MA fit the ITER conditions. The total integrated radiation dose possible in JET is too low to be representative for the ITER radiation environment. However, in terms of the dose-rate, the JET environment during future deuterium-plasma (DT) operation will be fully relevant, allowing studying the physics of dose-rate dependent effects on FOCS. The estimated neutron flux at JET of ∼10¹⁰ n/cm²/s on the external surface of the vacuum vessel [8] is comparable with those expected for ex-vessel magnetics, i.e. at the location of ex-vessel electro-magnetic coils, steady-state Hall sensors, and the FOCS at ITER [4]. It is also important that the impact of 14 MeV neutrons can be addressed during the DT campaign.

In the present publication we describe installation of the FOCS system on JET. The goals of our work are to evaluate the performance of the FOCS for plasma current measurements at the MA current range and to assess effects of a real tokamak environment, including cross-talks from various tokamak systems.

Section snippets

FOCS location at JET

The FOCS system on JET is basically a fibre wrapped around the JET vacuum vessel, in Octant 3. FOCS operation rely on the Faraday effect, an interaction between light and a magnetic field in the fibre. This interaction results in a rotation of the plane of polarization, which is linearly proportional to the component of the magnetic field in the direction of propagation. It follows from the Ampere theorem that if the FOCS fibre makes a loop around a current, for linearly polarized light the

Experimental data processing

Eq.1 is applicable for the case of ideal no-birefringence fibre. In a real situation the intrinsic linear birefringence of the fibre and the birefringence induced by external perturbations, such as bending and vibrations, make data analysis more complicated. It is convenient to visualize FOCS measurements using the Poincaré sphere (PS), which is a graphical representation of the normalized Stokes vector: $[\begin{matrix} S_{1} \\ S_{2} \\ S_{3} \end{matrix}] = [\begin{matrix} c o s 2 χ c o s 2 ψ \\ c o s 2 χ s i n 2 ψ \\ s i n 2 χ \end{matrix}]$ where χ and ψ are the ellipticity and orientation angle of

Effect of the toroidal coils

The assessment of the FOCS performance was carried out by comparing the FOCS data with the standard JET electromagnetic sensor data obtained during the 2016 JET campaign for the shots range 89870−92500.

The current enclosed by the FOCS fibre consists of the plasma current, the divertor coil current, and eddy currents in conductive structures such as the vacuum vessel, the divertor support structure, and the divertor coils. The continuous external Rogowski (CER) coil at JET (label DA/C2 IEXR)

Conclusion

The objective of our work was to describe the FOCS installation at JET and to analyse its performance. JET is a machine that provides the conditions most relevant for the future ITER implementation. As a first step in this direction we used FOCS operating in the transmission mode. There are several reasons for using this scheme:

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minimal number of components, which reduces the problem of a long-term system reliability;
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noise level independent on the current range;
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perturbations can be seen at the

CRediT authorship contribution statement

W. Leysen: Conceptualization, Investigation, Data curation, Software, Formal analysis, Writing - review & editing. A. Gusarov: Conceptualization, Data curation, Formal analysis, Software, Writing - original draft, Writing - review & editing. M. Wuilpart: Conceptualization, Formal analysis, Writing - review & editing. P. Beaumont: Conceptualization, Data curation, Investigation, Writing - review & editing. A. Boboc: Software, Data curation, Writing - review & editing. D. Croft: Data curation,

Declaration of Competing Interest

The authors declare that there are no conflicts of interest.

Acknowledgments

The research received financial support from the Federal Public Service of Economy of the Belgian Federal Government. This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 and 2019-2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.

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Citation Excerpt :
Reproduction of such conditions in a laboratory is practically impossible. To experimentally address the tokamak environment impact on the sensor performance we have installed two FOCS systems on JET and performed measurements in various machine operating scenarios for currents up to 3.5 MA [4]. Our FOCS measurement approach is based on polarimetry [5].
The Fibre Optic Current Sensor (FOCS) is a diagnostic system to be installed in ITER to perform plasma current measurements during long plasma discharges under intense nuclear radiation. The sensor performance must be experimentally assessed at ITER representative conditions. JET is a unique machine where this task can be performed. We have installed polarisation detection based FOCS systems at JET and performed measurements in various machine operating scenarios at currents up to 3.5 MA. The frequency analysis of the data was performed to investigate the sensitivity of FOCS to mechanical perturbations. A significant part of the noise originates from the optical link between the sensing fibre and the polarisation analyser. Operation with a Faraday mirror reduces this effect. It is confirmed that the FOCS system has performance characteristics compatible with the ITER requirements.
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Plasma current measurement at JET using polarimetry-based fibre optic current sensor

Highlights

Abstract

Introduction

Section snippets

FOCS location at JET

Experimental data processing

Effect of the toroidal coils

Conclusion

CRediT authorship contribution statement

Declaration of Competing Interest

Acknowledgments

Fusion Eng. Des.

Fusion Eng. Des.

Fusion Eng. Des.

J. Nucl. Mater.

Fusion Eng. Des.

Fusion Eng. Des.

Optical-fibre current measurements

Int. J. Optoelectron.

ITER Research Plan Within the Staged Approach (Level III – Provisional Version)