Abstract
An optical time-domain reflectometer sensitive to the polarization of light is proposed for the measurement of plasma current in the Tore Supra fusion reactor. The measurement principle relies on the Faraday effect i.e. on the generation of a circular birefringence along an optical fiber subject to an axial magnetic field. The circular birefringence induces a polarization rotation that can be mapped along the fiber thanks to an opticaltime domain reflectometer followed by an linear polarizer. A proper fitting of the measurement trace then allows determining the applied plasma current. The sensor has been experimentally validated on the Tore Supra tokamak fusion reactor for a plasma current range going from 0.6 to 1.5 MA. A maximum error of 13.50% has been observed for the lowest current.
Highlights
I N tokamak fusion reactors, magnetic fields are used to squeeze the plasma in the shape of a torus [1]
In the presence of a current surrounded by the fiber, the magnetic field aligned with the fiber axis has for effect to rotate the light state of polarization (SOP) via the so-called Faraday effect
Making the hypotheses that the magnetic field is constant and that there are neither twist-induced circular birefringence nor polarization mode coupling along the sensing fiber, the backscattered SOP observed at port 2 of the polarizer after Rayleigh backscattering at a distance z is given by JB(z) = MLT M(z)M(z)MLJin
Summary
I N tokamak fusion reactors, magnetic fields are used to squeeze the plasma in the shape of a torus [1]. It is possible to retrieve the surrounded current This approach presents two drawbacks in the field of tokamak fusion reactor diagnosis. To eliminate these issues, a POTDR (Polarization Optical Domain Reflectometer) can be used since a POTDR trace contains information about the SOP spatial evolution along the fibre. A POTDR (Polarization Optical Domain Reflectometer) can be used since a POTDR trace contains information about the SOP spatial evolution along the fibre Such an approach was proposed in [5] where currents up to 2.5 kA were tested. The sensor has been experimentally validated on the Tore Supra tokamak fusion reactor of the CEA (Cadarache, France) for a plasma current range going from 0.6 to 1.5 MA. Let us note that a theoretical background describing polarization effects in optical fiber and their modeling using the Jones formalism can be found in [11]
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