Development of a Jones vector based model for the measurement of a plasma current in a thermonuclear fusion reactor with a POTDR setup

Fibre optical current sensor (FOCS) is a promising alternative to inductive sensors for the measurement of the plasma current in future thermonuclear fusion reactors. Standard FOCS relies on the measurement of the state of polarisation (SOP) of light at the output of an optical fibre surrounding a current. Because of the Faraday effect, magnetic field induced by electrical current rotates the SOP of light travelling into the fibre. According to the Ampere's theorem this rotation is proportional to the surrounded current. In future tokamaks like ITER and DEMO, the plasma current will be sufficiently high to generate a rotation of the SOP higher than 2π radians. These conditions may lead to uncertainties on the determination of the plasma current if no post processing is performed. In this paper we propose a solution with a Polarisation Optical Time Domain Reectometer (POTDR) setup allowing both unambiguous plasma current measurement and also local magnetic field measurements. This measurement is based on the assessment of the SOP rotation of the Rayleigh backscattered POTDR signal. Thanks to the presence of an input polarizer, SOP variations are converted into power uctuations that contain information about the distribution of the magnetic field and therefore about the plasma current. Using the Jones formalism we have developed a model accounting for the modification of the SOP of light travelling into the optical fibre and the evolution of the POTDR signal. In parallel experimental PODTR measurements have been performed on the Tore Supra tokamak situated at CEA Cadarache in France. A comparison between the models and the experimental results confirms the capability of the system to measure the plasma current and the local magnetic field even if further data post processing are still required.