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

Abstract

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 bre surrounding a current. Because of the Faraday eect, magnetic eld induced by electrical current rotates the SOP of light travelling into the bre. 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 suciently high to generate a rotation of the SOP higher than 2 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 Re ectometer (POTDR) setup allowing both unambiguous plasma current measurement and also local magnetic eld 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 eld and therefore about the plasma current. Using the Jones formalism we have developed a model accounting for the modication of the SOP of light travelling into the optical bre 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 conrms the capability of the system to measure the plasma current and the local magnetic eld even if further data post processing are still required.