Theoretical assessment of the OTDR detector noise on plasma current measurement in tokamaks

In this paper, we propose a theoretical study dedicated to the assessment of plasma current measurement in magnetic confinement fusion reactors using a Polarization Optical Time-Domain Reflectometer (POTDR) setup with a low-birefringent fibre is used as the sensing fibre. We consider the general case of a non-uniform magnetic field distribution along the sensing fibre. The numerical simulations, based on Jones formalism with taking into account the OTDR noise, provide the measurement error as a function of the plasma current. The measurement performance is evaluated for an ITER-relevant sensor configuration. We demonstrate that a signal-to-noise ratio of 6 dB, achievable in modern POTDRs, allows to comply with the ITER requirements for plasma currents from 0 to 1 MA, while for the 1 to 20 MA range the level is relaxed to 4 dB.