The OTDR detector noise effect of the plasma current detection in tokamak-type fusion reactors

This paper presents a theoretical analysis dealing with the plasma current measurement in tokamaks. A Polarization Optical Time-Domain Reflectometer (POTDR) setup was used, where the general case of non-uniform magnetic field distribution along the sensing fibre takes place. As a sensing medium, a low birefringent fibre has been chosen. The work is based on the numerical simulations in therms of the Jones formalism and takes into account the OTDR detector noise reflecting the measurement error as a function of the plasma current. The measurement performance is evaluated for an ITER-relevant sensor configuration. Simulation results reveal that for plasma currents from the range of 0 to 1 MA, the signal-to-noise ratio (SNR) of 6 dB complies with the ITER requirements. Subsequently, the SNR level of 4 dB satisfies the plasma current range of 1 to 20 MA. The SNR level of 6 dB is achievable in modern POTDRs.