Theoretical Assessment of Plasma Current Measurement in Tokamaks in Relation with the OTDR Detector Noise

In this paper, we present a theoretical analysis of the plasma current measurement in tokamak-type fusion reactors using a Polarization Optical Time-Domain Reflectometer (POTDR) setup, where the general case of non-uniform distribution of a magnetic field along the sensing fibre is analyzed. A low birefringent fibre has been considered as the sensing fibre. The numerical simulations were based on the Jones formalism and take into account the OTDR detector noise reflecting the measurement error as a function of the plasma current. The measurement performance was evaluated for an ITER (International Thermonuclear Experimental Reactor)-relevant sensor configuration. Based on the simulation results we demonstrate that for plasma currents in the range of 0 to 1 MA, the signal-to-noise ratio (SNR) of 6 dB, achievable in modern POTDRs, complies with the ITER requirements. Subsequently, the SNR level of 4 dB satisfies the plasma current range of 1 to 20 MA.