Direct measurement of toroidal eddy current on the EXL-50U tokamak with a hardware-compensated fiber optic current sensor

Toroidal eddy currents induced in the tokamak vacuum vessel play a critical role in plasma startup, volt-second consumption, and magnetic field configuration. However, the direct and accurate measurement of these currents is challenged by the presence of strong background magnetic fields—on the order of several Tesla—generated by the central solenoid (CS) and poloidal field (PF) coils, with currents often exceeding several mega-ampere-turns. This paper presents a novel dual-loop fiber optic current sensor (FOCS) system developed specifically for the EXL-50U spherical tokamak to directly measure toroidal eddy currents. The diagnostic setup includes an inner FOCS loop dedicated to measuring the plasma current (I_p), and an outer loop for detecting the total enclosed toroidal current, which comprises contributions from the plasma, all external coils, and the eddy currents. A specially designed hardware compensation coil physically cancels the dominant magnetic flux generated by the CS leads in real time, significantly enhancing the signal-to-noise ratio. Experimental results demonstrate that the system is capable of measuring eddy currents even under strong background fields. The measured waveforms are in good agreement with numerical simulations, validating both the diagnostic approach and the electromagnetic model of the device. The proposed diagnostic system offers a robust and reliable tool for optimizing plasma startup scenarios in spherical tokamaks.