A Method for Diagnosis of Poloidal Field Coils’ Current Based on Inversion of Magnetic Field

Tokamak devices such as ITER, EAST, and HL-2M have been designed and constructed (or under construction) to challenge the magnetic-confined nuclear fusion technology for the sustainable energy problem of human beings. The magnet system of the tokamak devices is mainly composed with poloidal field (PF) coils, toroidal field (TF) coils, and central solenoid coils. Among them, the PF coils play a key role in adjusting the confining magnetic field for the control of the fusion plasma. In order to avoid the plasma disruption, it is necessary to know the correlation of the magnetic field in a vacuum vessel (VV) with the current in PF coils and to predict the current distribution of PF coils which can generate a required configuration of confining magnetic field. In this paper, taking the HL-2M tokamak device as an example, an inverse analysis scheme to predict and identify the perturbation of current in PF coils from the target magnetic field signals in the VV was proposed. First, based on the Biot-Savart law, a forward code to calculate the magnetic field due to PF coil currents and TF coil currents was developed. Second, an inversion code to predict and identify current distribution in PF coils from the target magnetic field was developed based on the conjugate gradient optimization method and the singular value decomposition method. The validity of the proposed inversion scheme and the corresponding numerical codes was investigated by reconstructing PF coil current distributions from several groups of perturbed magnetic fields. In addition, the effect of the measurement points and the measurement noises on the reconstruction results was also discussed. Finally, a simplified experiment on a coil system was conducted to verify the validity and efficiency of the proposed forward and inversion analysis code.