AC Loss Analysis of the Central Solenoid Charge and Discharge

High temperature superconducting (HTS) magnets in Tokamak devices are used to build strong magnetic fields for increasing plasma density and power, making the confinement system more compact. However, rapid changes in currents and magnetic fields can also result in ac losses in magnets. For example, the charge and discharge of the central solenoid (CS) induces ac losses within the CS magnet and the surrounding TF magnets. Therefore, using finite element method (FEM) modelling, this paper first analysed the inductance, magnetic field, and ac losses in the CS. Then, by equivalently transforming the magnetomotive force (MMF) of the CS, the impact of CS charge and discharge on the TF limbs in the centre column was investigated. Particularly, over the CS fast discharge, the self-field loss in the CS was 0.24 MJ, mainly attributed to hysteresis loss, and the induced loss in each TF limb inside the centre column was 2.5 kJ/m. Three different algorithms were discussed to model the large-scale CS: the
H
formulation,
H-Φ
formulation, and
T-A
formulation. The
H
formulation and
H-Φ
formulation yielded converged electromagnetic characteristics, and
H-Φ
formulation reduced computation time significantly. However, the
T-A
formulation, ignoring the axial magnetic field inside conductors, was not applicable for calculating ac losses in vertically stacked tapes (VST) cable magnets. This modelling work from the Spherical Tokamak for Energy Production (STEP) programme provides essential reference for optimizing HTS magnet designs in commercial Tokamak endeavours.