Numerical analysis of AC loss in central solenoid coils with twisted stacked-tape cables

Fast-ramping central solenoid (CS) coils in tokamak reactors present a major challenge for High Temperature Superconducting (HTS) cables in terms of AC loss. Twisted stacked-tape cables (TSTCs) such as PIT-VIPER and SECAS are promising candidates for these applications, yet AC loss characterisation under realistic CS operating conditions remains limited. In this work, full-scale TSTCs of up to 4 × 100 tapes are investigated under ramped magnetic field and transport current waveforms representative of fusion CS coils, including fields of up to 23 T at 20 K. A 2D scanning method is used to assess the influence of ramp rate, transport current, cable configuration, and modelling assumptions on AC loss behaviour during transient operation. As a final case study, AC losses in the SPARC CS1 module are estimated. Tape number and -based grading strategies are compared, with both approaches yielding similar hysteresis losses. Furthermore, peak hysteresis losses are shown to be approximately proportional to the ramp rate during coil discharge. Overall, the results provide insight into AC loss behaviour in large-scale TSTCs under realistic fusion operating conditions, supporting their application in fast-ramping fusion magnets.