Operating-margin assessment of rectangular Nb₃Sn CICCs under non-uniform AC-loss heat loads in a hybrid central solenoid

The central solenoid (CS) magnet of a next-generation Chinese fusion reactor adopts a hybrid configuration consisting of high-temperature superconducting (HTS) and low-temperature superconducting (LTS) coils. During pulsed plasma operation, the low-temperature superconducting cable-in-conduit conductor (CICC) is subjected to rapidly varying magnetic fields, resulting in AC-loss heat deposition and transient thermal-hydraulic disturbances. In this work, the transient thermal-hydraulic behavior of the LTS CICC was analyzed under a 5.3 MA plasma-current scenario. A two-dimensional axisymmetric electromagnetic model was first established to obtain the magnetic-field distribution and magnetic-field variation rate in each cooling channel. The hysteresis loss and coupling loss were then evaluated, with the coupling-loss contributions in the wide-face and narrow-face directions distinguished for the rectangular conductor cross-section. Based on the friction-factor correlation obtained from room-temperature nitrogen pressure-drop measurements, a one-dimensional transient thermal-hydraulic model was developed using GANDALF. The results show that the distributions of initial magnetic field and deposited AC-loss energy among the LTS cooling channels are significantly different. LTS1L-CH17 has the highest initial magnetic field of 11.57 T, whereas LTS2U-CH30 has the highest deposited AC-loss energy of 26.28 kJ. These two channels were selected as representative field-dominated and heat-load-dominated channels. The minimum temperature margin, Δ T marg , min , remains above 1.5 K in both representative channels, with the lowest value of approximately 2.2 K occurring in the heat-load-dominated channel. Overall, the adopted cooling condition can maintain effective forced-flow cooling for the LTS CICC under the analyzed excitation waveform, while the heat-load-dominated channel should be given particular attention in transient thermal design.