AC losses in the STEP TF magnet during plasma initiation

Abstract AC losses in the high temperature superconducting (HTS) toroidal field (TF) magnets of the Spherical Tokamak for Energy Production (STEP) tokamak are analysed, focusing on the transient electromagnetic response of the centre column to the charging and discharging of the central solenoid (CS) and poloidal field (PF) magnets during a plasma initiation scenario. An innovative H – H 0 – Φ simulation method is proposed to quantify the distribution of hysteresis losses and eddy current losses in the TF limb formed from HTS cables. The study examines the impacts of the peak shift present in the HTS tape critical current data and current sharing within the tapes. Furthermore, the magnitude of the coupling losses is estimated both by theoretical analysis and simulation, and the temperature rise caused by the AC losses under adiabatic conditions is evaluated. The results indicate that hysteresis losses dominate, with eddy current losses contributing an additional 40% on top of the hysteresis losses. Consideration of the peak shift in the tape’s <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>J</mml:mi> <mml:mrow> <mml:mtext>c</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> <mml:mrow> <mml:mo>(</mml:mo> <mml:mrow> <mml:mi>B</mml:mi> <mml:mo>,</mml:mo> <mml:mi>θ</mml:mi> </mml:mrow> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:math> curve reduces hysteresis losses by 13%, while current sharing in the copper stabiliser has no significant impact. Coupling losses add only about 3% on top of the hysteresis loss. The most notable local temperature rise of approximately 7 K occurs towards the ends of the CS and the S1 PF coil, a plasma shaping coil positioned in close proximity to the centre column. Through detailed simulation and analysis, this study demonstrates the rationality of the AC loss design of the STEP magnet system and provides theoretical support for the application of HTS materials in large fusion magnets, offering insights for future tokamak design and optimization.