Abstract Body

The rare-earth barium copper oxide (REBCO) conductors ideally have an excellent performance for compact electrical machines primarily due to their high current capacity and high mechanical tolerance. Recently, the toroidal field magnet development for nuclear fusion reactors by Commonwealth Fusion Systems (CFS) and Massachusetts Institute of Technology (MIT) has been attracting attention [1]. The successful development of the large-sized REBCO magnets is attributed to the no-insulation (NI) and the multi-bundle (MB) technologies. The NI technology eliminates the insulation between turns, resulting in the high thermal stability. Meanwhile, the MB technology, where multiple bundled REBCO tapes are wound into a coil shape, enables mitigation of the charging delay issue arising from the NI technique.

The majority of MB NI REBCO coils including the toroidal field coils is designed for DC operations. Nevertheless, even in these DC applications, the REBCO coils experience external field fluctuations, transient electromagnetic variation, and any kind of AC ripples. These field fluctuations cause magnetization losses, and it deteriorates the stability of MB NI REBCO coils. The MB NI REBCO coil has a well-balanced performance in thermal stability and charging speed; however, the current behaviors are complicated, and the AC losses are generated. This is due to the intricate current paths inside the coil, which depend on the tape-to-tape and the turn-to-turn contact resistances. The complicated current behaviors and the AC losses must be clarified to fully exploit the coil performances.

In the presentation, the developed simulation tool for the AC loss estimation of the MB NI REBCO coil will be shown, and the AC loss behaviors will be investigated numerically. Fig. 1 shows the developed partial element equivalent circuit (PEEC) model of the MB NI REBCO coil. It is noted the coil shown in Fig. 1 has 2 turns wound with 3 bundled tapes. The bare REBCO tapes have the inductances and the tape-longitudinal resistances. The turn-to-turn and the tape-to-tape contact resistances are distinguished and modeled separately. The “intra-turn” and “inter-turn” Joule heat losses are estimated from the PEEC simulation results. The hysteresis loss is also derived based on Bean’s model, and the calculated losses will be shown.

References

[1] Z. S. Hartwig et al., “The SPARC Toroidal Field Model Coil Program,” IEEE Trans. Appl. Supercond., vol. 34, no. 2, 2024, Art. no. 0600316.

Figure 1. Partial element equivalent circuit (PEEC) model of MB NI REBCO coil. The figure shows an example of the MB NI REBCO: 3-tape-bundled 2-turn MB NI coil.

Keywords: AC loss, multi-bundled REBCO coil, no-insulation winding