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Description
For REBCO magnets, the no-insulation (NI) winding technique, which eliminates turn-to-turn insulation, is considered a key approach. The NI technique offers advantages, particularly during quench events, as the absence of turn-to-turn insulation allows current to bypass localized heat sources or defects naturally. In fact, several studies have reported that NI magnets achieved higher operating currents and demonstrated more stable operation compared to insulated magnets. However, accurately reproducing the electromagnetic behavior (especially voltage) of NI magnets during quench using existing analytical methods has shown to be highly challenging. When circuit-based methods are used to analyze the voltage and magnetic field of NI magnets, the results match well under normal operating conditions when inductance and characteristic resistance are considered. However, applying the same parameters to quench analysis yields entirely inconsistent results.
This study proposes that the cause of the above issue is the ‘transition’ of contact resistivity at critical point. Some previous studies have indicated the potential existence of such a phenomenon based on indirect evidence. In this research, experimental results confirmed a drastic change in contact resistivity around the critical temperature. Quantitative measurements of resistance at different temperatures between REBCO conductors in contact were performed to observe this transition. Additionally, temperature-dependent characteristic resistance measurements of NI coils were conducted to assess how this transition manifests at the magnet level, and a revised analytical model was suggested. The findings of this study will enable more accurate analysis of NI magnet behavior during quench and provide criteria for magnet protection. Furthermore, it could lead to a deeper understanding of the origin of contact resistivity.
