Influence of Welding on the Transient Process of HTS Metal-as-Insulation Magnets

High-temperature superconducting (HTS) metal-as-insulation (MI) magnets are considered to have a higher contact resistance than HTS no-insulation (NI) magnets, and thus, have a lower charge-discharge delay. This is very beneficial for many applications. However, when the winding of the HTS MI coil is completed or the fabrication of the HTS MI magnet is completed, we find that the time constant of the HTS MI magnet is still very long, which is much different from predictions and expectations. In this article, we study this phenomenon in detail. It has been found that the welding operation of HTS MI coils and the multiple parallel HTS joints between coils in the HTS MI magnet can lead to closed low-resistance loops. The existence of the low-resistance loops will greatly affect the charge and discharge delay time of HTS MI magnets. Therefore, the measurement of the turn-to-turn contact resistivity of HTS MI coils and magnets will be greatly affected. In addition, the current distribution of the HTS MI magnet during the transient process will also be greatly affected, and large reverse currents will be induced on the low-resistance-loop turns, so that these turns may have a higher risk of quench.