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This paper presents experimental and analytical studies on the characteristic resistance of NI (no-insulation) ReBCO pancake coils, which are used in an equivalent circuit model to characterize 'radial as well as spiral' current paths within the NI coils. We identified turn-to-turn contact resistance as a major source of the characteristic resistance of an NI coil. In order to verify this, three single pancake NI HTS coils-60, 40, 20 turns-were fabricated with their winding tension carefully maintained constant. A sudden discharge test was performed on each coil to obtain its characteristic resistance, and the relation between the turn-to-turn contact and the characteristic resistance was investigated. Based on the characteristic resistance and the n-value model, an equivalent circuit model was proposed to characterize the time-varying response of the NI coils. Charging tests were performed on the three test coils and the experimental results were compared with the simulated ones to validate the proposed approach with the equivalent circuit model.

A 26 T 35 mm winding diameter all-GdBa2Cu3O (GdBCO) magnet was designed by the MIT Francis Bitter Magnet Laboratory, and constructed and tested by the SuNAM Co., Ltd. With the multi-width (MW) no-insulation (NI) high temperature superconductor (HTS) winding technique incorporated, the magnet is highly compact; its overall diameter and height are 172 and 327 mm, respectively. It consists of a stack of 26 NI double pancake coils wound with MW GdBCO tapes in five different widths ranged 4.1–8.1 mm. In a bath of liquid nitrogen at 77 K, the magnet had a charging time constant of 16 min due to the intrinsic NI characteristics. In liquid helium at 4.2 K, the magnet generated a 26.4 T field at the center, a record high in magnetic fields from all-HTS magnets. The results demonstrate a strong potential of MW-NI GdBCO magnets for direct current high-field applications.

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This paper presents the over-current quench test and post-quench operation results of a 7 T 78 mm winding diameter multi-width (MW), no-insulation (NI) magnet in a bath of liquid helium at 4.2 K. The MW-NI magnet consists of 13 double-pancake (DP) coils wound with GdBCO tapes having five different widths ranging from 4.1-8.1 mm. After the magnet reached 7.3 T at 253 A, the magnet current was further increased purposely until the magnet quenched at 312 A, corresponding to a current density of 895 A mm −2 for the central DP coils of the narrowest 4.1 mm tape. The NI DP coils showed a fast magnetically coupled quench propagation from the quenched DP to the rest of the 'healthy' DP coils. The stored magnetic energy of 25.4 kJ was completely dissipated in 0.3 s with an average dissipation power rate of 85 kW. The post-quench magnet, operated sequentially in baths of liquid nitrogen at 77 K and in liquid helium at 4.2 K, showed no discernable changes from the pre-quench magnet in their key parameters, except the magnet characteristic resistance, pre-1.4 mΩ versus post-3.6 mΩ. Thus, a forced quench of the magnet, thanks to the NI winding technique, kept the integrity-mechanical, electrical, and magnetic-of this NI magnet intact.

No-insulation (NI) REBCO magnets have many advantages. They are self-protecting, therefore do not need quench detection and protection which can be very challenging in a high Tc superconducting magnet. Moreover, by removing insulation and allowing thinner copper stabilizer, NI REBCO magnets have significantly higher engineering current density and higher mechanical strength. On the other hand, NI REBCO magnets have drawbacks of long magnet charging time and high field-ramp-loss. In principle, these drawbacks can be mitigated by managing the turn-to-turn contact resistivity (Rc). Evidently the first step toward managing Rc is to establish a reliable method of accurate Rc measurement. In this paper, we present experimental Rc measurements of REBCO tapes as a function of mechanical load up to 144 MPa and load cycles up to 14 times. We found that Rc is in the range of 26-100 µΩ -cm 2 ; it decreases with increasing pressure, and gradually increases with number of load cycles. The results are discussed in the framework of Holm's electric contact theory.