High Tc superconducting (HTS) dipole magnet is expected to upgrade the high-energy particle accelerator. The future LHC dipole in the framework of EuCARD-2 project aims to generate a magnetic field between 17 and 20 T including a 5 T contribution of the HTS magnet. The HTS magnet is inserted into a 100 mm aperture under the high field of low Tc superconducting (LTS) outsert magnet. This has to provide a good field quality inside a 40 mm beam aperture. The HTS magnet faces the extremely large electromagnetic force. The stack cable with a ReBCO tape, which has a block-coil layout, is the simplest way to acquire the large current and the strong mechanical cable. A twist for half turn in the coil-ends transposes the positions of the tape inside the stack cable and improves the current distribution. However, the twist stack cable exposes the ReBCO tape into the bad field orientation towards the degradation of critical current density Je. The HTS dipole magnet is designed with a 4mm square block-coil. The 23 block-coils achieved the good field quality and the good mechanical performance with a 650 A/mm2 of operating current density Jop. The block-coils are aligned to the field in order to direct the force on the correct wide surface of ReBCO tape. In the coil-ends, the different strength and direction of force cause the shear stress between the tapes and complicate the mechanical architecture. We investigate the mechanical structure of coil-ends especially at twist part with a numerical model. The position of coil-ends between HTS insert and LTS outsert helps to reduce the mechanical force and keep the large Je.

Due to superior mechanical performance and in-field current-carrying capacity under high fields, (RE)BCO tapes have played an important role to surpass the magnetic induction limit of LTS (Low Temperature Superconductor) wires. However, it also introduces a new challenge: the protection of (RE)BCO coil, much more complicated and costly than that of LTS counterpart, mainly because the slow normal zone propagation velocity of (RE)BCO. Also most (RE)BCO magnets with conventional insulated winding have been operated at a relatively low “overall magnet current density,” when compared with the short sample current density of (RE)BCO conductors, mainly due to the weak anisotropic in-field performance at a “local” spot of the (RE)BCO magnet.
Some new winding techniques, such as NI (Non-Insulated) and the NI-MW (NI-Multi-Width), are recently introduced for high-field (RE)BCO magnets, virtually with protection-free operation at an overall magnet current density close to the critical current density of a short sample. However, those techniques cannot be used for fast charge/discharge applications because of a long charging delay due to the bypassing currents through turn-to-turn contacts.
In this paper we are proposing an innovative winding technique: a metallic insulation (MI) technique for (RE)BCO coils, where metallic tapes are co-wound between coil turns in order to increase significantly the turn-to-turn contact resistances and reduce the charging delay, when compared to the NI counterpart. MI winding gives also a better control of the coil properties. Although the operation stability of MI coil is lower than that of NI, it is still better than its organic-insulated counterpart. Another advantage of the MI technique, when compared with its organic-insulated counterpart, is a much faster quench propagation velocity, even faster than that of typical LTS coils. The results presented here are: 1) a comparison of the overcurrent (OC) coil behaviors between NI and MI pancakes; 2) an in-depth study of electromagnetic characteristics of the MI pancake, including charging/discharging responses with various ramping rates, self-protection behaviors in case of fast pulse OC; 3) quench, stability and self-protection experimental studies with comprehensive instrumental setups at 77 K.