High-temperature superconducting ReBCO conductors are essential for realizing future accelerator type high-field magnets featuring 20+ tesla. The so-called cloverleaf configuration is a potential coil geometry for such magnets, as it utilizes the good magnetic field orientation of ReBCO tape and it allows for a magnet free bore for accelerator beam pipe without significant hard-way bending of the wide tape conductor.
For quench stability of ReBCO tape based coils, one of the solutions is using no-insulation technology. A study comparing four size-wise identical non-insulated pancake coils for anticipating accelerator magnet application was conducted. The pancake geometry was chosen as simplest geometry and an intermediate step towards a cloverleaf coil configuration, in order to study non-insulated coil technology. The coils, with inner and outer diameters of 52 mm and 102 mm, respectively, were wound with 10 mm wide Shanghai Superconducting Technology tape, with no insulation or with Sn62Pb36Ag2 cladding on the tape turns. Two versions of no-insulation coils were made using different techniques: one using a single tape and the other using a twin-tape.
Tests were performed in stand-alone mode at variable temperatures, ranging from 4.2 K to 70 K, and in liquid nitrogen at 77 K, as well as at 4.2 K in a background magnetic field of 7 T. The study derives: time constants and inter-turn resistances, voltages across coil sections in the radial direction, magnetic field behaviour during charging and discharging, as well as critical current and quench current of the coils. At a temperature of 10 K, the average time constant for the soldered filled coil is 1600 times bigger than that of the dry-wound coil. At all temperatures the soldered filled coils had a higher quench current margin relative to the critical current when compared to the dry-wound coils, indicating that soldered filled coils have, as expected, a much bigger time-constant paired with a much higher stability.
Next was to show the feasibility of the cloverleaf geometry with a practical, ReBCO tape. An exercise model and a half meter long demonstrator coil were constructed at CERN using specially developed tooling needed for this type of coil windings. The performance of the demonstrator coil was measured in liquid nitrogen at 77 K at the University of Twente.
The coil reached its predicted safe operating current of 400 A, showing no degradation that may have occurred during the coil winding and assembly process. The powering behaviour of this non-insulated cloverleaf coil has been studied in more detail. Its characteristic time constant is 2.7 s and the resistance in the windings perpendicular to the current flow is about 20 µΩ. The cloverleaf architecture exercised in this demonstrator did not show a showstopper so far for making a next step in the development and further exploration towards full-size high-field accelerator type magnets.