## 2 Superconducting Current Leads Development  
The set-up of a high current probe necessitates some compromise, needing to balance the different heat loads to the sample and the bath due to joule heating and thermal transfer from room temperature. The experimental apparatus utilized for high-current measurements of wires and tapes in our laboratory is based on a cryostat that encases a 14 T magnet with a useful bore 80 mm in diameter operating in a liquid helium bath. The liquid helium bath is shared with the sample, and this renders the efficient use of the cryogenic liquid (i.e., avoiding unnecessary consumption) of the utmost importance in order to safely operate the magnet while carrying out the measurements. For this reason, we decided to adopt the hybrid resistive-superconducting approach. We decided to avoid multiple sectioning of the current probe, keeping a simple design. The resistive part of the measurement probe is based on OFHC copper current leads (copper wires, 250 mm\({}^{2}\) total section for each lead) insulated between them and inserted into a steel tube. The resistive part is cooled through evaporation of liquid helium vapors. The length of the superconducting leads was chosen as the minimum needed to avoid the immersion of the resistive leads in the liquid helium bath. In our magnet, this corresponded to approximately 55 cm of length between the sample and the resistive portion. A 45 cm L-shaped stainless steel (0.4 mm thick) part was chosen as the mechanical support and brazed at the ends to copper terminals 20 mm wide and 10 mm thick. REBCO HTS tapes could be then allocated onto the flat 20 mm portion of the support, coupling eight tapes in four pairs on each lead, as shown in Figure 1.  
Regarding the choice of the tapes, we chose tapes produced by SuNAM Co., Ltd. (Anseong-si, Republic of Korea) It is well known that SuNAM tapes are highly efficient at  
Figure 1: Superconducting leads after the soldering process (**a**) with details of the copper terminals (**b**) and schematic illustration of the current leads’ cross-section describing the placement of the tapes on the top of the steel support and copper terminals (**c**). high temperatures rather than being optimized for low-temperature-high-field conditions. This controversial choice to produce a low-temperature-high-field measurement apparatus was based on two factors:  
1. SuNAM tapes are characterized by higher robustness thanks to the stainless steel substrate with respect to better-performing recent tapes that include Artificial Pinning Centers (APCs) to increase the critical current. This becomes useful when considering mechanical stresses induced by the thermal strain arising from the different materials and interfaces. 2. Failure of the superconducting section would, in our opinion, most likely be driven by thermal factors at the warmer end rather than lack of performance in the liquid helium bath, as is later discussed in the text. Previous experiments were also carried out with tapes from different producers, utilizing a brass support rather than the steel one. However, despite some positive factors--such as an easier soldering step for brass--the difference in thermal contractions led to failures due to the delamination of tapes and detachment at the copper terminals following thermal cycling. The final chosen configuration (SUNAM on steel support) demonstrated its robustness through several tens of thermal cycles, with no evidence of degradation or quenches. The superconducting leads after the soldering process are shown in Figure 1. As previously introduced, four pairs of tapes were aligned on the surface of the copper terminals and on the steel support, with the REBCO side towards the metal. The inner tape was cut 10 mm shorter than the outer tape to allow for direct contact of the latter with the copper terminals and to optimize the current redistribution. To protect the extremities, a thin copper foil was placed above the stack in correspondence to their ends.