## 3 Sample Holder Design  
The sample holder can play a crucial role concerning the measurement, having to provide a current to the sample while granting mechanical support without influencing the tape's performance. The interface between the sample and the current leads could be under pressure or soldered, depending on the design (e.g., [3] or [6]). While a pressure contact carries a higher level of simplicity during sample mounting procedures and avoids thermal treatments that could ruin the sample, soldering grants higher stability and lower contact resistance. To pursue higher efficiency and robustness, in our laboratory, we decided to select the soldering approach. We designed and tested several sample holders characterized by differences in the sample placement concept and in the supporting material. Some descriptive drawings are reported in Figure 4.  
* The sample holder in the panel (a) is constituted by a G10 board with a 4 mm large groove, which is rounded towards the edges with a flat section where the field is perpendicular to the tapes. It accommodates approximately 25-30 cm of tape, with 10 cm in the G10 portion and 3 cm in the flat part where the primary voltage taps are placed. This design, partially inspired by previous works of [PERSON] et al. [6; 7], is characterized by a good section of copper devoted to the sample soldering, but relies mostly on the groove and on the sample's pre-tension to support the tape. * The sample holder in panel (b) is constituted by a steel support bent and brazed on the copper terminals. The tape dimensions are similar to those obtained in the G10  
Figure 3: Picture of the assembled high-current probe. Figure 2: Voltage–current curves measured for the HTS leads in LN\({}_{2}\) (black and red points), with overimposed critical current fit (green curves) for 40 cm far voltage taps. support. This design preserves a large soldering surface and introduces a metallic support where the tape can be soldered along its entire length, enhancing mechanical support and granting good thermal stability for the sample. * The design in panel (c) relies instead on a flat steel support that is soldered simultaneously with the tape on the flat extremities of copper terminals. The sample length is limited to approximately 7.5 cm due to the bore size. It is characterized by extreme simplicity in the sample mounting, but reduces the soldering length to the length of side of the copper support (i.e., 25 mm in our case) and limits the dimension of the center part where the voltage probes are accommodated. * The design in panel (d) describes an angular measurement option. The sample holder exploits the flat approach, and the tape is soldered inside a groove obtained on a round support for its entire length of approximately 7.5 cm. This support is composed of copper extremities (i.e., 25 mm) and a central steel portion. The copper extremities are eventually clamped onto the copper terminals. The first difference between these supports is the length and the shape that the tape will assume, i.e., whether the tape will be bent or will remain flat. This has consequences for the field/tape orientation along the whole sample, both on the soldering area and on the probed lengths. Regarding the first aspect, a curved sample introduces a weak section into the tape, i.e., a section where the critical current is lower with respect to the other sample parts, localizing the transition. This should, in principle, correspond to the area where the field is perpendicular to the surface, i.e., the central part, due to the anisotropy of the REBCO superconducting properties. In this case, the transition would be far from the soldered part and should be less influenced by the thermal phenomena occurring there. The bending step, however, should be always caried out carefully. As was previously reported to be critical for the measurement's reproducibility [7], we also observed a critical role of the sample mounting step, which was more associated with the mechanical handling of the tapes rather than with other phases.