## 3 Sample Holder Design
On the contrary, a flat sample is easier to handle. For size constraints, however, its length is limited by the bore size, leading to a compromise between the soldering area and free section. Perturbation due to thermal effects at the terminals could potentially be  
Figure 4: Drawings of the tapes’ sample holders: (**a**) G10, (**b**) curved steel, (**c**) flat steel, (**d**) angular. enhanced due to the short lengths, and localization of the transition driven by the tape anisotropy is not possible. A second difference is the material of the support. The choice between a G10 support or a metallic fixture influences both the mechanical coupling and the presence of an electrical and thermal stabilizer. A G10 support, for example, has a limited role, both in keeping the tape mechanically stable when subjected to extremely high Lorentz forces and in preventing the tape from heating following the transition, potentially leading to burning of the tape. This is particularly true for the last generation of commercial tapes, which had low, thin mechanical stabilizers (e.g., 50 \(\upmu\)m thick Hastelloy). An example of such an occurrence is reported in Figure 5, which shows a tape irreversibly damaged after a quench event occurring few amperes above the critical current (voltage criteria were exceeded by only a factor of 2). When delivering large amounts of currents in high magnetic fields, Lorentz forces are, in fact, a factor that needs to be accounted for. Although, in the current leads, the current flows parallel to the magnetic field, thus exerting no net force, in the sections where the field and the current path are perpendicular, large transversal forces act on the system. In the worst case (i.e., high fields and high currents), forces on the order of hundreds of N are applied onto the few cm of the tape perpendicular to the field. The soldering of the tape on the metallic sample holders allows these stresses to be transferred onto the support, allowing for stable measurements thanks to the more robust configuration. The presence of a large electrical stabilizer could, however, influence the measure if the current is not injected efficiently into the superconductor. The presence of a non-insulating fixture, in fact, provides a parallel path for the currents. The amount that flows into the stabilizer is eventually driven by the relative ratio of the metal resistance and contact resistances, which play a major role in current redistribution. With our different set-ups and configurations, however, we did not observe a significant perturbation induced by this factor. Figure 6 depicts IV curves obtained for samples of the same batch of tape (from Shanghai Technologies) mounted on different sample holders. Apart from a small spread in critical currents varying between 165 A and 175 A, potentially due to small inhomogeneities in the sample batch, no significant differences can be observed on the transition shape or N values, which are close to 30 for all the samples. Figure 5: Picture of a REBCO tape mounted onto the G10 sample holder after a quench event, showing discoloration and evident mechanical deformation (red circle) caused by hot-spot development that led to irreversible degradation. The sample holder design and current flow path can, however, affect the measurement if the voltage taps are placed so as to take into account resistive voltages due to interfaces or resistive portions of the circuit. In an ideal experiment, in fact, voltage taps should be placed onto the tape surface, insulated from the rest of the experimental set-up. When adopting a metallic fixture, however, this may not be always possible. In our angular design, for example, as highlighted in Figure 7, the tape is inserted into a slot carved into the round support and soldered along its length. Figure 6: Field/current curves obtained for samples of the same Shanghai Technologies tape on the different sample holders in LN\({}_{2}\). A linear contribution has been subtracted from the curve related to the angular sample holder (see text for details).