Extended Data Fig. 1: Geometry of three single pancake test coils.

The test coils were made with the same 30-μm-thick substrate tape used in LBC3 and were built to simulate the outermost 25 turns of LBC3. The tapes in Coil A and Coil B have only one slit edge, where their orientation and position is varied from the supposed damage-free orientations of Fig. 4. Coil A is positioned with its slit edge facing inwards towards the magnet centre, whereas Coil B has the reverse and unsafe orientation. Coils A and B are placed into the same 31-T magnet used for the LBC3 coil test in a position similar to that of pancake P1, where the radial field is highest. Coil C is placed in the central field region, where the radial field is essentially zero and only hoop tension operates, mimicking the central pancakes P6 and P7.