BOX: an efficient benchmark facility for the study and mitigation of interface-induced training in accelerator type high-field superconducting magnets

For 30 years, training and unpredictable degradation in accelerator type high-field Nb3Sn magnets have seriously hampered Nb3Sn application. Training and deterioration have to be solved or at least better controlled. The global picture shows that most of the R&D and short model magnets start to train at some 40%-70% of the critical current and then creep up to almost the critical current within some 10-50 training steps. A typical class of failures leading to quenches is largely characterized by cracking and debonding at the interfaces between cable and glass-resin insulation, as well as between insulation and coil former. The study of training by means of testing demonstrator coils is rather expensive and time consuming. However, advances in magnet design and fabrication can also be assessed and benchmarked using BOX, the bonding experiment presented here, that produces maximum uniaxial Lorentz forces at some 7.5 T in a controlled experiment performed in 11 T solenoid facility at the University of Twente. BOX samples use only one meter of Nb3Sn cable inserted in a three-wave meandering slot in a flat metallic sample holder, reproducing magnet-relevant interactions between cable, insulation, impregnated materials and coil former. The meander shape exposes seven straight cable sections to a transverse magnetic field, thereby generating a representative level of shear stress at the interfaces. In this way, characteristic training curves of magnets can be mimicked and solutions studied. We aim to demonstrate with various samples failure mechanisms of high-field Nb3Sn magnets without the need to manufacture complete magnets. BOX may thus be expected to allow for quick and affordable testing of novel insulations, impregnation materials, coatings and interfaces for Nb3Sn magnets achieved by investigating various resins, fillers and more.