Study of the effects of high temperatures during quenches on the performance of a small Nb3Sn racetrack magnet

Several high field Nb3Sn magnets of different design are under development for future particle accelerators. The high levels of stored energy in these magnets and the high current densities in the conductor can cause high peak temperatures during a quench. The thermal gradients generated in the epoxy-Impregnated magnet coils during the fast temperature rise can result in high thermo-mechanical stresses. Considering the sensitivity of Nb3Sn to strain and epoxy cracks, it is important to define a maximum acceptable temperature in the coils during a quench which does not cause degradation of the magnet performance. A program was launched at Fermilab to study the effects of thermo-mechanical stress in Nb3Sn coils, supported by experiments and by analysis. In collaboration with LBNL, a sub-scaled magnet was built and instrumented to measure the effect of the thermo-mechanical shock during magnet quenches. The magnet consisted of two racetrack coils, assembled in a common coil configuration with a small gap in between. During the test, the magnet reached the maximum field of similar to11 T at the short sample current of 9100 A. Temperature excursions up to 400 K did not diminish the magnet quench performance; only after temperature excursions over 430 K did the magnet show detraining effects, which occasionally reduced the quench current to about 6%. Signs of irreversible degradation (reducing the maximum current of about 3%) appeared only after temperature excursions over 550 K.