Measurement of the Temperature Margin in HFDB02

In view of a future, post LHC hadron collider, Fermilab is developing high field accelerator magnets using Nb3Sn superconducto r. Two high field magnet designs have been developed and are currently in the prototyping stage. One of t hem, the one-layer, common coil dipole model, uses brittle Nb3Sn superconducto r in the react-and-wind approach. In view of the common coil magnet development a more general react-and- wind R&D program has been launched two years ago, aiming at a better understanding of the performance degradation of the brittle Nb3Sn during the multiple technological steps from virgin strand to the magnet. The racetrack magnet program is part of the react-and- wind development program, whereby the performance degradation is m easured in flat racetrack coils, assembled into a simple mechanical structure. The second racetrack magnet has been recently tested. The following note reports on temperature margin measurements performed on the second racetrack model. These measurements were obtained using a combination of a spot heater and a calibrated temperature sensor, fixed onto a turn of the magnet in close proximity to each other. The magnet was operated at a constant current below the short sample limit and the cable temperature was raised, sending a DC current through the spot heater, while measuring the temperature rise in the cable, until a quench occurred. The so found temperature margin can be related to the critical current in the particular spot where the spot heater and temperature sensor were located. Therefore the temperature margin measurement allows for an indirect measurement of the local performance limit of the magnet. The experimental results, as well as the critical surface implementations used to predict the temperature margin for a given strain state in the Nb3Sn conductor are discussed in this note.