Towards MQXFB Series Coils

The cold powering test of the first two MQXFB prototype quadrupoles, the Nb3Sn inner triplet magnets to be installed in the HL-LHC, has found performance limitations. This prompted putting on hold the coil fabrication, to review in depth the full manufacturing process, while in parallel performing destructive inspections on selected coils. In particular, the post-mortem analysis of the limiting coil in MQXFBP1 identified a recurrent defect on top of the inner layer pole turn in the 2D cross-section, in correspondence to the titanium pole-to-pole transitions in the longitudinal direction. The next two magnets, MQXFBP3 and MQXFB02, reached the required performance for operation in the machine at 1.9 K. However, they still showed a limitation at 4.5 K, with a phenomenology similar to the one observed on MQXFBP1 and MQXFBP2. The review of the coil manufacturing process pointed to some improvement axes, which are being treated in a systematic manner. With the aim of addressing the potential root causes, the coil production gradually restarted, with transition coils, where additional measurements were carried out and selected changes in the procedure implemented. This article presents the challenges of fabricating 7.2-m-long accelerator quality Nb3Sn coils, together with the solutions specifically adopted for MQXFB, which could be instrumental also for future projects.