Performance of a HL-LHC Nb3Sn Quadrupole Magnet in the 100-200 MPa Range of Azimuthal Stress

With the assembly and test results of four Nb3Sn short-model quadrupoles (MQXFS) for the High Luminosity Upgrade of the CERN Large Hadron Collider (LHC), an optimum pre-load level was established for the construction of the full-length, series magnets. Successive MQXFS magnets were used as testbeds for potential changes (including pre-load fine tuning) to be implemented in the series, and to better understand the stress dependence of Nb3Sn magnet performance. In this paper we report the findings of the short model MQXFS7, where we investigated the effect of higher azimuthal pre-load on the performance of this magnet assembled with coils wound from Nb3Sn Restacked-Rod-Process (RRP) conductors, which are the baseline for MQXF magnets, and Powder-In-Tube (PIT) conductors, which were initially considered as a potential candidate but subsequently set aside. Starting at the baseline level of 110 MPa azimuthal preload at 1.9 K (corresponding to a full preload at nominal current, which is 77% of the short sample limit at 1.9 K), we increased the pre-load in steps of 20 MPa up to 190 MPa. The magnet was able to operate above 90% of the short sample limit indicating a large range of possible preloads. Indications of performance degradation at 90-95% of the short sample limit were found in the PIT conductor at 170 and 190 MPa. The test included a significant set of observables, such as the ramp rate dependency on the quench current, and V-I measurements to see growing resistance in segments of the coil.