Performance of a HL-LHC Nb$_\text{3}$Sn Quadrupole Magnet in the 100–200 MPa Range of Azimuthal Stress

With the assembly and test results of four Nb$_\text{3}$Sn 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 Nb$_\text{3}$Sn 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 Nb$_\text{3}$Sn 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.