Mechanical Performance of the First Two Prototype 4.5 m Long Nb3Sn Low-β Quadrupole Magnets for the Hi-Lumi LHC Upgrade

The U.S. High-Luminosity LHC Accelerator Upgrade Project (HL-LHC AUP) team is collaborating with CERN in the design and fabrication of the first 4.5 m long MQXFA magnets, a 150 mm aperture high-field Nb₃Sn quadrupole magnet that uses the aluminum shell-based bladder-and-key technology. The first two prototype magnets, MQXFAP1 and MQXFAP2, were assembled and tested while the first pre-series structure (MQXFA03) was in fabrication. This paper summarizes the mechanical performance of these prototype structures based on the comparison of measured strain gauge data with finite element model analyses from all load steps to powering. The MQXFAP1 magnet almost reached ultimate current before a short to ground was detected and the test was stopped. The MQXFAP2 magnet experienced a low training performance due to a fractured aluminum shell. MQXFAP1b was rebuilt with a new replacement coil, but an old coil limited the magnet from achieving the ultimate current. The mitigations and analyses of these prototype magnets are discussed in the context of the transition to pre-series production.

I. Introduction

The development of high field Nb₃Sn magnets under the LHC Accelerator Research Program (LARP) [1] effort has resulted in the MQXF magnet [2], which is a Nb₃Sn quadrupole designed in partnership with CERN that will replace the present inner triplet quadrupoles of the LHC for the High Luminosity LHC project [3]. The MQXF design was scaled up from a series of technology development models [4]–[7] that used the bladder-and-key technology [8]. The short MQXF models (1.2 m long with a 150 mm aperture) are extensively reported on in [9], [10], and its cross section is shown in Fig. 1. The MQXFA magnet is a 4.5 m (4.2 m magnetic length) length scale up of the same cross section that is being produced by the U.S. High-Luminosity LHC Accelerator Upgrade Project (HL-LHC AUP). These magnets also have the same cross-section as the 7.2 m long MQXFB [11] magnets being produced at CERN. Once MQXFA magnets are successfully tested, two of them will be installed together in a coldmass, and subsequently inserted in a cryostat provided by CERN. Fig. 1.

MQXFA Cross section layout showing the main components. Note the LHe SST vessel is part of the cold mass.