Construction and analysis of a modified Nb3Sn 11 T Short Dipole Model

The 5.2-m-long, 11 T double-aperture Nb3Sn Dipole Magnet was designed to replace some of the standard, 15-m-long 8.3 T Nb-Ti main dipole magnets in the Large Hadron Collider. After the production and test of several of 2-m-long models, the test of full-length 11 T series magnets revealed signs of performance degradation in the coil ends after a sequence of electromagnetic and thermal cycles. The possible sources for performance degradation were categorized into two groups: issues internal to the coils and issues with the coil support structure, in particular in the magnet ends. To address the constraints external to the coil, two mitigation measures were proposed: (1) reducing peak stresses in the coil ends and (2) improving the coil end support and axial loading. To validate the mitigation measures, a double-aperture model magnet, MBHDP301, was designed, manufactured, and power tested. The magnet was manufactured with four available coils, three of which had been assembled and tested in previous magnets. Two new features were implemented in both apertures to reduce the peak stresses in the coil ends (mitigation measure 1). The first involved a change in material of the removable pole from titanium to austenitic stainless steel, while the second introduced a gradual reduction of prestress applied in the ends of the coil straight section. For one of the two apertures, in addition, an end cage system was installed to improve coil end support and axial loading (mitigation measure 2). The two apertures in the magnet can be independently powered, facilitating comparison studies. This paper describes the manufacturing, mechanical measurements analysis and cold powering test campaign results of the MBHDP301 model magnet. The lessons learned from this work can be useful and directly applicable to future cosθ high field dipole magnets.