An Experimental 11.5-T Nb$_3$Sn LHC type of dipole magnet

As part of the magnet development program for the LHC an experimental 1 m long 11.5 T single aperture Nb$_3$Sn dipole magnet has been designed and is now under construction. The design is focused on full utilisation of the high current density in the powder tube Nb$_3$Sn. A new field optimisation has led to a different winding layout and cable sizes as compared to the reference LHC design. Another important feature of the design is the implementation of a shrink fit ring collar system. An extensive study of the critical current of the Nb$_3$Sn cables as a function of the transverse stress on the cables shows a permanent degradation by the cabling process of about 20%, still leaving a safety margin at the operation field of 11.5 T of 15%. A revised glass/mica glass insulation system is applied which improves the thermal conductivity of the windings as well as the impregnation process considerably. This paper describes various design and production details of the magnet system as well as component tests