Quench performance of large radial build solenoids in the MECO detector magnet

The MECO detector magnet produces magnetic fields decreasing from 5 T to 1 T over a curvilinear volume with an axial length of about 26 m using a set of 96 NbTi solenoid coils distributed in 4 cryostats operating at about 4.5 K. At the high field end of the magnet, the solenoids have an inner radius of 0.88 m, and radial builds up to 0.125 m. In these relatively large radial build coils, quenches initiating at the high field region at the inner radius result in warm temperatures at the inner radial region while the outer radial region can remain substantially cooler, even after 60-70 s after most of the current has decayed from the quench-protective dump. Axial and radial tensions can develop in the turn-to-turn insulation from these temperature gradients. This paper discusses the quench modeling process, including the 3-D quench code that provides element temperature vs. time data to ANSYS for analysis of the coil stress. The paper evaluates methods of managing insulation surface-normal tensile stress, including use of copper sheets in single and multiple coils to distribute the transient energy by eddy currents and thermal conduction, which helps minimize temperature gradients in the winding pack. Hard-way winding with ID and OD copper sheets in multiple coils gives the best performance based on both finite element analysis and temperature contour plots.