Challenges in the Design of the Detector Solenoid for the Mu2e Experiment

The Mu2e experiment at Fermilab is being designed to measure the rare process of direct muon-to-electron conversion in the field of a nucleus. The experiment comprises a system of three superconducting solenoids, which focus secondary muons from the production target and transport them to the stopping target, while minimizing the associated background. The detector solenoid is the last magnet in the transport line and it consists of an axially graded-field section at the upstream end, where the stopping target is located, and a spectrometer section with uniform field at the downstream end for accurate momentum measurement of the conversion elections. The detector solenoid has a warm bore of 1.9 m and is 10.75 m long. The stored energy of the magnet is 30 MJ. The conceptual design of the magnet is presented, in particular the challenge of achieving tight magnetic field specification in a cost-effective design.