2.6 Tesla Cryogen Free Mu3e System for the Investigation of Muon Decay

The magnet produces a homogeneous field profile with a maximum 2.6 T over a large cylindrical volume of Ø70 cm × 200 cm. The field is generated using four individually controllable solenoids. Subtle changes in the field profile are possible by varying the individual coil currents.

A 25-tonne steel shield surrounds the magnet and cryostat to shunt the magnetic flux generated by the magnet, thus reducing the stray field outside the cryostat. The shield is constructed using two dodecagon endplates separated by twelve flux return bars running between the endplates. Access to the room temperature bore is via semi-circular steel doors located at both ends of the system.

The magnet and shield are supported using two cradles manufactured from aluminium. The cradles are themselves supported by a total of four wedges which enable the complete system mass of 28.5 tonnes to be raised and lowered by 10 mm.

The system uses four two-stage Gifford-McMahon cryocoolers to produce coil temperatures of approximately 3.2 K. The magnet coils are wound using NbTi conductor, supported by an aluminium former.