Design of the Superconducting Magnet System for the New $g-2$/EDM Experiment in the J-PARC

A superconducting solenoid system has been designed for the
$g$
-2/EDM experiment at the J-PARC, which is used to store muon beam of 0.3 GeV/c. High homogeneous magnetic field is required to be below
$\pm$
0.1 ppm at 3 T within a muon storage region of 33.3
$\pm$
1.5 cm in radius and
$\mathbf {\pm }$
5 cm in height around the magnet center. The muon beam is spirally injected to the storage region from the top end of the magnet, therefore the magnetic field distribution is carefully designed in the region where the muon beam pass through. In order to achieve both requirements, NbTi superconducting coils are surrounded by an iron yoke with pole tips, which can optimize the field distribution by adjusting the shape and size. The superconducting coils are cooled by pool boiling in liquid helium, and the evaporated helium gas is re-condensed by GM cryocoolers to keep the liquid helium level for a long time. The main coils are operated in a persistent current mode to remove the magnetic field fluctuation with time caused by a power supply. This paper presents the current design of the magnet system for the experiment at the J-PARC.