Cryogenic Design of a Superconducting Solenoid for Muonium Hyperfine Structure Measurement

Precise measurement of the muonium hyperfine structure (MuHFS) has been planned at J-PARC. The key component of this experiment is a superconducting solenoid magnet with field homogeneity as high as 1 ppm. In this paper, we focus on the cryogenic design of this magnet. The total heat input to the magnet system was estimated and a combination of cryocoolers was determined. To reduce the heat leakage to 4 K, the structure of a He exhaust pipe and different arrangements of the baffles were considered. Based on the ray-trace model, radiation heat input through a bent pipe was calculated. A cooling test was carried out for the current lead. We confirmed that the polyimide tape-insulated Cu lead with an applied current of 400 A can be conduction-cooled with temperature difference of 5 K from the cold stage of a cryocooler. Mechanical vibration is a critical issue to achieve the required field homogeneity. We have also started vibration measurements using a He re-condensation cryostat to consider support structure of the coils.