A checklist for designers of cryogen-free MgB₂coils

Intensive research has been directed at MgB2 since its discovery in 2001, focusing first on the material properties and conductor development and recently also on coil demonstrations. The relatively cheap and easy fabrication makes MgB2 a tempting material for superconducting applications. It can also be operated in the vicinity of 20 K, at which the commercial LTS materials are still in the normal state. However, commercial breakthrough requires practical applications and demonstrations. Therefore, we built a solenoidal react-and-wind MgB2 coil consisting of 46 m of commercially available MgB2 /Ni/Fe/Cu tape manufactured by Columbus Superconductors. We tested the coil in a cryogen-free environment and measured the effect of repeated cooldowns and current ramp rate on the coil critical current. Also, temperature homogeneity in the winding was studied. Based on the test results we point out features which should be checked when cryogen-free magnet systems are designed or their performance is discussed. For example, the coil critical current and n value can depend notably on the current ramp rate.