Eddy-current analysis of cold mass and thermal shield for series-connected hybrid magnet

The National High Magnetic Field Laboratory currently has three series-connected hybrid (SCH) magnet projects, where resistive coils are connected in series with superconducting coils using cable-in-conduit-conductor (CICC) underway: first for the magnet laboratory in Tallahassee, FL; second for the Helmholtz Zentrum Berlin for Materials and Energy (HZB), Germany; and the third for the Spallation Neutron Source at the Oak Ridge National Laboratory, TN. The one for HZB has a horizontal conical bore with a 30° opening angle for neutron scattering experiments. During power supply trip, superconducting magnet quench, resistive insert short, and insert fault, transient electromagnetic effects as a result of fast decay of the coil current introduce a significant amount of eddy current and Lorentz force on the conductive components of the cryostat such as the metallic cold-mass magnet frame and the 50-K thermal radiation shield. Although the eddy-current heating is not a concern during quench and fault operations, the eddy-current-induced Lorentz forces need to be taken into account in the structural design of the SCH cryostat. In this paper, a detailed eddy-current analysis for the HZB magnet during abnormal operations has been performed for its cryostat based on the dry magnet design concept. The nonuniform eddy-current distribution from the finite-element analysis implies that local hot spots may develop under abnormal operations. The eddy-current-induced Lorentz forces are quantified to ensure the strength and stability of the cryostat structure and, most importantly, safety of the SCH magnet during abnormal conditions.