The Role of Advanced Modeling in the Assembly and Operation of Tokamak Magnets

This paper provides an overview on the engineering analyses workflow typically executed in support of the assembly and operation of superconducting magnet systems, in particular for ITER. Accurate field computations are required to evaluate (1) Lorentz forces as primary loads on magnets (2) cable temperature margins (3) critical error field harmonics and (4) field lines tracing. Although the required accuracy increases with each of these magnetic computations, their common aim, beyond use during machine design, is to provide estimates on the final alignment of magnetic fields and, possibly, to guide and adjust magnet installation. Two global mechanical models (18 TF and 2 TF coils) featuring their interfaces to the Pre-Compression Rings, CS and PF coils are used to assess the impact of tolerances and misalignments on the final coils’ positions in operation so to guide, with the aid of metrology data, the assembly process. With these, we also evaluate the mechanical stresses during current tests at 4 K if performed before final installation. A global and several local models of the Pre-Compression Rings that must be mounted in the tokamak were developed and used to design the tightening sequence during PCRs assembly and the required assembly tooling. Magnet systems also require sophisticated models to predict conductor temperature margins and to assist during thermal transients (magnets cooldown). Thermal loads due to Joule losses in cold structures are evaluated with 3D eddy-current codes and nuclear heating with detailed Monte-Carlo models suited to simulate streaming and deep penetration 3D phenomena leading to nuclear power deposition in the cables.