The toroidal field (TF) coils used in MIT9s C-Mod tokamak are (20) picture-frame shaped coils with sliding finger joints at each corner. The copper conductor has a cross section which is roughly 0.19 m deep by 0.0254 m thick. Current is transferred between finger joint through copper felt metal pads.

As a pulsed machine, the TF coils are ramped up to their flat top currents on a 1-2 s time scale. Early in the transient, the current distribution across the conductor is far from uniform, as most of the current is carried by the conductor skin closest to the plasma (on the smallest toroidal radius). In addition, the diffusion of current into the conductor as the transient progresses is a function of the conductor temperature: high currents on the conductor skin drive up the temperature of the copper, and thus increase its resistivity. As the resistivity of the conductor increases, it forces the current to a larger minor toroidal radius, and a more uniform distribution. This presents a rather difficult design problem, as the actual current distribution in the finger joint felt metal must be solved as a 3D, transient, coupled electromagnetic-thermal analysis.

This paper presents a modeling technique which uses ANSYS to solve for the 3D, time-dependent current and temperature distribution in the region of the finger joints. A 3D global model of the TF magnet system is presented which is used to solve for the field and temperature throughout the magnet system. The finite-element model solves for the 3D, time-dependent vector potential, the time-integrated electric scalar potential, and temperature degrees of freedom. Time-dependent boundary conditions obtained from the global model are applied to a detailed submodel of the finger joint region. The submodel transient analysis is used to ultimately capture the detailed current and temperature distribution in the finger joint and felt metal, and thus characterize the operating parameters of this critical element of the magnet system.

* Work sponsored by USDOE contract # DE-AC02-78ET51013