FIRE/NSO toroidal field coil structural/thermal analyses

The toroidal field coil system of the FIRE tokamak utilizes LN₂ cooled, copper alloy Bitter plate type magnets. The baseline configuration is wedged, C17510 high strength, high conductivity BeCu alloy conductor developed for BPX is proposed for the conductor. These design choices were made after considering a number of alternative structural concepts and a variety of conductor materials. FIRE´s high elongation and double null plasma results in high overturning moments; it is primarily for this reason that the wedged configuration was adopted. Results of coupled electromagnetic-thermal diffusion analyses of the TF coil are presented along with parametric studies of the pulse length. A central tierod assembly is intended to contribute a small amount to reducing the inner leg vertical tension stress. Difficulties arise from OOP support for all of the in-plane load carrying concepts. The upper and lower inner corners of the TF de-wedge from the tension in the horizontal legs and the differential heat-up of the inner leg. Two mechanisms have been investigated to solve this problem. The first is to accept the de-wedged condition. For this case, the torsional shear concentrates near the equatorial plane of the inner leg, and the de-wedged regions must “flex” to allow the relative motion of the central column and the outer coil structure. Bending stresses of the “flexing” coils must be accommodated. Dividing the Bitter plates into multiple thin sections that flex easily with OOP displacements was investigated. This “flexure” approach required that there be no net torque on the central column, which, for nominal loading was not a problem. FIRE is proposed as a double null, up-down symmetric machine. Off-normal conditions, and disruptions, could, however produce net loads and torques on the central column. The second solution, which has been adopted for the baseline, is to add large rings to offset the horizontal leg tension. This is similar in concept to the large static ring used in IGNITOR, but the positioning of the ring in FIRE is intended to offset radially outward in-plane loads, rather than to offset vertical tension in the inner leg, as in IGNITOR