Design considerations for the TF coil center conductor post for the Ignition Spherical Torus (IST)

A trade-off study has been carried out to compare costs of using high-strength alloy copper versus oxygen-free, high-conductivity (OFHC) copper for the center legs of the toroidal field (TF) coils of an Ignition Spherical Torus (IST). The electrical heating, temperatures, stresses, cooling requirements, material costs, pump costs, and power to drive the TF coils and pumps are all assessed for both for a range of compact tokamak reactors. The alloy copper material is found to result in a more compact reactor and to allow use of current densities of up to 170 MA/m2 versus 40 MA/m2 for the OFHC copper. The OFHC conductor system with high current density is Dollar 24 million less expensive than more conventional copper systems with 30 MA/m2. The alloy copper system costs Dollar 32 million less than conventional systems. Therefore, the alloy system offers a net savings of Dollar 8 million compared to the 50% cold-worked OFHC copper system. Although the savings are a significant fraction of the center conductor post cost, they are relatively insignificant in terms of the total device cost. It is concluded that the use of alloy copper contributes very little to the economic or technical viability of the compact IST. It is recommended that a similar symstematic approach be applied to evaluating coil material and current density trade-offs for other compact copper-TF-coil tokamak designs. (orig.)