Conceptual design of a superconducting coil system cooled by forced flow He II for the central column of NET

The poloidal field and ohmic heating coils for the Next European Torus Tokamak are envisioned for operation at magnetic fields as high as 10-14 T. To increase the maximum flux sweep, present designs either use niobium tin for the superconducting coils or inset a copper coil cooled by liquid nitrogen in the NbTi coil or even both. Based on experience with the TORE SUPRA, the authors argue that cooling NbTi to a lower temperature is a highly competitive and safe alternative. Two solutions are investigated: the first uses a 40-kA cable wound in double pancakes. The second makes use of several conductors layer-wound with current supplied in parallel. The novel aspect of both solutions is that He II circulates inside the conductor itself, introducing the advantages of a force-flow-cooled conductor at the cost of the implementation of a circulating device.