A single-turn coil configuration is described with an unshielded central column, which must carry a high current density continuously whilst subjected to high neutron fluxes. A 3-dimensional thermal-hydraulic analysis shows that a current density of 5 kA/cm2 is practicable whilst 8 kA/cm2 is possible. A column with a waisted profile is seen to dissipate significantly less power. Oxide dispersion-strengthened copper is found to be the most suitable material for the column and provides sufficient strength to support the self-forces on the coils. Demountable joints between the column and the parallel return conductors facilitate its annual replacement, which limits the ohmic power dissipation and specific radioactivity. The use of a single-turn coil leads to very high power supply current requirements (typically 6 MA). A power supply with 12 modules positioned around the torus, supplying the magnet in parallel through radial busbars, is proposed. The trade-offs which must be made, if future experimental results indicate that multi-turn coils can be insulated, are discussed.
