High-current density coils for high-radiation environments

The limits on current density for continuous operation of indirectly cooled mineral-insulated coils are explored by thermal analysis, and the criteria for cable optimization are established. The insulation system, compacted magnesium oxide powder, has these advantages: 1) Radiation insensitivity - no detectable degradation at 10¹⁴rads, with no reason to expect this to be close to any limit. 2) Flexibility - enabling standard coil-winding practice to be followed, without the problems of fired ceramics. 3) Good thermal conductivity - 2.36 W/m.K at room temperature. 4) High-temperature stability - making the metal components of the coil determine the operational temperature limit, rather than the insulation system as in conventional coils. The cooling system avoids oxidation problems caused by radiolysis of water. Other potentially radiation-sensitive components also are discussed. An example is given of the design procedure. The feasibility of extending the technique to commercial fusion reactor coils is examined.