Mechanical Loss of HTS Coils Reinforced With Negative Thermal Expansion Fiber Materials

High-$T_ c$superconducting (HTS) coils with various types of reinforcement are investigated with respect to the reduction of mechanical loss. The total AC loss of the coils is measured under an external DC magnetic field at liquid nitrogen temperature, the reduction of the mechanical loss components are estimated. Tensioned stainless-steel tape is found to be the most effective for reducing the mechanical loss by improving the rigidity of the entire winding. The mechanical loss of the HTS coils is reduced by reinforcement with a bobbin of negative thermal expansion material, although the hoop stress caused by radial expansion of the bobbin is absorbed by a small number of winding turns. Bobbins of negative thermal expansion material are therefore unsuitable for pancake-type coils or multilayer windings, but can be expected to realize substantial reductions in mechanical loss for single-layer windings or thin solenoids.