Thermal Conductivity Test of YBCO Coated Conductor Tape Stacks Interleaved With Insulated Stainless Steel Tapes

A 32 Tesla, all-superconducting user magnet, which consists of two high temperature superconductor YBCO inner coils producing a field of 17 T in an low temperature superconductor Nb₃Sn and NbTi outer magnet producing a background field of 15 T, is being developed at the National High Magnetic Field Laboratory. The YBCO inner coils are pancake-wound with YBCO coated conductor tapes with an interleaved insulation of sol-gel coated stainless steel tapes. The coils are to be cooled directly in liquid helium bath. Heat losses in the windings, such as ac losses during ramping and heat loss in the internal joints, are supposed to be transferred to the coil external surfaces through heat conduction. Thus, thermal conductivity of the coil structure is critical for the internal cooling of the coil and also quench propagation if any. Thermal conductivity measurements were carried out in the radial direction on stacks of alternating YBCO tapes and stainless steel tapes. This paper presents the test results that showed a very low thermal conductivity in the radial direction. For comparison purposes, calculated thermal conductivities in the axial and azimuthal direction are also presented.