HeII heat transfer through superconducting cables electrical insulation

For NbTi magnets cooled by superfluid helium (He II), the most severe heat barrier comes from the electrical insulation of the cables. Tests on electr ical multi-layer insulations, made of Kapton(R), dry fiber and epoxy resin impregnated fiberglass tapes, indicate that heat transfer is influenced by He II contained in the insulation. Electrical insulation can be considered as a composite material made of a solid matrix with a complicated helium ch annels network. For several insulations, this network is characterized by s teady-state heat transfer experiment through an elementary insulation patte rn. Measurements in Landau regime for low temperature difference (10(-5)-10 (-3) K) and in Gorter-Mellink (GM) regime for higher temperature difference s permit to determine an equivalent He II channel cross-section (10(-6) m(2 )) with an equivalent channel thickness (25 mu m). We use the assumptions t hat He II heat transfer through the channels network and conduction in the insulation are decoupled and that the channels length is determined from th e insulation overlap. It is observed that He II heat transfer is competing with conduction in the insulation. Furthermore, the mea surements reveal an anomaly of heat transfer in the vicinity of the lambda temperature which i s associated to the phenomenon of lambda-point depression. (C) 2000 Elsevie r Science Ltd. All rights reserved.