Helium II heat transfer in LHC magnets: Polyimide cable insulation

Today’s large particle accelerators like the LHC at CERN are using superconducting materials as a construction material for magnets. These magnets need to be cooled constantly to temperatures below the critical surface of the superconducting material. In the LHC this is achieved by using liquid Helium at a temperature of 1.9K. At this temperature Helium is in a liquid state that is called Helium II, which is characterized by its low viscosity and its high effective thermal conductivity. In this work heat transfer in Helium II between the superconductor and the He II bath is investigated in steady-state as well as in transient conditions. The geometry investigated in this work is a result of the design requirements of the LHC and consists of microchannels formed by the polyimide insulation of the superconducting Rutherford cable. As results of the work it is found that two distinct channels are formed by the polyimide insulation, where one channel has a significant contribution at low heating power and the second channel becomes dominant at higher heating power. To further confirm this result the cable experimental data is obtained in the temperature range 1.7K to 2.1K and the heating power is varied between 0.61 mW/cm3 and 4.9 mW/cm3. Additionally a model is developed for the effective thermal conductivity of He II in order to replicate the measurement data. In this effective thermal conductivity model a critical temperature gradient in introduced to model the transition between the laminar heat transfer regime and the turbulent heat transfer regime. The experimental data can be reproduced very well using a two channel model for the geometry and the effective thermal conductivity model.