Heat transfer through cable insulation of Nb–Ti superconducting magnets operating in He II

P.P Granieri

doi:10.1016/j.cryogenics.2012.06.007

Abstract

The operation of Nb–Ti superconducting magnets in He II relies on superfluidity to overcome the severe thermal barrier represented by the cable electrical insulation. In wrapped cable insulations, like those used for the main magnets of the Large Hadron Collider (LHC) particle accelerator, the micro-channels network created by the insulation wrappings allows to efficiently transfer the heat deposited or generated in the cable to the He bath.In this paper, available experimental data of heat transfer through polyimide electrical insulation schemes are analyzed. A steady-state thermal model is developed to describe the insulation of the LHC main dipole magnets and the Enhanced Insulation proposed for the High Luminosity LHC upgrade (HL-LHC), according to the relevant geometric parameters. The model is based on the coupled mechanisms of heat transfer through the bulk of the dielectric insulation and through micro-channels between the insulation tapes.A good agreement is found between calculations and tests performed at different applied pressures and heating configurations. The model allows identifying the heat fluxes in the cable cross-section as well as the dimensions of the micro-channels. These dimensions are confirmed by microscope images of the two insulations schemes.

Highlights

Superconducting accelerator magnets are confronted with heat deposit and heat generation due to several mechanisms such as beam losses and AC losses, respectively
In the Nb–Ti conductors constituting the main magnets of the Large Hadron Collider (LHC), the choice of superfluid helium (He II) coolant and of a porous insulation scheme allowed improving the heat transfer [3] with respect to sealed insulation schemes as that of the Superconducting Super Collider [4]
Adding the slits in the cable large faces provides a solution to this problem, since the corresponding heat extraction is larger if the cables adjacent to the central one are not heated

Summary

Introduction

Superconducting accelerator magnets are confronted with heat deposit and heat generation due to several mechanisms such as beam losses and AC losses, respectively. In case of the foreseen High Luminosity LHC (HL-LHC) upgrade, the required magnets will have to withstand larger heat loads than in the current LHC operation [5] For this reason they will make use either of the Nb3Sn technology that provides a large temperature margin, or of the Nb–Ti technology featuring an increased heat extraction capability thanks to a thermally enhanced insulation scheme [6,7]. The thermal characterization of electrical insulations of superconducting cables has been mainly addressed using the stack method, an experimental setup reproducing the coil configuration This kind of measurement was first performed at CEA-Saclay using solid conductors during the R&D phase of the LHC main magnets [3,8], as well as at KEK using resistive cables with the same geometry of the superconducting ones to study the LHC MQXA interaction region quadrupole [9,10]. This allowed a first decoupled modeling of the polyimide and helium heat transfer contributions

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