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

Abstract

a b s t r a c tThe 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.

Finite Element Modeling in 3D of the Impact of Superfluid Helium Filled Micro-Channels on the Heat Transfer through LHC Type Cable Insulation

“…This also implies that below the lambda temperature adjacent cables are largely thermally decoupled. Above 2.17 K, thermal conduction through the Kapton becomes more important [9] and a much stronger coupling occurs. For simplicity, the thermal conductivity of the cable is taken as 1 kW/mK, a value comparable to copper at 1.9 K.…”

“…From an engineering point of view it is only interesting to know what the temperature increase of a cable is when subjected to a certain heat deposition. Since the temperature rises quickly once in the micro-channels the lambda temperature is reached [9], the focus here is on the temperature range between 1.9 and 2.17 K. Fig. 4 shows schematically how the insulation layer with micro-channels is implemented in the software and how the mesh looks like.…”

“…Also an analytic and a numerical model have been made to calculate the heat transfer [10]- [11]. Here a new and much more detailed 3D FEM model is presented and used to demonstrate the beneficial features of the enhanced insulation by comparison to measured data [9].…”

“…Many years are spent to better understand heat transfer through the cable insulation [5]- [8]. An enhanced cable insulation has been proposed [7] and subjected to several tests [8]- [9]. Also an analytic and a numerical model have been made to calculate the heat transfer [10]- [11].…”

Unsteady heat dissipation in accelerator superconducting coils insulated with porous ceramic insulation in normal and supercritical helium conditions

“…In the literature there is a lot of attention paid to researches on thermal processes in superconducting cables and their numerical analyses. Recently, these research focused mainly on the electrical insulation made of polyimide in different configurations of tapes for NbTi magnets [3][4][5][6][7]. Additionally numerical analyses were performed mostly for magnets immersed in superfluid helium during steady state conditions [4].…”