Thermal and mechanical properties of advanced impregnation materials for HTS cables and coils

N Bagrets,A Kario,K-P Weiss,W Goldacker,S Otten

doi:10.1088/1757-899x/102/1/012021

Abstract

In the growing field of high-temperature superconducting (HTS) applications, finding an appropriate impregnation material for cables and coils remains a challenging task. In HTS cables and coils, tapes have to be able to withstand mechanical loads during operation. Impregnation is playing a role as mechanical stabilization. However, material properties usually change significantly when going to low temperatures which can decrease performance of superconducting devices. For example, a large mismatch in thermal expansion between a conductor and impregnation material at low temperatures can lead to delamination and to degradation of the critical current. Impregnation materials can insulate tapes thermally which can lead to damage of the superconducting device in case of quench. Thus, thermal conductivity is an important property which is responsible for the temperature distribution in a superconducting cable or in a coil. Due to Lorentz forces acting on structural materials in a superconducting device, the mechanical properties of these materials should be investigated at operating temperatures of this device. Therefore, it is important to identify an advanced impregnation material meeting all specific requirements. In this paper, thermal and mechanical properties of impregnation material candidates with added fillers are presented in a temperature range from 300 K to 4 K.

Highlights

Impregnation of high-temperature superconducting (HTS) devices is applied for mechanical reinforcement against Lorentz forces or to facilitate heat conduction in conduction–cooled coils
Attempts to impregnate HTS devices with such resins, showed that the thermal expansion mismatch can lead to damage of the conductor [1]
Filler particles decrease the thermal expansion mismatch with the conductor; beeswax, paraffin, or epoxy with shrink tube as barrier do not bond to the conductor and, in this way, prevent transmission of thermal stresses

Summary

Introduction

Impregnation of HTS devices is applied for mechanical reinforcement against Lorentz forces or to facilitate heat conduction in conduction–cooled coils. Attempts to impregnate HTS devices with such resins, showed that the thermal expansion mismatch can lead to damage of the conductor [1]. Alternative impregnation materials including epoxy resins with filler particles [2], paraffin [3], or introducing barrier between conductor and epoxy resin [4] are considered and applied. It was shown that transverse mechanical loads lead to degradation of critical current of the Roebel cable due to its complex architecture [5]. Such transverse stresses are unavoidable in superconducting magnets, appropriate impregnation of the cable is needed

Objectives

Methods

Results

Conclusion