The evaporated helium in superconducting coils is liquefied by an on-board refrigerator for re-circulation, with a power source of about 1kW required to liquefy helium evaporated at a heating value of 1W. It is therefore important to suppress heat generation in superconducting coils on running vehicles in order to cut operating costs and environmental burdens.
   While focusing on friction (a cause of heat generation) in superconducting coils, the RTRI tested minor displacement friction elements immersed in liquid helium. The test results indicate that a large quantity of friction heat is generated at the contact face between the spacer and the polyimide tape covering the surface of molded coils impregnated with resin.
   Keeping an eye on the relationship between the friction heat generated and the surface hardness of the molded coil, the RTRI also performed element tests, confirming that the surface hardness greatly contributes to heat generation. Based on this finding, the RTRI placed metal foil (e.g. aluminum or stainless steel) on the surface of the molded coil to increase its hardness, and found that the quantity of friction heat generated was approximately halved (Fig. 1). It is thought that the generation of friction heat will be suppressed at the fixing point in actual superconducting coils if the contact face between the spacer and the epoxy resin is completely wrapped in a belt of metal foil and covered with polyimide tape (Fig. 2). This study was promoted with a subsidy from the Ministry of Land, Infrastructure and Transport.