A. Iwamoto

R. Maekawa

T. Mito

National Institute for Fusion Science (NIFS-Repository)

Fig.l. Schematic illustration of the sample::1:::'::::1::::"8.07.0gQ) 6.0I-;:,10I- 5.0Q)0-E2 4.0Q)03.0;:,CJ)2.01.00Heat transfer 020surface GFRPFlangeLO --.Stainlesssteel flangeGFRP0NLO Thermometer<D0 OFCN cylinderThermofoilheater10000 20000 30000 40000 50000Heat flux [W/rrf]Fig.2. Kapitza conductance of the oxidized copper surfaceat 1.97 K in subcooled and saturated He IIReference1) Iwamoto, A., Maekawa, R. and Mito, T., Cryogenics 41,(2001) 367.The Kapitza conductance of the oxidized surface insubcooled and saturated He II was measured. Fig. 2 showsthe surface temperature variation. The heat flux wasprovided up to 17.3 kW/m2 and 36.6 kW/m2 in subcooledand saturated He II, respectively. The variations in themaximum heat fluxes resulted from the limitedrefrigeration capacity in each measurement. There is noapparent difference in Kapitza conductance for eithersubcooled or saturated He II. The variation is the maximumof 1.2 % in the range of the heat flux from 0 to 17.3 kW1m2 •Even though the surface morphology is quite different forthe chemically oxidized surface, the Kapitza conductance isnot affected by the pressure difference in He II; like othermetals considered in the same temperature region.Kapitza conductance is an essential characteristic forheat transfer from a metal to He II. Temperaturediscontinuity occurs at the metal-He II interface. Manystudies have been performed to investigate the dependenceon various metals because of engineering requirements. Onthe other hand, superconducting magnets make use ofsubcooled or saturated He II, around 1.8 K, as the coolant.It has been already reported that the Kapitza conductanceof a copper surface around 1.8 K, which corresponds theoperation temperature of superconducting magnet, isindependent of the He II pressure.The helical coil system of the Large Helical Device(LHD) might be cooled by subcooled He II in the future toincrease the magnetic field. The helical coil conductor isthe aluminum-stabilized, pool-boiling, superconductor witha copper sheath. Its surface is treated by chemical oxidationto improve the heat transfer characteristics in He I. Theknowledge of the Kapitza conductance of the chemicallyoxidized copper surface is required for the stability analysisand it had been measured in saturated He II.l) However,the oxidized surface has quite different appearance from acopper surface; it must be considered that the difference ofsurface condition possibly affects the Kapitza conductancein subcooled He II.In the present study, the heat transfer of thechemically oxidized copper surfaces has been measured.The dependence of the Kapitza conductance of the oxidizedsurface on subcooled and saturated He II is discussed.The configuration was designed for steady state heattransfer measurement as shown in Fig. 1. The sample is acylindrical shape, 20 mm in diameter and 70 mm in length,with three holes for thermometers and was machined froman oxygen-free copper (OFC) block, RRR=80±20. Thesurface was treated by chemicals. Three thermometers,germanium sensors, were placed into the holes withCry-Con thermal conductive grease for good thermalcontact at the interval of 20 mm to measure the temperaturedistribution. A thermofoil heater, 74.5 ohms at roomtemperature, was glued on the opposite side of the heattransfer surface by Stycast 1266. In order to minimize theheat leak via a flange, the sample with a glass fiberreinforced plastic(GFRP) flange was assembled to astainless steel flange which is a part of a vacuum can.Indium seal was used at the interface between the flanges.The cryostat was evacuated and filled with gaseoushelium. After the vacuum for thermal insulation of thesample was achieved less than 1.33x10-2 Pa, the cryostatwas charged with He I. Then, the sample was cooled downto 1.97 K of subcooled or saturated He II.§ 7. Kapitza Conductance of an OxidizedCopper Surface in Subcooled andSaturated He IIIwamoto, A., Maekawa, R., Mito, T.99

Kapitza Conductance of an Oxidized Copper Surface in Subcooled and Saturated He II

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