N. Yanagi

S. Imagawa

T. Mita

Y. Hishinuma

National Institute for Fusion Science (NIFS-Repository)

6.512.512One-sidepropagation10.5#1-012 . "'2.72 TC:;() 11H-I ( 3)Heater: H1(L1T12S13), Heater duration: 0.02 s !Pressure of Current-lead Tank: 0.12 MPa .1'10'20100 , ..,c>·:··t··.. ·· .... :.{lIHl'!.'."'....... DOL> Not propagated§ 4.4 K : 4.2 K .... Propagated80 .OlCI3a.£ 60.2S 40a.E@Fig. 2. Flow diagram of the subcooled system.11 11.5Current (kA)Fig. 3 Minimum heat input for the propagation of anormal-zone in the R&D coil in a subcooled helium.5.5 6Bias Magnetic Field (T)Fig. 4. Dynamic normal-zone propagating current I(m.p.)*of the composite conductor for the LHD helical coil.1.;;-' :;-,@ --t?J (0) Spacer positionFig. 1. Schematic drawing of the R&D coils.§ 1. Results of Stability Test in SubcooledHelium with the R&D Coil for the LHDHel ical Coi IThe operating currents of the LHD helical coils arerestricted below about 90% of the design current because anormal zone has propagated dynamically at several times atalmost the same current [I]. An R&D coil, as shown inFig. 1, was made of the same conductor to estimate theeffect of lowering temperatures on the cryogenic stability.It was wound by layer winding of 24 turns and 12 layers.The magnetic field becomes the highest at the middle turnof the first layer, which is the testing region for thecryogenic stability. The value is 6.9 T at 13 kA, same asthe LHD helical coil. The wetting surface fraction of thefirst layer is 67%, same as the turns at the highest field ofLHD helical coil. Thermometers are installed not only onthe conductors but also in the cryogen to examine thetemperature differences in the coil.The cryogenic stability of the R&D coil was examinedin the saturated and subcooled helium. The flow diagram ofthe R&D system is shown in Fig. 2. Liquid helium issupplied from the bottom of the R&D coil. It is exhaustedthrough current leads to the current-leads tank. The inlethelium is subcooled by a pre-cooler, which is evacuated bytwo cold-compressors. Tape heaters are inserted between theconductor and the layer to layer spacer for initiating anormal zone. The heating duration is set to 20 ms to put asmuch energy as possible adiabatically. The propagationwas detected by voltage taps. The heater at the bottom ofthe middle turn of the first layer was mainly used.In saturated helium of 4.4 K and 0.12 MPa, theminimum current to begin propagation is 10.7 kA. Thenecessary heat input for the propagation becomes less atthe higher currents, as shown in Fig. 3. The normal zoneswere propagated only one side at less than 11.4 kA. Thedirection is, same as JsxB, where the J s is the vector ofthe transfer current from the superconducting strands to thealuminum stabilizer, and B is the magnetic field vector. At11.4 and 11.5 kA, the normal zones were propagated bothside, and recovered from the upper area in the middle turn.Namely, finite length of normal zones propagated bothsides. The propagating velocity at the direction of JsxB isalmost twice as the opposite direction. At 11.6 kA, thenormal zones started recovering once from the middle turn,but rapid propagation was followed. The cooling conditionwas considered to be deteriorated by accumulation ofhelium bubbles.In subcooled helium at 0.12 MPa, the minimumcurrents to begin propagation are increased up to 11.4 and11.6 kA at 4.2 and 3.8 K, respectively, as the temperaturesin the R&D coil. Figure 4 shows these points comparedwith the LHD helical coil. The necessary heat input isincreased by 10 to 20% for the saturated helium. The rapidpropagation was followed at 12.1 and 12.2 kA at 4.2 and3.8 K, respectively, in spite of one-side propagation.Imagawa, S., Yanagi, N., Hishinuma, Y., Mita, T.Reference1) S. Imagawa, et aI., IEEE Trans. Appl. Supercond.,Vol. 11 (2001) 1889-1892.93

Results of Stability Test in Subcooled Helium with the R&D Coil for the LHD Helical Coil

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