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APPLIED SUPERCONDUCTIVITY 2009Superconductivity of C and TiC doped multi-filamentary MgB 2 wiresRecently a great deal of both fundamental and technical researchhas been carried out on carbide-doped MgB 2 dueto its high upper critical field (H c2 ) which presents considerableinterest for the fabrication of practical magnets, especiallythe GM-cryocooled MRI magnet. For engineeringapplications, it is necessary to make MgB 2 superconductorsinto multi-filamentary wires or tapes. The in-situ and exsitupowder-in-tube (PIT) processes have become the dominantor standard methods due to their commercial potentialfor large-scale and low-cost production of MgB 2 wires andtapes.Figure 145: Cross section of the different multi-filamentMgB 2 /NbCu wires.In this work, various multi-filament MgB 2 /NbCu roundwires with carbon or TiC doping have been fabricated byin-situ PIT method at the Northwest Institute for NonferrousMetal Research (NIN). The typical diameter of the finalwire is 1 mm. The MgB 2 coil was fabricated using thewind and react method with a 1.5 m long wires and a heattreatment at 680 ◦ C for 1.5 h. The transport critical currentof the MgB 2 coils was measured with magnetic fields upto 10 T at various temperatures using a standard four probemethod. The critical current density, J c , of the coil was calculatedfrom the measured critical current I c divided by thecross sectional area of the MgB 2 core.Figure 145 shows the 6-, 12- and 36-filamentaryMgB 2 /NbCu wires with amorphous carbon doping. Thevolume of MgB 2 in whole wires is around 20%, 14% and14%, respectively. Figure 146 shows the transport criticalcurrent density J c values as a function of applied field at20 K for the multi-filamentary MgB 2 coils with amorphouscarbon or TiC doping. The 6-filament wires with carbondoping has the largest J c , as high as 4.5×10 4 A/cm 2 at 2 T.The large value of J c in this sample may be due to the goodgrain connectivity and strong flux pinning force. This resultindicates that carbon doping is beneficial for the fabricationof high performance long length multi filamentary MgB 2wires.Figure 146: Transport critical current density of different multi–filamentary MgB 2 coils at T = 20 K.Figure 147 shows the result of the transport J c versus Hmeasurements at temperatures from 4.2 to 30 K measuredfor 6-filament wires with carbon doping. This value islower than the best values found in the literature, but webelieve that the transport J c could be improved by optimizingvarious processing parameters.In summary, the J c of multi-filamentary long length MgB 2wires were enhanced by amorphous carbon doping. Thehighly reactive amorphous carbon can easily substitute intothe lattice of MgB 2 even with a heat-treatment at lower temperature.On the other hand, the lower heat-treatment temperatureresults in a smaller MgB 2 grain size, which introduceda high density of flux-pinning centers.Figure 147: Transport critical current density of a 6-filamentMgB 2 coil at various temperatures.E. MossangQ.Y. Wang, G. Yan (Northwest Institute for Non-Ferrous Metal Research, Xi’an, China), A. Sulpice (Institut Neel,Grenoble102