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2009 APPLIED SUPERCONDUCTIVITYMagnetic field behaviour of ex-situ processed MgB 2 multifilamentary wiresIn order to make MgB 2 useful not only for dc but also for acapplications further conductor development and optimizationare still needed, in particular to reduce the ac lossescaused by magnetic hysteresis in the MgB 2 core, filamentcoupling and eddy currents flowing through the metallicmatrix. In this context research work should be focused onmultifilamentary strands with a large number of very finefilaments, twisted filaments and non-magnetic and high resistivitysheath.We have focused our work on obtaining multifilamentaryconductors with a large number of very fine filaments. Inthis context, the powder’s granulometry can play a crucialrole. In this experiment we have prepared two MgB 2 startingpowders which are either not milled (NM) or milled(M) with different granulometries (NM= 1.5 µm and M=450 nm) and by the ex-situ powder in tube (PIT) methodwe have realized multifilamentary wires with 19, 91 and361 filaments and an average size of each filament of 279,110 and 30 µm respectively. In figure 143 the cross sectionsof the three wire types are shown.We have studied the relationship between grain and filamentsize in terms of transport properties. The measuredcritical current densities (J C ) for the samples with NM powderand M powder are reported in figure 144. The criticalcurrent density improves with milling for all samples asreported in our previous work [A. Malagoli et al J. Appl.Phys. 104, 103908 (2008)]. Focusing on the behaviour ofthe not milled samples, passing from 19 to 91 filaments aremarkable critical current density degradation is evident,that is partially recovered going to 361 filaments. On thecontrary for the milled samples 19M and 91M have almostidentical critical current density - a slightly better behaviourin field being observed in 91M. When the number of filamentsincreases up to 361, critical current density decreasesstaying though above the 361NM.Such a behaviour of the critical current density in field cannotbe explained or well understood simply considering theeffects of milling. In these complex conductors several factorshave to be taken into account which have an effect onthe transport properties: the starting granulometry of theMgB 2 powders, the cold deformation force and the final filamentsize. Therefore, in the final analysis, the capabilityof these conductors to transport high critical currents cruciallydepends on a proper balance of these parameters. Inthis work we have obtained the best ratio filament size/grainsize on a 91 filaments wire with an average filament size ofabout 110 µm and a powder starting average grain diameterof about 450 nm. A finer MgB 2 granulometry seems to beneeded to realize very thin filaments (10−30 µm) with highcritical current density.Figure 143: Images of different cross sections through the wireswith 19, 91 and 361 filaments respectively.Figure 144: Transport critical current density (J C ) for differentmilled (M) and not milled (NM) samples measured up to magneticfields of 13 T using a wide bore resistive magnet and a Heliumbath cryostat (T = 4.2 K).E. MossangA. Malagoli, G. Romano, M. Vignolo, C. Ferdeghini, M. Putti (CNR-INFM LAMIA, Genova, Italy), S. Brisigotti, G.Grasso, A. Tumino (Columbus Superconductors S.p.A., Genova, Italy)101