Laboratory for Particle Physics ( MCS ) CARE-Pub-07-003 LOWFIELD INSTABILITIES IN N b 3 S n MULTIFILAMENTARY WIRES : THE POSSIBLE ROLE OF UNREACTED Nb

We report an experimental study aiming to demonstrate the not negligible role of unreacted Nb on the magnetic instabilities in superconducting Nb3Sn multifilamentary wires, observable through partial flux jumps at magnetic field values below 0.5 T. The analysed wires were recently developed for use as dipoles required in future high-energy proton accelerators and are based on powder-in-tube technology. We studied both unreacted (only involving Nb filaments) and reacted wires, finding flux jump instabilities in both cases when performing magnetic measurements. The results can be interpreted on the basis of the critical state model and are coherent with the intrinsic stability criterion. (Some figures in this article are in colour only in the electronic version) In recent years we have observed renewed interest in Nb3Sn superconductors. In spite of the progress with high-Tc superconductors optimized for applications (e.g. YBCOcoated conductors) and of very promising developments of magnesium diboride wires, large projects involving highfield superconducting magnets are still looking for conductors based on Nb3Sn. The ITER project, aiming to develop a Tokamak fusion reactor, envisages a cable in a conduit Nb3Sn conductor for toroidal and poloidal coils and for the central solenoid. American and European teams are developing highfield-gradient quadrupole and high-field dipole magnets (15 T) for the interaction regions of the Large Hadron Collider at CERN, based on a multistrand conductor, made of Nb3Sn multifilamentary wires [1, 2]. The basic feature making Nb3Sn so appealing is its ability to carry very high current density in practical wires, typically 2400 A mm−2 at a temperature of 4.5 K and applied magnetic field of 12 T. Unfortunately, recent developments in high-field accelerator magnets showed the Achilles’ heel of wires carrying high current density [3, 4]. It is a well-known problem since the early developments of superconducting wires in the 1960s: a local temperature increase, due to a disturbance, causes a sudden magnetic flux penetration into the superconductor, generating a further heat dissipation [5, 6]. This avalanche process can be controlled (no transition to normal state occurs) if the wire is thin enough according to the formula b < √ 3γ Cp(Tc(B) − Top) μ0 J 2 c (B, Top) , (1) where b is the wire diameter, γ is the mass density, Cp is the specific heat, Tc is the critical temperature at a given magnetic field B, Top is the operating temperature and Jc is the critical current density at the operating temperature. On the basis of this simple formula (the adiabatic stability criterion) the need to develop multifilamentary wires was understood with 0953-2048/07/060034+04$30.00 © 2007 IOP Publishing Ltd Printed in the UK