Impact of instabilities on Nb3Sn magnets

Thursday 24 Mar 2022, 16:00 → 17:30 Europe/Zurich

ZOOM

Emanuela Z Barzi (Fermi National Accelerator Laboratory - FNAL)

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Abstract:

Magnetic instabilities have impaired Nb3Sn magnets for decades. In high-field accelerator magnets their effect ranged from the magnets not reaching 100% of their short sample limit (SSL) to performing at 50% of SSL. About twenty years ago, instabilities studies were started at Fermilab by analyzing Nb3Sn wires of various dimensions using systematic macroscopic and microscopic studies of the effect of wire deformations. By using the technique of flat-rolling wires, which allows to impart various levels of homogeneous deformation along the wire, it was found that above some threshold level of plastic deformation, the Nb3Sn filaments coalesced with each other. This "merging" effect sometimes produced equivalent filament sizes several times larger than the original one. This same phenomenon was then verified also in Rutherford cables. The next step was to change the design of the billet, made at the time by Oxford Instruments Superconducting Technology (OST), in increasing the Cu spacing between the Nb3Sn subelements. In this talk, we will show how in Restacked Rod Process strands, contrary to Powder-in-Tube, filaments sometimes merge into each other, creating larger ones with a continuous barrier. If filaments are fused together, the strand sees a larger filament size and its instability can dramatically increase locally. It was experimentally found that the mechanism by which the extra Cu thickness in the new OST design is effective is that of providing a barrier to merging, not as much during the deformation process as during reaction. These experimental findings were followed by finite element models and further studies of Nb3Sn wires under plastic deformation. Following these developments, OST adopted the spaced-filaments design as standard for HEP wires in 2007. The new RRP conductor with extra spaced filaments has been adopted by the LHC Accelerator Research Program in 2008, as well as by the FNAL-CERN collaboration on the 11 Tesla Dipole Program for the LHC upgrades. However, some cases remain in which poor magnet performance is to be attributed to magnetic instabilities. In the second part of this talk, we will propose a few cases of recent Nb3Sn dipoles and quads which suffered from this problem.

Instabilities_CERN_march24_Barzi.pdf