Nb3Sn Magnet Development Breakthrough

1. Nb3Sn Magnet Development Breakthrough Alexander Zlobin Technical Division, Fermilab

2. Introduction Nb3Sn vs. NbTi • Bc2~28T (NbTi: 14T) => higher operation fields • Tc~18K (NbTi: 9K) => larger temperature margin • Jc(12T)~3 kA/mm2 (=Jc(5T) NbTi) => efficient coils Issues • Nb3Sn is brittle material sensitive to stress and strain => special materials, fabrication technologies, handling, coil support during operation • Nb3Sn strands are unstable wrt “flux jumps” due to large Jc and Deff => conductor optimization Nb3Sn accelerator magnet development • started in 70’s (BNL, Saclay) • last 10 years centered in U.S. => magnets: LBNL, Fermilab, BNL, TAMU; conductor: Labs, universities, industry) • Focused R&D + Adequate resources + Enthusiasm => breakthrough in Nb3Sn magnet development Zlobin Nb3Sn Magnet Development Breakthrough

3. Nb3Sn coil technology 1m Q coils Process: • W&R approach (reaction at ~650C during ~50 hrs) • high-temperature insulation – ceramic, S2 or E-glass • metallic coil components – water-jet method • ceramic binder – critical invention • coil vacuum impregnation with epoxy • coil size control – field quality Coil production: • 20 dipole and 34 quadrupole 1-m long coils • Good size reproducibility • Short fabrication time • 2 dipole and 11 quadrupole 4-m long coils • Technology scale up Handling and test: • Multiple reassembly without degradation with different structures • Coil and magnet handling and transportation across the country => Production quality Nb3Sn coil technology! 4m D coil Zlobin Nb3Sn Magnet Development Breakthrough

4. Mechanical structures • Coil pre-load and support reduce turn motion • Large Lorentz forces + Stress limit for Nb3Sn cable (150 MPa) => possible degradation during assembly and operation of brittle Nb3Sn coils • Model magnets (D and Q) were assembled and successfully tested with three different structures! • good performance of collar-based structure => solid base for accelerator quality Nb3Sn magnets! SS shell w/o collar (FNAL-HFDA) Al shell w/o collar (LBNL-TQS) SS shell + SS collar (FNAL-TQC) Zlobin Nb3Sn Magnet Development Breakthrough

5. Nb3Sn strand optimization • Conductor determines the SC magnet performance • Stable, high Jc Nb3Sn strand (RRP-108/127 with increased spacing) has been developed by Fermilab and OST • RRP-108/127 TQ coil successfully tested in quadrupole mirror • first time demonstration of stable operation at 4.5 and 1.9 K • Bmax~12 T (4.5K) and ~13 T (1.9K) => RRP-108/127 is baseline conductor for 11T Nb3Sn magnets! Zlobin Nb3Sn Magnet Development Breakthrough

6. Summary • Most important breakthroughs • Development of production-quality Nb3Sn coil technology • Demonstration of collar-based mechanical structures • Development of high-performance Nb3Sn strand => accelerator-quality Nb3Sn magnets (D and Q) • Fermilab HFM program made key contributions to all these breakthroughs • 10-11 T accelerator quality Nb3Sn magnets are real and can be considered now for practical applications • Solid base for higher field (~15 T) Nb3Sn accelerator magnets needed for Muon Collider and some other applications Zlobin Nb3Sn Magnet Development Breakthrough